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Internet Engineering Task Force (IETF) W. Hardaker
Request for Comments: 6353 SPARTA, Inc.
Obsoletes: 5953 July 2011
Category: Standards Track
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 an
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/rfc6353.
Hardaker Standards Track [Page 1]
RFC 6353 TLS Transport Model for SNMP July 2011
Copyright Notice
Copyright (c) 2011 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
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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
1.2. Changes Since RFC 5953 . . . . . . . . . . . . . . . . . . 8
2. The Transport Layer Security Protocol . . . . . . . . . . . . 8
3. How the TLSTM Fits into the Transport Subsystem . . . . . . . 8
3.1. Security Capabilities of This Model . . . . . . . . . . . 11
3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.2. Message Protection . . . . . . . . . . . . . . . . . . 12
3.1.3. (D)TLS Connections . . . . . . . . . . . . . . . . . . 13
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 14
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 14
4. Elements of the Model . . . . . . . . . . . . . . . . . . . . 15
4.1. X.509 Certificates . . . . . . . . . . . . . . . . . . . . 15
4.1.1. Provisioning for the Certificate . . . . . . . . . . . 15
4.2. (D)TLS Usage . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. SNMP Services . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1. SNMP Services for an Outgoing Message . . . . . . . . 18
4.3.2. SNMP Services for an Incoming Message . . . . . . . . 19
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RFC 6353 TLS Transport Model for SNMP July 2011
4.4. Cached Information and References . . . . . . . . . . . . 20
4.4.1. TLS Transport Model Cached Information . . . . . . . . 20
4.4.1.1. tmSecurityName . . . . . . . . . . . . . . . . . . 20
4.4.1.2. tmSessionID . . . . . . . . . . . . . . . . . . . 21
4.4.1.3. Session State . . . . . . . . . . . . . . . . . . 21
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 21
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 21
5.1.1. DTLS over UDP Processing for Incoming Messages . . . . 22
5.1.2. Transport Processing for Incoming SNMP Messages . . . 23
5.2. Procedures for an Outgoing SNMP Message . . . . . . . . . 25
5.3. Establishing or Accepting a Session . . . . . . . . . . . 26
5.3.1. Establishing a Session as a Client . . . . . . . . . . 26
5.3.2. Accepting a Session as a Server . . . . . . . . . . . 28
5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 29
6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 30
6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 30
6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 30
6.3. Statistical Counters . . . . . . . . . . . . . . . . . . . 30
6.4. Configuration Tables . . . . . . . . . . . . . . . . . . . 30
6.4.1. Notifications . . . . . . . . . . . . . . . . . . . . 31
6.5. Relationship to Other MIB Modules . . . . . . . . . . . . 31
6.5.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 31
7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 31
8. Operational Considerations . . . . . . . . . . . . . . . . . . 54
8.1. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 54
8.2. Notification Receiver Credential Selection . . . . . . . . 54
8.3. contextEngineID Discovery . . . . . . . . . . . . . . . . 55
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 . . . . . . . . . . . . . . . 57
9.3. Use with SNMPv1/SNMPv2c Messages . . . . . . . . . . . . . 57
9.4. MIB Module Security . . . . . . . . . . . . . . . . . . . 57
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59
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
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RFC 6353 TLS Transport Model for SNMP July 2011
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].
Hardaker Standards Track [Page 4]
RFC 6353 TLS Transport Model for SNMP July 2011
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].
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RFC 6353 TLS Transport Model for SNMP July 2011
+---------------------------------------------------------------------+
| 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 |
+---------------------------------+ +---------------------------------+
Hardaker Standards Track [Page 6]
RFC 6353 TLS Transport Model for SNMP July 2011
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.
Hardaker Standards Track [Page 7]
RFC 6353 TLS Transport Model for SNMP July 2011
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].
1.2. Changes Since RFC 5953
This document obsoletes [RFC5953].
Since the publication of RFC 5953, a few editorial errata have been
noted. These errata are posted on the RFC Editor web site. These
errors have been corrected in this document.
This document updates the references to RFC 3490 (IDNA 2003) to
[RFC5890] (IDNA 2008), because RFC 3490 was obsoleted by RFC 5890.
References to RFC 1033 were replaced with references to [RFC1123].
Added informative reference to 5953.
Updated MIB dates and revision date.
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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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 |
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RFC 6353 TLS Transport Model for SNMP July 2011
| +-------------+ +---------+ +--------------+ +-------------+ |
| | 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
Hardaker Standards Track [Page 11]
RFC 6353 TLS Transport Model for SNMP July 2011
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
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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 are 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
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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 to 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 an 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
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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 an 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.
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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.
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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.
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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.
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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.
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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.
5. Elements of Procedure
Abstract service interfaces have been defined by [RFC3411] and
further augmented by [RFC5590] 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.
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.
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Section 5.1.1 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 describes the transport processing required once the
(D)TLS processing has been completed. This will be needed for all
(D)TLS-based connections.
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. For DTLS over UDP, this
demultiplexing will either need to be done within the DTLS
implementation, if supported, 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], 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
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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 are 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 to complete the processing of the incoming message.
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. 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.
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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 prior to this point or they may
perform them now, but the procedures described in Section 5.3.2
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 from which the message
originated.
tmSecurityLevel = The derived tmSecurityLevel for the session,
as discussed in Sections 3.1.2 and 5.3.
tmSecurityName = The derived tmSecurityName for the session as
discussed in Section 5.3. 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
)
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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 tmSessionID is either 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 an snmpTlstmSessionNoSessions error to
the calling module and stop the 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.
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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). 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
the 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.
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.
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 an
SNMP over a (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.
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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 an 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 a 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
an snmpTlstmServerCertificateUnknown or
snmpTlstmServerInvalidCertificate notification SHOULD be sent as
appropriate. Reasons for server certificate invalidation
include, but are 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.
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If the connection is being established from a 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, and the
snmpTlstmSessionInvalidServerCertificates object is incremented.
If the session cannot 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).
5.3.2. Accepting a Session as a Server
A (D)TLS server should accept new session connections from any client
for which it is able to verify the client's credentials. This is
done by authenticating the client's presented certificate through a
certificate path validation process (e.g., [RFC5280]) 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.
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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 cannot 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]. Supporting this will allow, for
example, sending notifications to a specific principal at a given TCP
or UDP port.
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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
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.
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.
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:
o A new TransportAddress format for describing (D)TLS connection
addressing requirements.
o A certificate fingerprint allowing MIB module objects to
generically refer to a stored X.509 certificate using a
cryptographic hash as a reference pointer.
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.
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 tmSecurityNames.
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RFC 6353 TLS Transport Model for SNMP July 2011
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).
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], the SNMP-FRAMEWORK-MIB [RFC3411],
the SNMP-TARGET-MIB [RFC3413], the SNMP-NOTIFICATION-MIB [RFC3413],
and the SNMP-VIEW-BASED-ACM-MIB [RFC3415].
The SNMP-TLS-TM-MIB module contained in this document is for managing
TLS Transport Model information.
6.5.1. MIB Modules Required for IMPORTS
The SNMP-TLS-TM-MIB module imports items from SNMPv2-SMI [RFC2578],
SNMPv2-TC [RFC2579], SNMP-FRAMEWORK-MIB [RFC3411], SNMP-TARGET-MIB
[RFC3413], and SNMPv2-CONF [RFC2580].
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 -- RFC 2578 or any update thereof
TEXTUAL-CONVENTION, TimeStamp, RowStatus, StorageType,
AutonomousType
FROM SNMPv2-TC -- RFC 2579 or any update thereof
MODULE-COMPLIANCE, OBJECT-GROUP, NOTIFICATION-GROUP
FROM SNMPv2-CONF -- RFC 2580 or any update thereof
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB -- RFC 3411 or any update thereof
snmpTargetParamsName, snmpTargetAddrName
FROM SNMP-TARGET-MIB -- RFC 3413 or any update thereof
;
snmpTlstmMIB MODULE-IDENTITY
LAST-UPDATED "201107190000Z"
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RFC 6353 TLS Transport Model for SNMP July 2011
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) 2010-2011 IETF Trust and the persons identified
as authors of the code. 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 "201107190000Z"
DESCRIPTION "This version of this MIB module is part of
RFC 6353; see the RFC itself for full legal
notices. The only change was to introduce
new wording to reflect require changes for
IDNA addresses in the SnmpTLSAddress TC."
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RFC 6353 TLS Transport Model for SNMP July 2011
REVISION "201005070000Z"
DESCRIPTION "This version of this MIB module is part of
RFC 5953; see the RFC itself for full legal
notices."
::= { mib-2 198 }
-- ************************************************
-- 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 via TCP 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 8 }
snmpDTLSUDPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over DTLS via 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 9 }
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RFC 6353 TLS Transport Model for SNMP July 2011
SnmpTLSAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"Represents an IPv4 address, an IPv6 address, or a
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 RFC 5952), 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 RFC 1123);
internationalized hostnames are encoded as A-labels as specified
in RFC 5890. 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
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RFC 6353 TLS Transport Model for SNMP July 2011
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 1123: Requirements for Internet Hosts - Application and
Support
RFC 5890: Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework
RFC 5952: A Recommendation for IPv6 Address Text Representation
"
SYNTAX OCTET STRING (SIZE (1..255))
SnmpTLSFingerprint ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1x:1x"
STATUS current
DESCRIPTION
"A fingerprint value that can be used to uniquely reference
other data of potentially arbitrary length.
An 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 (RFC 5246). 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
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RFC 6353 TLS Transport Model for SNMP July 2011
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 lowercase. 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
lowercase.
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
lowercase (RFC 5280 does not specify converting to
lowercase so this involves an extra step). This
mapping results in a 1:1 correspondence between
subjectAltName dNSName values and the tmSecurityName
values."
REFERENCE "RFC 5280 - 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.
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RFC 6353 TLS Transport Model for SNMP July 2011
This mapping results in a 1:1 correspondence between
subjectAltName iPAddress values and the
tmSecurityName values.
The resulting length of an encoded IPv6 address 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
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RFC 6353 TLS Transport Model for SNMP July 2011
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 a (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 a (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 }
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmSessionServerCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as a (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 a (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 a (D)TLS client because the server certificate presented
by an SNMP over (D)TLS server was invalid because no
configured fingerprint or Certification Authority (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 CA."
::= { snmpTlstmSession 8 }
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmSessionInvalidServerCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on a (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. That
is, 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
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RFC 6353 TLS Transport Model for SNMP July 2011
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
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RFC 6353 TLS Transport Model for SNMP July 2011
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 (for example,
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 }
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RFC 6353 TLS Transport Model for SNMP July 2011
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 an 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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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,
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RFC 6353 TLS Transport Model for SNMP July 2011
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
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RFC 6353 TLS Transport Model for SNMP July 2011
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
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RFC 6353 TLS Transport Model for SNMP July 2011
"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
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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 an snmpTargetAddrEntry row, then the connection
SHOULD still proceed if some other certificate validation path
algorithm (e.g., RFC 5280) 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 }
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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.
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RFC 6353 TLS Transport Model for SNMP July 2011
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 an 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 Certification
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. That is, 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
-- ************************************************
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RFC 6353 TLS Transport Model for SNMP July 2011
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 that
implements the SNMP TLS Transport Model."
::= { snmpTlstmGroups 1 }
snmpTlstmIncomingGroup OBJECT-GROUP
OBJECTS {
snmpTlstmCertToTSNCount,
snmpTlstmCertToTSNTableLastChanged,
snmpTlstmCertToTSNFingerprint,
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmCertToTSNMapType,
snmpTlstmCertToTSNData,
snmpTlstmCertToTSNStorageType,
snmpTlstmCertToTSNRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
incoming connection certificate mappings to
tmSecurityNames of an SNMP engine that 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
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8. Operational Considerations
This section discusses various operational aspects of deploying
TLSTM.
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
[HEARTBEAT] 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.
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]. 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
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RFC 6353 TLS Transport Model for SNMP July 2011
implementation specific method of selecting a server certificate to
present to clients.
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] 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].
8.4. Transport Considerations
This document defines how SNMP messages can be transmitted over the
TLS- and DTLS-based protocols. Each of these protocols is
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.
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] and security considerations for TLS are
described in Section 11 and Appendices D, E, and F of TLS 1.2
[RFC5246]. 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.
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
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RFC 6353 TLS Transport Model for SNMP July 2011
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 a 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 an 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.
9.2. (D)TLS Security Considerations
This section discusses security considerations specific to the usage
of (D)TLS.
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] for further details.
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RFC 6353 TLS Transport Model for SNMP July 2011
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].
9.3. Use with SNMPv1/SNMPv2c Messages
The SNMPv1 and SNMPv2c message processing described in [RFC3584] (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], Section 7.1 for additional
details on the coexistence of security-aware transports and non-
transport-aware security models.
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:
o The snmpTlstmParamsTable can be used to change the outgoing X.509
certificate used to establish a (D)TLS connection. Modifications
to objects in this table need to be adequately authenticated since
modifying the 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.
o The snmpTlstmAddrTable can be used to change the expectations of
the certificates presented by a remote (D)TLS server.
Modifications to objects in this table need to be adequately
authenticated since modifying the values in this table will have
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
o The snmpTlstmCertToTSNTable is used to specify the mapping of
incoming X.509 certificates to tmSecurityNames, which eventually
get mapped to an SNMPv3 securityName. Modifications to objects in
this table need to be adequately authenticated since modifying the
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:
o 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 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], 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
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RFC 6353 TLS Transport Model for SNMP July 2011
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
10. IANA Considerations
IANA has assigned:
1. Two TCP/UDP port numbers from the "Registered Ports" range of the
Port Numbers registry, with the following keywords:
Keyword Decimal Description References
------- ------- ----------- ----------
snmptls 10161/tcp SNMP-TLS [RFC6353]
snmpdtls 10161/udp SNMP-DTLS [RFC6353]
snmptls-trap 10162/tcp SNMP-Trap-TLS [RFC6353]
snmpdtls-trap 10162/udp SNMP-Trap-DTLS [RFC6353]
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.
2. An SMI number (8) under snmpDomains for the snmpTLSTCPDomain
object identifier
3. An SMI number (9) under snmpDomains for the snmpDTLSUDPDomain
object identifier
4. An SMI number (198) under mib-2, for the MIB module in this
document
5. "tls" as the corresponding prefix for the snmpTLSTCPDomain in the
SNMP Transport Domains registry
6. "dtls" as the corresponding prefix for the snmpDTLSUDPDomain in
the SNMP Transport Domains registry
11. Acknowledgements
This document closely follows and copies the Secure Shell Transport
Model for SNMP documented by David Harrington and Joseph Salowey in
[RFC5592].
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,
Juergen Schoenwaelder, Dave Shield, and Robert Story.
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RFC 6353 TLS Transport Model for SNMP July 2011
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.
12. References
12.1. Normative References
[RFC1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62,
RFC 3411, December 2002.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3415,
December 2002.
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[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[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",
BCP 74, RFC 3584, August 2003.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[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", RFC 5280, May 2008.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport
Subsystem for the Simple Network Management Protocol
(SNMP)", RFC 5590, June 2009.
[RFC5591] Harrington, D. and W. Hardaker, "Transport Security
Model for the Simple Network Management Protocol
(SNMP)", RFC 5591, June 2009.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for
IPv6 Address Text Representation", RFC 5952,
August 2010.
12.2. Informative References
[HEARTBEAT] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer
Security (DTLS) Heartbeat Extension", Work in Progress,
July 2011.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
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[RFC5343] Schoenwaelder, J., "Simple Network Management Protocol
(SNMP) Context EngineID Discovery", RFC 5343,
September 2008.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC5953] Hardaker, W., "Transport Layer Security (TLS) Transport
Model for the Simple Network Management Protocol
(SNMP)", RFC 5953, August 2010.
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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.
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 = "192.0.2.1:10162"
snmpTargetAddrTimeout = 1500
snmpTargetAddrRetryCount = 3
snmpTargetAddrTagList = "toNRTag"
snmpTargetAddrParams = "toNR" (MUST match below)
snmpTargetAddrStorageType = 3 (nonVolatile)
snmpTargetAddrRowStatus = 4 (createAndGo)
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The following row configures the snmpTargetParamsTable to send the
notifications to "Joe Cool", using authPriv SNMPv3 notifications
through the TransportSecurityModel [RFC5591]:
snmpTargetParamsName = "toNR" (must match above)
snmpTargetParamsMPModel = 3 (SNMPv3)
snmpTargetParamsSecurityModel = 4 (TransportSecurityModel)
snmpTargetParamsSecurityName = "Joe Cool"
snmpTargetParamsSecurityLevel = 3 (authPriv)
snmpTargetParamsStorageType = 3 (nonVolatile)
snmpTargetParamsRowStatus = 4 (createAndGo)
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) Certification Authorities are
well understood, so two different principals cannot inadvertently be
identified by the same derived tmSecurityName.
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 = 2
(chosen by ordering preference)
snmpTlstmCertToTSNFingerprint = HASH (appropriate fingerprint)
snmpTlstmCertToTSNMapType = snmpTlstmCertSpecified
snmpTlstmCertToTSNSecurityName = "Joe Cool"
snmpTlstmCertToTSNStorageType = 3 (nonVolatile)
snmpTlstmCertToTSNRowStatus = 4 (createAndGo)
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Author's Address
Wes Hardaker
SPARTA, Inc.
P.O. Box 382
Davis, CA 95617
USA
Phone: +1 530 792 1913
EMail: ietf@hardakers.net
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