ISMS W. Hardaker
Internet-Draft Sparta, Inc.
Intended status: Informational December 10, 2008
Expires: June 13, 2009
Datagram Transport Layer Security Transport Model for SNMP
draft-hardaker-isms-dtls-tm-02.txt
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Abstract
This document describes a Transport Model for the Simple Network
Management Protocol (SNMP), that uses the Datagram Transport Layer
Security (DTLS) protocol. The DTLS protocol provides authentication
and privacy services for SNMP applications. This document describes
how the DTLS Transport Model (DTLSTM) implements the needed features
of a SNMP Transport Subsystem to make this protection possible in an
interoperable way.
This transport model is designed to meet the security and operational
needs of network administrators, operate in environments where a
connectionless (UDP) transport is preferred, and integrates well into
existing public keying infrastructures.
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This document also defines a portion of the Management Information
Base (MIB) for monitoring and managing the DTLS Transport Model for
SNMP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Terminology . . . . . . . . . . . . . . . . . 6
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 6
2. The Datagram Transport Layer Security Protocol . . . . . . . . 6
2.1. The DTLS Record Protocol . . . . . . . . . . . . . . . . . 7
2.2. The DTLS Handshake Protocol . . . . . . . . . . . . . . . 7
3. How the DTLSTM fits into the Transport Subsystem . . . . . . . 8
3.1. Security Capabilities of this Model . . . . . . . . . . . 10
3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Message Protection . . . . . . . . . . . . . . . . . . 12
3.1.3. DTLS Sessions . . . . . . . . . . . . . . . . . . . . 13
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 14
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 14
4. Elements of the Model . . . . . . . . . . . . . . . . . . . . 15
4.1. Certificates . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.1. The Certificate Infrastructure . . . . . . . . . . . . 15
4.1.2. Provisioning for the Certificate . . . . . . . . . . . 16
4.2. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. SNMP Services . . . . . . . . . . . . . . . . . . . . . . 17
4.3.1. SNMP Services for an Outgoing Message . . . . . . . . 18
4.3.2. SNMP Services for an Incoming Message . . . . . . . . 19
4.4. DTLS Services . . . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Services for Establishing a Session . . . . . . . . . 20
4.4.2. DTLS Services for an Incoming Message . . . . . . . . 21
4.4.3. DTLS Services for an Outgoing Message . . . . . . . . 22
4.5. Cached Information and References . . . . . . . . . . . . 22
4.5.1. securityStateReference . . . . . . . . . . . . . . . . 23
4.5.2. tmStateReference . . . . . . . . . . . . . . . . . . . 23
4.5.2.1. Transport information . . . . . . . . . . . . . . 24
4.5.2.2. securityName . . . . . . . . . . . . . . . . . . . 24
4.5.2.3. securityLevel . . . . . . . . . . . . . . . . . . 25
4.5.2.4. Session Information . . . . . . . . . . . . . . . 25
4.5.3. DTLS Transport Model Cached Information . . . . . . . 26
4.5.3.1. Transport Information . . . . . . . . . . . . . . 26
4.5.3.2. tmRequestedSecurityLevel . . . . . . . . . . . . . 27
4.5.3.3. tmSecurityLevel . . . . . . . . . . . . . . . . . 27
4.5.3.4. tmSecurityName . . . . . . . . . . . . . . . . . . 27
4.5.4. Transport Model LCD . . . . . . . . . . . . . . . . . 27
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 28
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 28
5.1.1. DTLS Processing for Incoming Messages . . . . . . . . 28
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5.1.2. Transport Processing for Incoming Messages . . . . . . 30
5.2. Procedures for an Outgoing Message . . . . . . . . . . . . 31
5.3. Establishing a Session . . . . . . . . . . . . . . . . . . 32
5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 34
6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 35
6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 35
6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 35
6.3. Statistical Counters . . . . . . . . . . . . . . . . . . . 35
6.4. Configuration Tables . . . . . . . . . . . . . . . . . . . 35
6.5. Relationship to Other MIB Modules . . . . . . . . . . . . 35
6.5.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 36
7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 36
8. Operational Considerations . . . . . . . . . . . . . . . . . . 49
8.1. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.2. Notification Receiver Credential Selection . . . . . . . . 50
8.3. contextEngineID Discovery . . . . . . . . . . . . . . . . 50
9. Security Considerations . . . . . . . . . . . . . . . . . . . 50
9.1. Certificates, Authentication, and Authorization . . . . . 51
9.2. Use with SNMPv1/SNMPv2c Messages . . . . . . . . . . . . . 52
9.3. MIB Module Security . . . . . . . . . . . . . . . . . . . 52
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 53
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
12.1. Normative References . . . . . . . . . . . . . . . . . . . 53
12.2. Informative References . . . . . . . . . . . . . . . . . . 55
Appendix A. Target and Notificaton Configuration Example . . . . 56
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . . . . 59
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1. Introduction
It is important to understand the SNMPv3 architecture [RFC3411], as
enhanced by the Transport Subsystem [I-D.ietf-isms-tmsm]. 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
Datagram Transport Layer Security (DTLS) Protocol [RFC4347], the
datagram variant of the existing and commonly deployed Transport
Layer Security (TLS) protocol [RFC4346], within a transport subsystem
[I-D.ietf-isms-tmsm]. The Transport Model in this document is
referred to as the Datagram Transport Layer Security Transport Model
(DTLSTM). DTLS takes advantage of the X.509 public keying
infrastructure [X509]. This transport model is designed to meet the
security and operational needs of network administrators, operate in
environments where a connectionless (UDP) transport is preferred, and
integrate well into existing public keying infrastructures.
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 document specifies a
MIB module that is compliant to the SMIv2, which is described in STD
58, RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC
2580 [RFC2580].
This document also defines a portion of the Management Information
Base (MIB) for use with network management protocols in IP based
networks. In particular it defines objects for monitoring and
managing the DTLS Transport Model for SNMP.
The diagram shown below gives a conceptual overview of two SNMP
entities communicating using the DTLS Transport Model. One entity
contains a Command Responder and Notification Originator application,
and the other a Command Generator and Notification Responder
application. It should be understood that this particular mix of
application types is an example only and other combinations are
equally as legitimate.
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+----------------------------------------------------------------+
| Network |
+----------------------------------------------------------------+
^ ^ ^ ^
|Notifications |Commands |Commands |Notifications
+---|---------------------|--------+ +--|---------------|-------------+
| V V | | V V |
| +------------+ +------------+ | | +-----------+ +----------+ |
| | DTLS | | DTLS | | | | DTLS | | DTLS | |
| | Service | | Service | | | | Service | | Service | |
| | (Client) | | (Server) | | | | (Client) | | (Server)| |
| +------------+ +------------+ | | +-----------+ +----------+ |
| ^ ^ | | ^ ^ |
| +--+----------+ | | +-+--------------+ |
| +-----|---------+----+ | | +---|--------+----+ |
| | V |LCD | +-------+ | | | V |LCD | +--------+ |
| | +------+ +----+ | | | | | +------+ +----+ | | |
| | | DTLS | <---------->| Cache | | | | | DTLS | <---->| Cache | |
| | | TM | | | | | | | | TM | | | | |
| | +------+ | +-------+ | | | +------+ | +--------+ |
| |Transport Subsystem | ^ | | |Transport Sub. | ^ |
| +--------------------+ | | | +-----------------+ | |
| ^ +----+ | | ^ | |
| | | | | | | |
| v | | | V | |
| +-------+ +----------+ +-----+ | | | +-----+ +------+ +-----+ | |
| | | |Message | |Sec. | | | | | | | MP | |Sec. | | |
| | Disp. | |Processing| |Sub- | | | | |Disp.| | Sub- | |Sub- | | |
| | | |Subsystem | |sys. | | | | | | |system| |sys. | | |
| | | | | | | | | | | | | | | | | |
| | | | | |+---+| | | | | | | | |+---+| | |
| | | | +-----+ | || || | | | | | |+----+| || || | |
| | <--->|v3MP |<-->||TSM|<-+ | | | <-->|v3MP|<->|TSM|<-+ |
| | | | +-----+ | || || | | | | |+----+| || || |
| +-------+ | | |+---+| | | +-----+ | | |+---+| |
| ^ | | | | | | ^ | | | | |
| | +----------+ +-----+ | | | +------+ +-----+ |
| +-+----------+ | | +-+------------+ |
| ^ ^ | | ^ ^ |
| | | | | | | |
| v v | | V V |
| +-------------+ +--------------+ | | +-----------+ +--------------+ |
| | COMMAND | | NOTIFICATION | | | | COMMAND | | NOTIFICATION | |
| | RESPONDER | | ORIGINATOR | | | | GENERATOR | | RESPONDER | |
| | application | | applications | | | |application| | application | |
| +-------------+ +--------------+ | | +-----------+ +--------------+ |
| SNMP entity | | SNMP entity |
+----------------------------------+ +--------------------------------+
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1.1. Requirements Terminology
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. Conventions
For consistency with SNMP-related specifications, this document
favors terminology as defined in STD62 rather than favoring
terminology that is consistent with non-SNMP specifications that use
different variations of the same terminology. This is consistent
with the IESG decision to not require the SNMPv3 terminology be
modified to match the usage of other non-SNMP specifications when
SNMPv3 was advanced to Full Standard.
In particular, where distinction is required the application names of
"Command Generator", "Command Responder", "Notification Originator",
"Notification Receiver", and "Proxy Forwarder" are used. See the
"SNMP Applications" in [RFC3413] for further information.
Authentication in this document typically refers to source
authentication or peer identity authentication performed in the
transport subsystem.
Throughout this document, the terms "client" and "server" are used to
refer to the two ends of the DTLS session. The client actively opens
the DTLS session, and the server passively listens for the incoming
DTLS session. Any SNMP entity may act as client or as server.
While security protocols frequently refer to a "user", the term used
in RFC3411 [RFC3411] and in this document is "principal". A
principal is the "who" on whose behalf services are provided or
processing takes place. A principal can be, among other things, an
individual acting in a particular role; a set of individuals, with
each acting in a particular role; an application or a set of
applications, or a combination of these within an administrative
domain.
Throughout this document, the term "session" is used to refer to a
secure association between two DTLS Transport Models that permits the
transmission of one or more SNMP messages within the lifetime of the
session.
2. The Datagram Transport Layer Security Protocol
The DTLS protocol is a datagram-compatible variant of the commonly
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used Transport Layer Security (TLS) protocol. DTLS provides
authentication, data message integrity, and privacy at the transport
layer. (See [RFC4347])
The primary goals of the DTLS Transport Model are to provide privacy,
source authentication and data integrity between two communicating
SNMP entities. The DTLS protocol is composed of two layers: the DTLS
Record Protocol and the DTLS Handshake Protocol. The following
sections provide an overview of these two layers. Please refer to
[RFC4347] for a complete description of the protocol.
2.1. The DTLS Record Protocol
At the lowest layer, layered on top of the transport protocol (UDP)
is the DTLS Record Protocol.
The DTLS Record Protocol provides security that has three basic
properties:
o The session can be confidential. Symmetric cryptography is used
for data encryption (e.g., AES [AES], DES [DES] etc.). The keys
for this symmetric encryption are generated uniquely for each
session and are based on a secret negotiated by another protocol
(such as the DTLS Handshake Protocol). The Record Protocol can
also be used without encryption.
o Messages can have data integrity. Message transport includes a
message integrity check using a keyed MAC. Secure hash functions
(e.g., SHA, MD5, etc.) are used for MAC computations. The Record
Protocol can operate without a MAC, but is generally only used in
this mode while another protocol is using the Record Protocol as a
transport for negotiating security parameters.
o Messages are protected against replay. DTLS uses explicit
sequence numbers, integrity checks, and a sliding window to
protect against replay of messages within a session.
DTLS also provides protection against replay of entire sessions. In
a properly-implemented keying material exchange, both sides will
generate new random numbers for each exchange. This results in
different encryption and integrity keys for every session.
2.2. The DTLS Handshake Protocol
The DTLS Record Protocol is used for encapsulation of various higher-
level protocols. One such encapsulated protocol, the DTLS Handshake
Protocol, allows server and client to authenticate each other and to
negotiate an encryption algorithm and cryptographic keys before the
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application protocol transmits or receives its first octet of data.
Only the DTLS client can initiate the handshake protocol. The DTLS
Handshake Protocol provides security that has three basic properties:
o The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required by
at least one of the peers.
DTLS supports three authentication modes: authentication of both the
server and the client, server authentication with an unauthenticated
client, and total anonymity. For authentication of both entities,
each entity provides a valid certificate chain leading to an
acceptable certificate authority. Each entity is responsible for
verifying that the other's certificate is valid and has not expired
or been revoked. The DTLS Transport Model SHOULD always use
authentication of both the server and the client. At a minimum the
DTLS Transport MUST support authentication of the Command Generator
principals to guarantee the authenticity of the securityName (a
parameter used to pass the authenticated identity name from the
transport model to security model for even later use by the access
control subsystem. See Section 4.5.3.4). The DTLS Transport SHOULD
support the message encryption to protect sensitive data from
eavesdropping attacks. See [I-D.hodges-server-ident-check] for
further details on standardized processing when checking Server
certificate identities.
o The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
handshake the secret cannot be obtained, even by an attacker who
can place himself in the middle of the session.
o The negotiation is not vulnerable to malicious modification: it is
infeasible for an attacker to modify negotiation communication
without being detected by the parties to the communication.
o DTLS uses a stateless cookie exchange to protect against anonymous
denial of service attacks and has retransmission timers, sequence
numbers, and counters to handle message loss, reordering, and
fragmentation.
3. How the DTLSTM fits into the Transport Subsystem
A transport model is a component of the Transport Subsystem. The
DTLS Transport Model thus fits between the underlying DTLS transport
layer and the message Dispatcher [RFC3411] component of the SNMP
engine and the Transport Subsystem [I-D.ietf-isms-tmsm].
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The DTLS Transport Model will establish a session between itself and
the DTLS Transport Model of another SNMP engine. The sending
transport model passes unprotected messages from the dispatcher to
DTLS to be protected, and the receiving transport model accepts
decrypted and authenticated/integrity-checked incoming messages from
DTLS and passes them to the dispatcher.
After a DTLS 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
SHOULD be passed through the same session. A DTLSTM implementation
engine MAY choose to close a DTLS session to conserve resources.
The DTLS Transport Model of an SNMP engine will perform the
translation between DTLS-specific security parameters and SNMP-
specific, model-independent parameters.
The diagram below depicts where the DTLS Transport Model fits into
the architecture described in RFC3411 and the Transport Subsystem:
+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-------------------------------------------------------------------+
| +--------------------------------------------------+ |
| | Transport Subsystem | +--------+ |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | | | |
| | | UDP | | TCP | | SSH | |DTLS | . . . | other |<--->| Cache | |
| | | | | | | TM | TM | | | | | | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | +--------+ |
| +--------------------------------------------------+ ^ |
| ^ | |
| | | |
| Dispatcher v | |
| +--------------+ +---------------------+ +----------------+ | |
| | Transport | | Message Processing | | Security | | |
| | Dispatch | | Subsystem | | Subsystem | | |
| | | | +------------+ | | +------------+ | | |
| | | | +->| v1MP |<--->| | USM | | | |
| | | | | +------------+ | | +------------+ | | |
| | | | | +------------+ | | +------------+ | | |
| | | | +->| v2cMP |<--->| | Transport | | | |
| | Message | | | +------------+ | | | Security |<--+ |
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| | Dispatch <---->| +------------+ | | | Model | | |
| | | | +->| v3MP |<--->| +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | PDU Dispatch | | | +------------+ | | | Other | | |
| +--------------+ | +->| otherMP |<--->| | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
3.1. Security Capabilities of this Model
3.1.1. Threats
The DTLS 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 some 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.
DTLS 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 a principal with appropriate
authorizations.
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The DTLSTM provides for authentication of the Principal Command
Generator and Notification Generator and for authentication of
the Command Responder, Notification Responder and Proxy Forwarder
through the use of X.509 certificates.
The masquerade threat can be mitigated against by using an
appropriate Access Control Model (ACM) such as the View-based
Access Control Module (VACM) [RFC3415]. In addition, it is
important to authenticate and verify both the authenticated
identity of the DTLS client and the DTLS server to protect
against this threat. (See Section 9 for more detail.)
3. Message stream modification - The re-ordering, delay or replay of
messages can and does occur through the natural operation of many
connectionless transport services. The message stream
modification threat is the danger that messages may be
maliciously re-ordered, delayed or replayed to an extent which is
greater than can occur through the natural operation of
connectionless transport services, in order to effect
unauthorized management operations.
DTLS provides replay protection with a MAC that includes a
sequence number. DTLS uses a sliding window protocol with the
sequence number for replay protection, see [RFC4347]. The
technique used is the same as in IPsec AH/ESP [RFC4302]
[RFC4303], by maintaining a bitmap window of received records.
Records that are too old to fit in the window and records that
have previously been received are silently discarded. The replay
detection feature is optional, since packet duplication can also
occur naturally due to routing errors and does not necessarily
indicate an active attack. Applications may conceivably detect
duplicate packets and accordingly modify their data transmission
strategy.
4. Disclosure - The disclosure threat is the danger of eavesdropping
on the exchanges between SNMP engines. Protecting against this
threat may be required as a matter of local policy.
Symmetric cryptography (e.g., AES [AES], DES [DES] etc.) can be
used by DTLS for data privacy. The keys for this symmetric
encryption are generated uniquely for each session and are based
on a secret negotiated by another protocol (such as the DTLS
Handshake Protocol).
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, datagram security protocols are
susceptible to a variety of denial of service attacks. Two
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attacks are of particular concern:
o An attacker can consume excessive resources on the server by
transmitting a series of handshake initiation requests,
causing the server to allocate state and potentially to
perform expensive cryptographic operations.
o An attacker can use the server as an amplifier by sending
session initiation messages with a forged source of the
victim. The server then sends its next message (in DTLS, a
Certificate message, which can be quite large) to the victim
machine, thus flooding it.
In order to counter both of these attacks, DTLS borrows the
stateless cookie technique used by Photuris [RFC2522] and IKEv2
[RFC4306]. When the client sends its ClientHello message to the
server, the server MAY respond with a HelloVerifyRequest message.
This message contains a stateless cookie generated using the
technique of [RFC2522]. The client MUST retransmit the
ClientHello with the cookie added. The server then verifies the
cookie and proceeds with the handshake only if it is valid. This
mechanism forces the attacker/client to be able to receive the
cookie, which makes DoS attacks with spoofed IP addresses
difficult. This mechanism does not provide any defense against
denial of service attacks mounted from valid IP addresses.
Implementations are not required to perform the stateless cookie
exchange for every DTLS handshakes but in environments where
amplification could be an issue it is RECOMMENDED that the cookie
exchange is utilized.
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 DTLS Transport Model determines from DTLS the identity of the
authenticated principal, and the type and address associated with an
incoming message, and the DTLS Transport Model provides this
information to DTLS for an outgoing message.
When an application requests a session for a message, through the
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cache, the application requests a security level for that session.
The DTLS Transport Model MUST ensure that the DTLS session provides
security at least as high as the requested level of security. How
the security level is translated into the algorithms used to provide
data integrity and privacy is implementation-dependent. However, the
NULL integrity and encryption algorithms MUST NOT be used to fulfill
security level requests for authentication or privacy.
Implementations MAY choose to force DTLS to only allow cipher_suites
that provide both authentication and privacy to guarantee this
assertion.
If a suitable interface between the DTLS Transport Model and the DTLS
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. However, different port numbers will
need to be used by at least one side of the connection to
differentiate between the DTLS sessions. This is the only way to
ensured proper selection of a session ID for an incoming DTLS
message.
The authentication, integrity and privacy algorithms used by the DTLS
Protocol [RFC4347] may vary over time as the science of cryptography
continues to evolve and the development of DTLS continues over time.
Implementers are encouraged to plan for changes in operator trust of
particular algorithms and implementations should offer configuration
settings for mapping algorithms to SNMPv3 security levels.
3.1.3. DTLS Sessions
DTLS sessions are opened by the DTLS Transport Model during the
elements of procedure for an outgoing SNMP message. Since the sender
of a message initiates the creation of a DTLS session if needed, the
DTLS session will already exist for an incoming message.
Implementations MAY choose to instantiate DTLS sessions in
anticipation of outgoing messages. This approach might be useful to
ensure that a DTLS session to a given target can be established
before it becomes important to send a message over the DTLS session.
Of course, there is no guarantee that a pre-established session will
still be valid when needed.
DTLS sessions are uniquely identified within the DTLS Transport Model
by the combination of transportDomain, transportAddress,
securityName, and requestedSecurityLevel associated with each
session. Each unique combination of these parameters MUST have a
locally-chosen unique dtlsSessionID associated for active sessions.
For further information see Section 4.4 and Section 5.
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3.2. Security Parameter Passing
For the DTLS server-side, DTLS-specific security parameters (i.e.,
cipher_suites, X.509 certificate fields, IP address and port) are
translated by the DTLS Transport Model into security parameters for
the DTLS Transport Model and security model (i.e., securityLevel,
securityName, transportDomain, transportAddress). The transport-
related and DTLS-security-related information, including the
authenticated identity, are stored in a cache referenced by
tmStateReference.
For the DTLS client-side, the DTLS Transport Model takes input
provided by the dispatcher in the sendMessage() Abstract Service
Interface (ASI) and input from the tmStateReference cache. The DTLS
Transport Model converts that information into suitable security
parameters for DTLS and establishes sessions as needed.
The elements of procedure in Section 5 discuss these concepts in much
greater detail.
3.3. Notifications and Proxy
DTLS sessions may be initiated by DTLS clients on behalf of command
generators or notification originators. Command generators are
frequently operated by a human, but notification originators are
usually unmanned automated processes. The targets to whom
notifications should be sent is typically determined and configured
by a network administrator.
The SNMP-TARGET-MIB module [RFC3413] contains objects for defining
management targets, including transportDomain, transportAddress,
securityName, securityModel, and securityLevel parameters, for
Notification Generator, Proxy Forwarder, and SNMP-controllable
Command Generator applications. Transport domain and transport
address are configured in the snmpTargetAddrTable, and the
securityModel, securityName, and securityLevel parameters are
configured in the snmpTargetParamsTable. This document defines a MIB
module that extends the SNMP-TARGET-MIB's snmpTargetParamsTable to
specify a DTLS client-side certificate to use for the connection.
When configuring a DTLS target, the snmpTargetAddrTDomain and
snmpTargetAddrTAddress parameters in snmpTargetAddrTable should be
set to the snmpDTLSDomain object and an appropriate snmpDTLSAddress
value respectively. The snmpTargetParamsMPModel column of the
snmpTargetParamsTable should be set to a value of 3 to indicate the
SNMPv3 message processing model. The snmpTargetParamsSecurityName
should be set to an appropriate securityName value and the
dtlstmParamsHashType and dtlstmParamsHashValue parameters of the
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dtlstmParamsTable should be set to values that refer to a locally
held certificate to be used. Other parameters, for example
cryptographic configuration such as cipher suites to use, must come
from configuration mechanisms not defined in this document. The
other needed configuration may be configured using SNMP or other
implementation-dependent mechanisms (for example, via a CLI). This
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 DTLS
Transport Model defined by this document.
4.1. Certificates
DTLS makes use of X.509 certificates for authentication of both sides
of the transport. This section discusses the use of certificates in
DTLS and the its effects on SNMP over DTLS.
4.1.1. The Certificate Infrastructure
Users of a public key SHALL be confident that the associated private
key is owned by the correct remote subject (person or system) with
which an encryption or digital signature mechanism will be used.
This confidence is obtained through the use of public key
certificates, which are data structures that bind public key values
to subjects. The binding is asserted by having a trusted CA
digitally sign each certificate. The CA may base this assertion upon
technical means (i.e., proof of possession through a challenge-
response protocol), presentation of the private key, or on an
assertion by the subject. A certificate has a limited valid lifetime
which is indicated in its signed contents. Because a certificate's
signature and timeliness can be independently checked by a
certificate-using client, certificates can be distributed via
untrusted communications and server systems, and can be cached in
unsecured storage in certificate-using systems.
ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was
first published in 1988 as part of the X.500 Directory
recommendations, defines a standard certificate format [X509] which
is a certificate which binds a subject (principal) to a public key
value. This was later further documented in [RFC5280].
A X.509 certificate is a sequence of three required fields:
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tbsCertificate: The field contains the names of the subject and
issuer, a public key associated with the subject, a validity
period, and other associated information. This field may also
contain extensions.
signatureAlgorithm: The signatureAlgorithm field contains the
identifier for the cryptographic algorithm used by the certificate
authority (CA) to sign this certificate.
signatureValue: The signatureValue field contains a digital
signature computed upon the ASN.1 DER encoded tbsCertificate
field. The ASN.1 DER encoded tbsCertificate is used as the input
to the signature function. This signature value is then ASN.1 DER
encoded as a BIT STRING and included in the Certificate's
signature field. By generating this signature, a CA certifies the
validity of the information in the tbsCertificate field. In
particular, the CA certifies the binding between the public key
material and the subject of the certificate.
The basic X.509 authentication procedure is as follows: A system is
initialized with a number of root certificates that contain the
public keys of a number of trusted CAs. When a system receives a
X.509 certificate, signed by one of those CAs, the certificate has to
be verified. It first checks the signatureValue field by using the
public key of the corresponding trusted CA. Then it compares the
decrypted information with a digest of the tbsCertificate field. If
they match, then the subject in the tbsCertificate field is
authenticated.
4.1.2. Provisioning for the Certificate
Authentication using DTLS will require that SNMP entities are
provisioned with certificates, which are signed by trusted
certificate authorities. Furthermore, SNMP entities will most
commonly need to be provisioned with root certificates which
represent the list of trusted certificate authorities that an SNMP
entity can use for certificate verification. SNMP entities MAY also
be provisioned with a X.509 certificate revocation mechanism which
can be used to verify that a certificate has not been revoked.
The authenticated tmSecurityName of the principal is looked up using
the dtlstmCertificateToSNTable. This table either:
o Maps a certificate's fingerprint hash type and value to a directly
specified tmSecurityName.
o Identifies a certificate issuer's fingerprint hash type and value
and allows child certificate's subjectAltName or CommonName to
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directly used as the tmSecurityNome.
The certificate trust anchors, being either CA certificates or public
keys for use by self-signed certificates, must be installed through
an out of band trusted mechanism into the server and its authenticity
MUST be verified before access is granted. Implementations SHOULD
choose to discard any connections for which no potential
dtlstmCertificateToSNTable mapping exists before performing
certificate verification to avoid expending computational resources
associated with certificate verification.
The typical enterprise configuration will map the "subjectAltName"
component of the tbsCertificate to the DTLSSM specific
tmSecurityName. Thus, the authenticated identity can be obtained by
the DTLS Transport Model by extracting the subjectAltName from the
peer's certificate and the receiving application will have an
appropriate tmSecurityName for use by components like an access
control model. This setup requires very little configuration: a
single row in the dtlstmCertificateToSNTable referencing a
certificate authority.
An example mapping setup can be found in Appendix A
This tmSecurityName may be later translated from a DTLSSM specific
tmSecurityName to a SNMP engine securityName by the security model.
A security model, like the TSM security model, may perform an
identity mapping or a more complex mapping to derive the securityName
from the tmSecurityName offered by the DTLS Transport Model.
4.2. Messages
As stated in RFC4347, each DTLS message must fit within a single
datagram. The DTLSTM MUST prohibit SNMP messages from being set that
exceed the MTU size. The DTLSTM implementation MUST return an error
when the MTU size would be exceeded and the message won't be sent.
For Ethernet the MTU size is 1500 and thus the maximum allowable SNMP
message that can be sent over DTLSTM after UDP/IP/DTLS overhead is
taken into account will be 1455 for IPv6 (MTU:1500 - IPv6:40 - UDP:8
- DTLS:13) and 1475 for IPv4 (MTU:1500 - IPv4:20 - UDP:8 - DTLS:13).
From this integrity and encryption overhead also needs to be
subtracted, which are integrity and encryption algorithm specific.
4.3. SNMP Services
This section describes the services provided by the DTLS Transport
Model with their inputs and outputs. The services are between the
Transport Model and the Dispatcher.
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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 DTLS 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 passed as parameters in 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.
destTransportDomain: The transport domain for the associated
destTransportAddress. The Transport Model uses this parameter to
determine the transport type of the associated
destTransportAddress. This parameter may also be used by the
transport subsystem to route the message to the appropriate
Transport Model.
destTransportAddress: The transport address of the destination DTLS
Transport Model in a format specified by the SnmpDTLSAddress
TEXTUAL-CONVENTION.
outgoingMessage: The outgoing message to send to DTLS for
encapsulation.
outgoingMessageLength: The length of the outgoing message.
tmStateReference: A handle/reference to tmSecurityData to be used
when securing outgoing messages.
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4.3.2. SNMP Services for an Incoming Message
The DTLS Transport Model processes the received message from the
network using the DTLS service and then passes it to the Dispatcher
using the following ASI:
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 passed as parameters in 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.
transportAddress: The transport address of the source of the
received message in a format specified by the SnmpDTLSAddress
TEXTUAL-CONVENTION.
incomingMessage: The whole SNMP message stripped of all DTLS
protection data.
incomingMessageLength: The length of the SNMP message after being
processed by DTLS.
tmStateReference: A handle/reference to tmSecurityData to be used by
the security model.
4.4. DTLS Services
This section describes the services provided by the DTLS Transport
Model with their inputs and outputs. These services are between the
DTLS Transport Model and the DTLS transport layer. The following
sections describe services for establishing and closing a session and
for passing messages between the DTLS transport layer and the DTLS
Transport Model.
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4.4.1. Services for Establishing a Session
The DTLS Transport Model provides the following ASI to describe the
data passed between the Transport Model and the DTLS transport layer
for session establishment.
statusInformation = -- errorIndication or success
openSession(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
OUT dtlsSessionID -- Session identifier for DTLS
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation: An indication of whether the process was
successful or not. If not, then the status information will
include the error indication provided by DTLS.
destTransportDomain: The transport domain for the associated
destTransportAddress. The DTLS Transport Model uses this
parameter to determine the transport type of the associated
destTransportAddress.
destTransportAddress: The transport address of the destination DTLS
Transport Model in a format specified by the SnmpDTLSAddress
TEXTUAL-CONVENTION.
securityName: The security name representing the principal on whose
behalf the message will be sent.
securityLevel: The level of security requested by the application.
dtlsSessionID: An implementation-dependent session identifier to
reference the specific DTLS session.
DTLS and UDP do not provide a session de-multiplexing mechanism and
it is possible that implementations will only be able to identify a
unique session based on a unique combination of source address,
destination address, source UDP port number and destination UDP port
number. Because of this, when establishing a new sessions
implementations MUST use a different UDP source port number for each
connection to a remote destination IP-address/port-number combination
to ensure the remote entity can properly disambiguate between
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multiple sessions from a host to the same port on a server.
The procedural details for establishing a session are further
described in Section 5.3.
Upon completion of the process the DTLS Transport Model returns
status information and, if the process was successful the
dtlsSessionID and other implementation-dependent data from DTLS are
also returned. The dtlsSessionID is stored in an implementation-
dependent manner and tied to the tmSecurityData for future use of
this session.
4.4.2. DTLS Services for an Incoming Message
When the DTLS Transport Model invokes the DTLS record layer to verify
proper security for the incoming message, it must use the following
ASI:
statusInformation = -- errorIndication or success
dtlsRead(
IN dtlsSessionID -- Session identifier for DTLS
IN wholeDtlsMsg -- as received on the wire
IN wholeDtlsMsgLength -- length as received on the wire
OUT incomingMessage -- the whole SNMP message from DTLS
OUT incomingMessageLength -- the length of the SNMP message
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation: An indication of whether the process was
successful or not. If not, then the status information will
include the error indication provided by DTLS.
dtlsSessionID: An implementation-dependent session identifier to
reference the specific DTLS session. How the DTLS session ID is
obtained for each message is implementation-dependent. As an
implementation hint, the DTLS Transport Model can examine incoming
messages to determine the source IP address and port number and
use these values to look up the local DTLS session ID in the list
of active sessions.
wholeDtlsMsg: The whole message as received on the wire.
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wholeDtlsMsgLength: The length of the message as it was received on
the wire.
incomingMessage: The whole SNMP message stripped of all DTLS privacy
and integrity data.
incomingMessageLength: The length of the SNMP message stripped of
all DTLS privacy and integrity data.
4.4.3. DTLS Services for an Outgoing Message
When the DTLS Transport Model invokes the DTLS record layer to
encapsulate and transmit a SNMP message, it must use the following
ASI.
statusInformation = -- errorIndication or success
dtlsWrite(
IN dtlsSessionID -- Session identifier for DTLS
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation: An indication of whether the process was
successful or not. If not, then the status information will
include the error indication provided by DTLS.
dtlsSessionID: An implementation-dependent session identifier to
reference the specific DTLS session that the message should be
sent using.
outgoingMessage: The outgoing message to send to DTLS for
encapsulation.
outgoingMessageLength: The length of the outgoing message.
4.5. 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.
The RFC3411 architecture uses caches to maintain the short-term
message state, and uses references in the ASIs to pass this
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information between subsystems.
This document defines the requirements for a cache to handle the
longer-term transport state information, using a tmStateReference
parameter to pass this information 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 being processed gets
discarded, the state related to that message SHOULD also be
discarded. If state information is available when a relationship
between engines is severed, such as the closing of a transport
session, the state information for that relationship SHOULD also be
discarded.
Since the contents of a cache are meaningful only within an
implementation, and not on-the-wire, the format of the cache and the
LCD are implementation-specific.
4.5.1. securityStateReference
The securityStateReference parameter is defined in RFC3411. Its
primary purpose is to provide a mapping between a request and the
corresponding response. This cache is not accessible to Transport
Models, and an entry is typically only retained for the lifetime of a
request-response pair of messages.
4.5.2. tmStateReference
For each transport session, information about the transport security
is stored in a cache. The tmStateReference parameter is used to pass
model-specific and mechanism-specific parameters between the
Transport subsystem and transport-aware Security Models.
The tmStateReference cache will typically remain valid for the
duration of the transport session, and hence may be used for several
messages.
Since this cache is only used within an implementation, and not on-
the-wire, the precise contents and format are implementation-
dependent. However, for interoperability between Transport Models
and transport-aware Security Models, entries in this cache must
include at least the following fields:
transportDomain
transportAddress
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tmSecurityName
tmRequestedSecurityLevel
tmTransportSecurityLevel
tmSameSecurity
tmSessionID
4.5.2.1. Transport information
Information about the source of an incoming SNMP message is passed up
from the Transport subsystem as far as the Message Processing
subsystem. However these parameters are not included in the
processIncomingMsg ASI defined in RFC3411, and hence this information
is not directly available to the Security Model.
A transport-aware Security Model might wish to take account of the
transport protocol and originating address when authenticating the
request, and setting up the authorization parameters. It is
therefore necessary for the Transport Model to include this
information in the tmStateReference cache, so that it is accessible
to the Security Model.
o transportDomain: the transport protocol (and hence the Transport
Model) used to receive the incoming message
o transportAddress: the source of the incoming message.
The ASIs used for processing an outgoing message all include explicit
transportDomain and transportAddress parameters. The values within
the securityStateReference cache might override these parameters for
outgoing messages.
4.5.2.2. securityName
There are actually three distinct "identities" that can be identified
during the processing of an SNMP request over a secure transport:
o transport principal: the transport-authenticated identity, on
whose behalf the secure transport connection was (or should be)
established. This value is transport-, mechanism- and
implementation- specific, and is only used within a given
Transport Model.
o tmSecurityName: a human-readable name (in snmpAdminString format)
representing this transport identity. This value is transport-
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and implementation-specific, and is only used (directly) by the
Transport and Security Models.
o securityName: a human-readable name (in snmpAdminString format)
representing the SNMP principal in a model-independent manner.
The transport principal may or may not be the same as the
tmSecurityName. Similarly, the tmSecurityName may or may not be the
same as the securityName as seen by the Application and Access
Control subsystems. In particular, a non-transport-aware Security
Model will ignore tmSecurityName completely when determining the SNMP
securityName.
However it is important that the mapping between the transport
principal and the SNMP securityName (for transport-aware Security
Models) is consistent and predictable, to allow configuration of
suitable access control and the establishment of transport
connections.
4.5.2.3. securityLevel
There are two distinct issues relating to security level as applied
to secure transports. For clarity, these are handled by separate
fields in the tmStateReference cache:
o tmTransportSecurityLevel: an indication from the Transport Model
of the level of security offered by this session. The Security
Model can use this to ensure that incoming messages were suitably
protected before acting on them.
o tmRequestedSecurityLevel: an indication from the Security Model of
the level of security required to be provided by the transport
protocol. The Transport Model can use this to ensure that
outgoing messages will not be sent over an insufficiently secure
session.
4.5.2.4. Session Information
For security reasons, if a secure transport session is closed between
the time a request message is received and the corresponding response
message is sent, then the response message SHOULD be discarded, even
if a new session has been established. The SNMPv3 WG decided that
this should be a SHOULD architecturally, and it is a security-model-
specific decision whether to REQUIRE this.
o tmSameSecurity: this flag is used by a transport-aware Security
Model to indicate whether the Transport Model MUST enforce this
restriction.
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o tmSessionID: in order to verify whether the session has changed,
the Transport Model must be able to compare the session used to
receive the original request with the one to be used to send the
response. This typically requires some form of session
identifier. This value is only ever used by the Transport Model,
so the format and interpretation of this field are model-specific
and implementation-dependent.
When processing an outgoing message, if tmSameSecurity is true, then
the tmSessionID MUST match the current transport session, otherwise
the message MUST be discarded, and the dispatcher notified that
sending the message failed.
4.5.3. DTLS Transport Model Cached Information
For the DTLS Transport Model, the session state is maintained using
tmStateReference. Upon opening each DTLS session, the DTLS Transport
Model stores model- and mechanism-specific information about the
session in a cache, referenced by tmStateReference. An
implementation might store the contents of the cache in a Local
Configuration Datastore (LCD).
At a minimum, the following parameters are stored in the cache:
tmTransportDomain = Specified by the application tmSameSecurity =
boolean value set by the security model or false by default
tmTransportAddress = Specified by the application
tmRequestedSecurityLevel = ["noAuthNoPriv" | "authNoPriv" |
"authPriv" ] the security level requested by the application
tmSecurityLevel = ["noAuthNoPriv" | "authNoPriv" | "authPriv" ] the
security level of the established DTLS session
tmSecurityName = the security name associated with a principal
The tmStateReference cache is used to pass a reference to these
values between the DTLS Transport Model and the security model.
The DTLS Transport Model has the responsibility for releasing the
complete tmStateReference and deleting the associated information
when the session is destroyed.
4.5.3.1. Transport Information
The tmTransportDomain and tmTransportAddress identify the type and
address of the remote DTLS transport endpoint. The domain for
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address types for DTLS sessions SHOULD be "snmpDTLSDomain" and the
address SHOULD be one that conforms to the details specified in the
"SnmpDTLSAddress" textual convention.
4.5.3.2. tmRequestedSecurityLevel
The tmRequestedSecurityLevel is the security level requested by the
application. This parameter is set in the cache by the security
model and used by DTLS Transport Model initiating a session to select
the appropriate cipher_suites and other configuration needed settings
for establishing the session. The DTLS Transport Model MUST ensure
that the actual security provided by the session (tmSecurityLevel) is
at least as high as the requested security level
(tmRequestedSecurityLevel).
4.5.3.3. tmSecurityLevel
The tmSecurityLevel is the actual security level of the established
session. See Section 3.1.2 for more detail about security levels.
How the chosen cipher_suites and other DTLS session parameters are
translated into a security level at the DTLS Transport Model is
implementation dependent and/or policy specific. Implementations
MUST NOT use NULL algorithms for fulfilling authentication or
encryption needs indicated by the tmSecurityLevel.
4.5.3.4. tmSecurityName
The tmSecurityName is the name of the principal on whose behalf the
message is being sent. This field is set via the mapping defined in
the dtlstmCertificateToSNTable when mapping incoming client
connection certificates to a tmSecurityName. For outgoing
connections, the application will specify the value that should be
placed in this field (possibly by extracting it from the SNMP-TARGET-
MIB's snmpTargetParamsSecurityName value).
4.5.4. Transport Model LCD
Implementations may store DTLS-specific and model-specific
information in a LCD. The DTLS session ID is one such parameter that
could be stored in the LCD. When messages are to be routed for
encapsulation or for integrity verification and decryption the
message and the DTLS session ID must be passed to the DTLS transport
layer for processing. Therefore, the DTLS Transport Model MUST
maintain a one-to-one mapping between the DTLS session ID and the
tmStateReference cache entry for that session. Implementations will
need to store the DTLS session ID in the tmStateReference cache to
simplify the procedure.
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5. Elements of Procedure
Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an
SNMP entity. The DTLSTM uses some of these conceptual data flows
when communicating between subsystems. These RFC 3411-defined data
flows are referred to here as public interfaces.
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 with zero value or random
value data prior to being released.
An error indication may return an OID and value for an incremented
counter if the information is available at the point where the error
is detected.
5.1. Procedures for an Incoming Message
The following section describes the procedures followed by the DTLS
Transport Model when it receives a DTLS protected packet. The steps
are broken into two different sections. The first section describes
the needed steps for de-multiplexing multiple DTLS sessions and the
second section describes the steps which are specific to transport
processing once the DTLS processing has been completed.
5.1.1. DTLS Processing for Incoming Messages
DTLS is significantly different in terms of session handling than
SSH, TLS or other TCP-based session streams. The DTLS protocol,
which is UDP-based, does not have a session identifier that allows
implementations to determine through which session a packet is
arriving like TCP-based streams have. Thus, a process for de-
multiplexing sessions must be incorporated into the procedures for an
incoming message. The steps in this section describe how this can be
accomplished, although any implementation dependent method for doing
so should be suitable as long as the results are consistently
deterministic. The important results from the steps in this section
are the transportDomain, the transportAddress, the wholeMessage, the
wholeMessageLength, and a unique implementation-dependent session
identifier.
This procedure assumes that upon session establishment, an entry in a
local transport mapping table is created in the Transport Model's
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LCD. This transport mapping table entry should be able to map a
unique combination of the remote address, remote port number, local
address and local port number to a implementation-dependent
dtlsSessionID.
1) The DTLS Transport Model examines the raw UDP message, in an
implementation-dependent manner. If the message is not a DTLS
message then it should be discarded. If the message is not a
(D)TLS Application Data message then the message should be
processed by the underlying DTLS framework as it is (for example)
a session initialization or session modification message and no
further steps below should be taken by the DTLS Transport.
2) The DTLS Transport Model queries the LCD using the transport
parameters to determine if a session already exists and its
dtlsSessionID. As noted previously, the source and destination
addresses and ports of the message should uniquely assign the
message to a specific session identifier. However, another
implementation-dependent method may be used if so desired.
3) If a matching entry in the LCD does not exist then the message is
discarded. Increment the dtlstmSessionNoAvailableSessions
counter and stop processing the message.
Note that an entry would already exist if the client and server's
session establishment procedures had been successfully completed
(as described both above and in Section 5.3) even if no message
had yet been sent through the newly established session. An
entry may not exist, however, if a "rogue" message was routed to
the SNMP entity by mistake. An entry might also be missing
because of a "broken" session (see operational considerations).
4) Retrieve the dtlsSessionID from the LCD.
5) The dtlsWholeMsg, and the dtlsSessionID are passed to DTLS for
integrity checking and decryption using the dtlsRead() ASI.
6) If the message fails integrity checks or other DTLS security
processing then the dtlstmDTLSProtectionErrors counter is
incremented, the message is discarded and processing of the
message is stopped.
7) The output of the dtlsRead results in an incomingMessage and an
incomingMessageLength. These results and the dtlsSessionID are
used below in the Section 5.1.2 to complete the processing of the
incoming message.
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5.1.2. Transport Processing for Incoming Messages
The procedures in this section describe how the DTLS Transport should
process messages that have already been properly extracted from the
DTLS stream, as described in Section 5.1.1.
1) Create a tmStateReference cache for the subsequent reference and
assign the following values within it:
tmTransportDomain = snmpDTLSDomain
tmTransportAddress = The address the message originated from,
determined in an implementation dependent way.
tmSecurityLevel = The derived tmSecurityLevel for the session,
as discussed in Section 3.1.2 and Section 5.3.
tmSecurityName = The derived tmSecurityName for the session as
discussed in and Section 5.3.
tmSessionID = The dtlsSessionID, which MUST be A unique session
identifier for this DTLS session. 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 dtlsSessionID discussed in
Section 5.1.1. tmSessionID refers to the session identifier
when stored in the tmStateReference and dtlsSessionID refers
to the session identifier when stored in the LCD. They MUST
always be equal when processing a given session's traffic.
2) The wholeMessage and the wholeMessageLength are assigned values
from the incomingMessage and incomingMessageLength values from
the DTLS processing.
3) The DTLS Transport Model passes the transportDomain,
transportAddress, wholeMessage, and wholeMessageLength to the
Dispatcher using the receiveMessage ASI:
statusInformation =
receiveMessage(
IN transportDomain -- snmpSSHDomain
IN transportAddress -- address for the received message
IN wholeMessage -- the whole SNMP message from SSH
IN wholeMessageLength -- the length of the SNMP message
IN tmStateReference -- (NEW) transport info
)
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5.2. Procedures for an Outgoing Message
The Dispatcher sends a message to the DTLS 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 -- (NEW) transport info
)
This section describes the procedure followed by the DTLS Transport
Model whenever it is requested through this ASI to send a message.
1) Extract tmSessionID, tmTransportAddress, tmSecurityName,
tmRequestedSecurityLevel. and tmSameSecurity from the
tmStateReference. Note: The tmSessionID value may be undefined
if session exists yet.
2) If tmSameSecurity is true and either tmSessionID is undefined or
refers to a session that is no longer open then increment the
dtlstmSessionNoAvailableSessions counter, discard the message and
return the error indication in the statusInformation. Processing
of this message stops.
3) 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 as described below in
step 3b. An implementation MAY choose to return an error to the
calling module.
4) If tmSessionID is undefined, then use tmTransportAddress,
tmSecurityName and tmRequestedSecurityLevel to see if there is a
corresponding entry in the LCD suitable to send the message over.
3a) If there is a corresponding LCD entry, then this session
will be used to send the message.
3b) If there is not a corresponding LCD entry, then open a
session using the openSession() ASI (discussed further in
Section 4.4.1). 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, increment the
dtlstmSessionOpenErrors, and return an error indication to
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the calling module.
5) Using either the session indicated by the tmSessionID if there
was one or the session resulting in the previous step, pass the
outgoingMessage to DTLS for encapsulation and transmission.
5.3. Establishing a Session
The DTLS Transport Model provides the following primitive to
establish a new DTLS session (previously discussed in Section 4.4.1):
statusInformation = -- errorIndication or success
openSession(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
OUT dtlsSessionID -- Session identifier for DTLS
)
The following sections describe the procedures followed by a DTLS
Transport Model when establishing a session as a Command Generator, a
Notification Originator or as part of a Proxy Forwarder.
The following describes the procedure to follow to establish a
session between SNMP engines to exchange SNMP messages. This process
is followed by any SNMP engine establishing a session for subsequent
use.
This MAY done automatically for SNMP messages which are not Response
or Report messages.
DTLS provides no explicit manner for transmitting an identity the
client wishes to connect to during or prior to key exchange to
facilitate certificate selection at the server (e.g. at a
Notification Receiver). I.E., there is no available mechanism for
sending notifications to a specific principal at a given UDP/port
combination. Therefore, implementations MAY support responding with
multiple identities using separate UDP port numbers to indicate the
desired principal or some other implementation-dependent solution.
1) The client selects the appropriate certificate and cipher_suites
for the key agreement based on the tmSecurityName and the
tmRequestedSecurityLevel for the session. For sessions being
established as a result of a SNMP-TARGET-MIB based operation, the
certificate will potentially have been identified via the
dtlstmParamsTable mapping and the cipher_suites will have to be
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taken from system-wide or implementation-specific configuration.
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 DTLS session.
However this is done, the security capabilities provided by DTLS
MUST be at least as high as the level of security indicated by
the tmRequestedSecurityLevel parameter. The actual security
level of the session should be reported in the tmStateReference
cache as tmSecurityLevel. For DTLS to provide strong
authentication, each principal acting as a Command Generator
SHOULD have its own certificate.
2) Using the destTransportDomain and destTransportAddress values,
the client will initiate the DTLS handshake protocol to establish
session keys for message integrity and encryption.
If the attempt to establish a session is unsuccessful, then
dtlstmSessionOpenErrors is incremented, an error indication is
returned, and session establishment processing stops.
3) Once the secure session is established and both sides have been
authenticated, certificate validation and identity expectations
are performed.
a) The DTLS server side of the connection identifies the
authenticated identity from the DTLS client's principal
certificate using the dtlstmCertificateToSNTable mapping
table and records this in the tmStateReference cache as
tmSecurityName. The details of the lookup process are fully
described in the DESCRIPTION clause of the
dtlstmCertificateToSNTable MIB object. If this verification
fails in any way (for example because of failures in
cryptographic verification or the lack of an appropriate row
in the dtlstmCertificateToSNTable) then the session
establishment MUST fail, the
dtlstmSessionInvalidClientCertificates object is incremented
and processing is stopped.
b) The DTLS client side of the connection SHOULD verify that
authenticated identity of the DTLS server's certificate is
the expected identity and MUST do so if the client
application is a Notification Generator. If strong
authentication is desired then the DTLS server certificate
MUST always be verified and checked against the expected
identity. Methods for doing this are described in
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[I-D.hodges-server-ident-check]. DTLS provides assurance
that the authenticated identity has been signed by a trusted
configured certificate authority. If verification of the
server's certificate fails in any way (for example because of
failures in cryptographic verification or the presented
identity was not the expected identity) then the session
establishment MUST fail, the
dtlstmSessionInvalidServerCertificates object is incremented
and processing is stopped.
4) The DTLS-specific session identifier is passed to the DTLS
Transport Model and associated with the tmStateReference cache
entry to indicate that the session has been established
successfully and to point to a specific DTLS session for future
use.
5.4. Closing a Session
The DTLS Transport Model provides the following primitive to close a
session:
statusInformation =
closeSession(
IN tmStateReference -- transport info
)
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) Look up the session in the cache and the LCD using the
tmStateReference.
2) If there is no session open associated with the tmStateReference,
then closeSession processing is completed.
3) Delete the entry from the cache and any other implementation-
dependent information in the LCD.
4) Have DTLS close the specified session. This SHOULD include
sending a close_notify TLS Alert to inform the other side that
session cleanup may be performed.
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6. MIB Module Overview
This MIB module provides management of the DTLS Transport Model. It
defines needed textual conventions, statistical counters 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 two new Textual Conventions: a new
TransportDomain and TransportAddress format for describing DTLS
connection addressing requirements.
6.3. Statistical Counters
The DTLSTM-MIB defines some statical counters that can provide
network managers with feedback about DTLS session usage and potential
errors that a MIB-instrumented device may be experiencing.
6.4. Configuration Tables
The DTLSTM-MIB defines configuration tables that a manager can use
for help in configuring a MIB-instrumented device for sending and
receiving SNMP messages over DTLS. In particular, there is a MIB
table that extends the SNMP-TARGET-MIB for configuring certificates
to be used and a MIB table for mapping incoming DTLS client
certificates to securityNames.
6.5. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing the DTLS Transport Model. In particular,
it is assumed that an entity implementing the DTLSTM-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].
This MIB module is for managing DTLS Transport Model information.
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6.5.1. MIB Modules Required for IMPORTS
The following MIB module imports items from SNMPV2-SMI [RFC2578],
SNMPV2-TC [RFC2579], SNMP-FRAMEWORK-MIB [RFC3411], SNMP-TARGET-MIB
[RFC3413] and SNMP-CONF [RFC2580].
7. MIB Module Definition
DTLSTM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, snmpModules, snmpDomains,
Counter32, Unsigned32
FROM SNMPv2-SMI
TEXTUAL-CONVENTION, TimeStamp, RowStatus, StorageType
FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB
snmpTargetParamsEntry
FROM SNMP-TARGET-MIB
;
dtlstmMIB MODULE-IDENTITY
LAST-UPDATED "200807070000Z"
ORGANIZATION " "
CONTACT-INFO "WG-EMail:
Subscribe:
Chairs:
Co-editors:
"
DESCRIPTION "The DTLS Transport Model MIB
Copyright (C) The IETF Trust (2008). This
version of this MIB module is part of RFC XXXX;
see the RFC itself for full legal notices."
-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
REVISION "200807070000Z"
DESCRIPTION "The initial version, published in RFC XXXX."
-- NOTE to RFC editor: replace XXXX with actual RFC number
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-- for this document and remove this note
::= { snmpModules xxxx }
-- RFC Ed.: replace xxxx with IANA-assigned number and
-- remove this note
-- ************************************************
-- subtrees of the SNMP-DTLS-TM-MIB
-- ************************************************
dtlstmNotifications OBJECT IDENTIFIER ::= { dtlstmMIB 0 }
dtlstmObjects OBJECT IDENTIFIER ::= { dtlstmMIB 1 }
dtlstmConformance OBJECT IDENTIFIER ::= { dtlstmMIB 2 }
-- ************************************************
-- Objects
-- ************************************************
snmpDTLSDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over DTLS transport domain. The corresponding
transport address is of type SnmpDTLSAddress.
When an SNMP entity uses the snmpDTLSDomain transport
model, it must be capable of accepting messages up to
the maximum MTU size for an interface it supports, minus the
needed IP, UDP, DTLS and other protocol overheads.
The securityName prefix to be associated with the
snmpDTLSDomain is 'dtls'. This prefix may be used by
security models or other components to identify what secure
transport infrastructure authenticated a securityName."
::= { snmpDomains yy }
-- RFC Ed.: Please replace the I-D reference with a proper one once it
-- has been published. Note: xml2rfc doesn't handle refs within artwork
-- RFC Ed.: replace yy with IANA-assigned number and
-- remove this note
-- RFC Ed.: replace 'dtls' with the actual IANA assigned prefix string
-- if 'dtls' is not assigned to this document.
SnmpDTLSAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
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STATUS current
DESCRIPTION
"Represents a UDP connection address for an IPv4 address, an
IPv6 address or an ASCII encoded host name and port number.
The hostname must be encoded in ASCII, as specified in RFC3490
(Internationalizing Domain Names in Applications) followed by
a colon ':' (ASCII character 0x3A) and a decimal port number
in ASCII. The name SHOULD be fully qualified whenever
possible.
An IPv4 address must be a dotted decimal format followed by a
colon ':' (ASCII character 0x3A) and a decimal port number in
ASCII.
An IPv6 address must be a colon separated format, surrounded
by square brackets (ASCII characters 0x5B and 0x5D), followed
by a colon ':' (ASCII character 0x3A) and a decimal port
number in ASCII.
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 snmpDTLSAddress values must fully describe how (and when)
such names are to be resolved to IP addresses and vice versa.
This textual convention SHOULD NOT be used directly in object
definitions since it restricts addresses to a specific
format. However, if it is used, it MAY be used either on its
own or in conjunction with TransportAddressType or
TransportDomain as a pair.
When this textual convention is used as a syntax of an index
object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2, STD 58. It is RECOMMENDED
that all MIB documents using this textual convention make
explicit any limitations on index component lengths that
management software must observe. This may be done either by
including SIZE constraints on the index components or by
specifying applicable constraints in the conceptual row
DESCRIPTION clause or in the surrounding documentation."
SYNTAX OCTET STRING (SIZE (1..255))
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X509IdentifierHashType ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Identifies a hashing algorithm type that will be used for
identifying an X.509 certificate.
The md5(1) value SHOULD NOT be used."
SYNTAX INTEGER { md5(1), sha1(2), sha256(3) }
X509IdentifierHash ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"A hash value that uniquely identifies a certificate within a
systems local certificate store. The length of the value
stored in an object of type X509IdentifierHash is dependent on
the hashing algorithm that produced the hash.
MIB structures making use of this textual convention should
have an accompanying object of type X509IdentifierHashType.
"
SYNTAX OCTET STRING
-- The dtlstmSession Group
dtlstmSession OBJECT IDENTIFIER ::= { dtlstmObjects 1 }
dtlstmSessionOpens OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an openSession() request has been
executed as an SSH client, whether it succeeded or failed."
::= { dtlstmSession 1 }
dtlstmSessionCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as an SSH client, whether it succeeded or failed."
::= { dtlstmSession 2 }
dtlstmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
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DESCRIPTION
"The number of times an openSession() request failed to open a
session as a SSH client, for any reason."
::= { dtlstmSession 3 }
dtlstmSessionNoAvailableSessions 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."
::= { dtlstmSession 4 }
dtlstmSessionInvalidClientCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an incoming session was not established
on an SSH server because the presented client certificate was
invalid. Reasons for invalidation includes, but is not
limited to, crypographic validation failures and lack of a
suitable mapping row in the dtlstmCertificateToSNTable."
::= { dtlstmSession 5 }
dtlstmSessionInvalidServerCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on an SSH client because the presented server certificate was
invalid. Reasons for invalidation includes, but is not
limited to, crypographic validation failures and an unexpected
presented certificate identity."
::= { dtlstmSession 6 }
dtlstmDTLSProtectionErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times DTLS processing resulted in a message
being discarded because it failed its integrity test,
decryption processing or other DTLS processing."
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::= { dtlstmSession 7 }
-- Configuration Objects
dtlstmConfig OBJECT IDENTIFIER ::= { dtlstmObjects 2 }
-- Certificate mapping
dtlstmCertificateMapping OBJECT IDENTIFIER ::= { dtlstmConfig 1 }
dtlstmCertificateToSNCount OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the
dtlstmCertificateToSNTable"
::= { dtlstmCertificateMapping 1 }
dtlstmCertificateToSNTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the dtlstmCertificateToSNTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { dtlstmCertificateMapping 2 }
dtlstmCertificateToSNTable OBJECT-TYPE
SYNTAX SEQUENCE OF DtlstmCertificateToSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table listing the X.509 certificates known to the entity
and the associated method for determining the SNMPv3 security
name from a certificate.
On an incoming DTLS/SNMP connection the client's presented
certificate should be examined and validated based on an
established trusted CA certificate or self-signed public
certificate. This table does not provide a mechanism for
uploading the certificates as that is expected to occur
through an out-of-band transfer.
Once the authenticity of the certificate has been verified,
this table can be consulted to determine the appropriate
securityName to identify the remote connection. This is done
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by comparing the issuer's fingerprint hash type and value and
the certificate's fingerprint hash type and value against the
dtlstmCertHashType and dtlstmCertHashValue values in each
entry of this table. If a matching entry is found then the
securityName is selected based on the dtlstmCertMapType,
dtlstmCertHashType, dtlstmCertHashValue and
dtlstmCertSecurityName fields and the resulting securityName
is used to identify the other side of the DTLS connection.
This table should be treated as an ordered list of mapping
rules to check. The first mapping rule appropriately matching
a certificate in the local certificate store with a
corresponding hash type (dtlstmCertHashType) and hash value
(dtlstmCertHashValue) will be used to perform the mapping from
X.509 certificate values to a securityName. If, after a
matching row is found but the mapping can not succeed for some
other reason then further attempts to perform the mapping MUST
NOT be taken. For example, if the entry being checked
contains a dtlstmCertMapType of bySubjectAltName(2) and an
incoming connection uses a certificate with an issuer
certificate matching the dtlstmCertHashType and
dtlstmCertHashValue fields but the connecting certificate does
not contain a subjectAltName field then the lookup operation
must be treated as a failure. No further rows are examined for
other potential mappings.
Missing values of dtlstmCertID are acceptable and
implementations should treat missing entries as a failed match
and should continue to the next highest numbered row. E.G.,
the table may legally contain only two rows with dtlstmCertID
values of 10 and 20.
Users are encouraged to make use of certificates with
subjectAltName fields that can be used as securityNames so
that a single root CA certificate can allow all child
certificate's subjectAltName to map directly to a securityName
via a 1:1 transformation. However, this table is flexible
enough to allow for situations where existing deployed
certificate infrastructures do not provide adequate
subjectAltName values for use as SNMPv3 securityNames.
Certificates may also be mapped to securityNames using the
CommonName portion of the Subject field which is also a
scalable method of mapping certificate components to
securityNames. Finally, direct mapping from each individual
certificate fingerprint to a securityName is possible but
requires one entry in the table per securityName."
::= { dtlstmCertificateMapping 3 }
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dtlstmCertificateToSNEntry OBJECT-TYPE
SYNTAX DtlstmCertificateToSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A row in the dtlstmCertificateToSNTable that specifies a
mapping for an incoming DTLS certificate to a securityName to
use for the connection."
INDEX { dtlstmCertID }
::= { dtlstmCertificateToSNTable 1 }
DtlstmCertificateToSNEntry ::= SEQUENCE {
dtlstmCertID Unsigned32,
dtlstmCertHashType X509IdentifierHashType,
dtlstmCertHashValue X509IdentifierHash,
dtlstmCertMapType INTEGER,
dtlstmCertSecurityName SnmpAdminString,
dtlstmCertStorageType StorageType,
dtlstmCertRowStatus RowStatus
}
dtlstmCertID OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A unique arbitrary number index for a given certificate
entry."
::= { dtlstmCertificateToSNEntry 1 }
dtlstmCertHashType OBJECT-TYPE
SYNTAX X509IdentifierHashType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The hash algorithm to use when applying a hash to a X.509
certificate for purposes of referring to it from the
dtlstmCertHashValue column.
The md5(1) value SHOULD NOT be used."
DEFVAL { sha256 }
::= { dtlstmCertificateToSNEntry 2 }
dtlstmCertHashValue OBJECT-TYPE
SYNTAX X509IdentifierHash
MAX-ACCESS read-create
STATUS current
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DESCRIPTION
"A cryptographic hash of a X.509 certificate. The use of this
hash is dictated by the dtlstmCertMapType column.
"
::= { dtlstmCertificateToSNEntry 3 }
dtlstmCertMapType OBJECT-TYPE
SYNTAX INTEGER { specified(1), bySubjectAltName(2), byCN(3) }
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The mapping type used to obtain the securityName from the
certificate. The possible values of use and their usage
methods are defined as follows:
specified(1): The securityName that should be used locally to
identify the remote entity is directly specified
in the dtlstmCertSecurityName column from this
table. The dtlstmCertHashValue MUST refer to a
X.509 client certificate that will be mapped
directly to the securityName specified in the
dtlstmCertSecurityName column.
bySubjectAltName(2):
The securityName that should be used locally to
identify the remote entity should be taken from
the subjectAltName portion of the X.509
certificate. The dtlstmCertHashValue MUST refer
to a trust anchor certificate that is
responsible for issuing certificates with
carefully controlled subjectAltName fields.
byCN(3): The securityName that should be used locally to
identify the remote entity should be taken from
the CommonName portion of the Subject field from
the X.509 certificate. The dtlstmCertHashValue
MUST refer to a trust anchor certificate that is
responsible for issuing certificates with
carefully controlled CommonName fields."
DEFVAL { specified }
::= { dtlstmCertificateToSNEntry 4 }
dtlstmCertSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The securityName that the session should use if the
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dtlstmCertMapType is set to specified(1), otherwise the value
in this column should be ignored. If dtlstmCertMapType is set
to specifed(1) and this column contains a zero-length string
(which is not a legal securityName value) this row is
effectively disabled and the match will not be considered
successful."
DEFVAL { "" }
::= { dtlstmCertificateToSNEntry 5 }
dtlstmCertStorageType 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 }
::= { dtlstmCertificateToSNEntry 6 }
dtlstmCertRowStatus 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.
The value of this object has no effect on whether
other objects in this conceptual row can be modified."
::= { dtlstmCertificateToSNEntry 7 }
-- Maps securityNames to certificates for use by the SNMP-TARGET-MIB
dtlstmParamsCount OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the
dtlstmParamsTable"
::= { dtlstmCertificateMapping 4 }
dtlstmParamsTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
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DESCRIPTION
"The value of sysUpTime.0 when the dtlstmParamsTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { dtlstmCertificateMapping 5 }
dtlstmParamsTable OBJECT-TYPE
SYNTAX SEQUENCE OF DtlstmParamsEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table augments the SNMP-TARGET-MIB's
snmpTargetParamsTable with additional a DTLS client-side
certificate certificate identifier to use when establishing
new DTLS connections."
::= { dtlstmCertificateMapping 6 }
dtlstmParamsEntry OBJECT-TYPE
SYNTAX DtlstmParamsEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A conceptual row containing a locally held certificate's hash
type and hash value for a given snmpTargetParamsEntry. The
values in this row should be ignored if not the connection
that needs to be established, as indicated by the
SNMP-TARGET-MIB infrastructure, is not a DTLS based
connection."
AUGMENTS { snmpTargetParamsEntry }
::= { dtlstmParamsTable 1 }
DtlstmParamsEntry ::= SEQUENCE {
dtlstmParamsHashType X509IdentifierHashType,
dtlstmParamsHashValue X509IdentifierHash,
dtlstmParamsStorageType StorageType,
dtlstmParamsRowStatus RowStatus
}
dtlstmParamsHashType OBJECT-TYPE
SYNTAX X509IdentifierHashType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The hash algorithm type for the hash stored in the
dtlstmParamsHash column to identify a locally-held X.509
certificate that should be used when initiating a DTLS
connection as a DTLS client."
DEFVAL { sha256 }
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::= { dtlstmParamsEntry 1 }
dtlstmParamsHashValue OBJECT-TYPE
SYNTAX X509IdentifierHash
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A cryptographic hash of a X.509 certificate. This object
should store the hash of a locally held X.509 certificate that
should be used when initiating a DTLS connection as a DTLS
client."
::= { dtlstmParamsEntry 2 }
dtlstmParamsStorageType 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 }
::= { dtlstmParamsEntry 3 }
dtlstmParamsRowStatus 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.
The value of this object has no effect on whether
other objects in this conceptual row can be modified."
::= { dtlstmParamsEntry 4 }
-- ************************************************
-- dtlstmMIB - Conformance Information
-- ************************************************
dtlstmCompliances OBJECT IDENTIFIER ::= { dtlstmConformance 1 }
dtlstmGroups OBJECT IDENTIFIER ::= { dtlstmConformance 2 }
-- ************************************************
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-- Compliance statements
-- ************************************************
dtlstmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
SNMP-DTLS-TM-MIB"
MODULE
MANDATORY-GROUPS { dtlstmStatsGroup,
dtlstmIncomingGroup, dtlstmOutgoingGroup }
::= { dtlstmCompliances 1 }
-- ************************************************
-- Units of conformance
-- ************************************************
dtlstmStatsGroup OBJECT-GROUP
OBJECTS {
dtlstmSessionOpens,
dtlstmSessionCloses,
dtlstmSessionOpenErrors,
dtlstmSessionNoAvailableSessions,
dtlstmSessionInvalidClientCertificates,
dtlstmSessionInvalidServerCertificates,
dtlstmDTLSProtectionErrors
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
statistical information of an SNMP engine which
implements the SNMP DTLS Transport Model."
::= { dtlstmGroups 1 }
dtlstmIncomingGroup OBJECT-GROUP
OBJECTS {
dtlstmCertificateToSNCount,
dtlstmCertificateToSNTableLastChanged,
dtlstmCertHashType,
dtlstmCertHashValue,
dtlstmCertMapType,
dtlstmCertSecurityName,
dtlstmCertStorageType,
dtlstmCertRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
incoming connection certificate mappings to
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securityNames of an SNMP engine which implements the
SNMP DTLS Transport Model."
::= { dtlstmGroups 2 }
dtlstmOutgoingGroup OBJECT-GROUP
OBJECTS {
dtlstmParamsCount,
dtlstmParamsTableLastChanged,
dtlstmParamsHashType,
dtlstmParamsHashValue,
dtlstmParamsStorageType,
dtlstmParamsRowStatus
}
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."
::= { dtlstmGroups 3 }
END
8. Operational Considerations
This section discusses various operational aspects of the solution
8.1. Sessions
A session is discussed throughout this document as meaning a security
association between the DTLS client and the DTLS server. State
information for the sessions are maintained in each DTLSTM and this
information is created and destroyed as sessions are opened and
closed. Because of the connectionless nature of UDP, a "broken"
session, one side up one side down, could 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 disconnect messages. Implementations
SHOULD also have a system in place for dealing with "broken"
sessions. Implementations SHOULD support the session resumption
feature of TLS.
To simplify session management it is RECOMMENDED that implementations
utilize two separate ports, one for Notification sessions and one for
Command sessions. If this implementation recommendation is followed,
DTLS clients will always send REQUEST messages and DTLS servers will
always send RESPONSE messages. With this assertion, implementations
may be able to simplify "broken" session handling, session
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resumption, and other aspects of session management such as
guaranteeing that Request- Response pairs use the same session.
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, a time-to-live (TTL)
value SHOULD be established for sessions. An upper limit of 24 hours
is suggested for this TTL value. The TTL value could be stored in
the LCD and checked before passing a message to the DTLS session.
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. However, the receiving
SNMP engine (Server) does not know which DTLS certificate to offer to
the Client so that the tmSecurityName identity-authentication will be
successful. The best solution would be to maintain a one-to-one
mapping between certificates and incoming ports for notification
receivers, although other implementation dependent mechanisms may be
used instead. This can be handled at the Notification Originator by
configuring the snmpTargetAddrTable (snmpTargetAddrTDomain and
snmpTargetAddrTAddress) and then 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 TRAPS and INFORMS.
8.3. contextEngineID Discovery
Because most Command Responders have contextEngineIDs that are
identical to the USM securityEngineID, the USM provides Command
Generators with the ability to discover a default contextEngineID to
use. Because the DTLS transport does not make use of a discoverable
securityEngineID like the USM does, it may be difficult for Command
Generators to discover a suitable default contextEngineID.
Implementations should consider offering another engineID discovery
mechanism to continue providing Command Generators with a
contextEngineID discovery mechanism. A recommended discovery
solution is documented in [RFC5343].
9. Security Considerations
This document describes a transport model that permits SNMP to
utilize DTLS security services. The security threats and how the
DTLS transport model mitigates these threats are covered in detail
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throughout this document. Security considerations for DTLS are
covered in [RFC4347] and security considerations for TLS are
described in Appendices D, E, and F of TLS 1.1 [RFC4346]. DTLS adds
to the security considerations of TLS only because it is more
vulnerable to denial of service attacks. A random cookie exchange
was added to the handshake to prevent anonymous denial of service
attacks. RFC 4347 recommends that the cookie exchange is utilized
for all handshakes and therefore it is RECOMMENDED that implementers
also support this cookie exchange.
9.1. Certificates, Authentication, and Authorization
Implementations are responsible for providing a security certificate
configuration installation . Implementations SHOULD support
certificate revocation lists and expiration of certificates or other
access control mechanisms.
DTLS provides for both authentication of the identity of the DTLS
server and authentication of the identity of the DTLS client. Access
to MIB objects for the authenticated principal MUST be enforced by an
access control subsystem (e.g. the VACM).
Authentication of the Command Generator principal's identity is
important for use with the SNMP access control subsystem to ensure
that only authorized principals have access to potentially sensitive
data. The authenticated identity of the Command Generator
principal's certificate is mapped to an SNMP model-independent
securityName for use with SNMP access control, as discussed in
Section 4.5.3.4, Section 7 and other sections.
Furthermore, the DTLS handshake only provides assurance that the
certificate of the authenticated identity has been signed by an
configured accepted Certificate Authority. DTLS 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 Responder's authorization to
receive Notifications from a Notification Originator. However to
avoid man-in-the-middle attacks both ends of the DTLS based
connection MUST check the certificate presented by the other side
against what was expected. For example, Command Generators must
check that the Command Responder presented and authenticated itself
with a X.509 certificate that was expected. Not doing so would allow
an impostor, at a minimum, to present false data, receive sensitive
information and/or provide a false-positive belief that configuration
was actually received and acted upon. Authenticating and verifying
the identity of the DTLS server and the DTLS client for all
operations ensures the authenticity of the SNMP engine that provides
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MIB data.
The instructions found in the DESCRIPTION clause of the
dtlstmCertificateToSNTable object must be followed exactly.
Specifically, it is important that if a row matching a certificate or
a certificate's issuer is found but the translation to a securityName
using the row fails that the lookup process stops and no further rows
are consulted. It is also important that the rows of the table be
search in order starting with the row containing the lowest numbered
dtlstmCertID value.
9.2. Use with SNMPv1/SNMPv2c Messages
The SNMPv1 and SNMPv2c message processing described in RFC3484 (BCP
74) [RFC3584] always selects the SNMPv1(1) Security Model for an
SNMPv1 message, or the SNMPv2c(2) Security Model for an SNMPv2c
message. When running SNMPv1/SNMPv2c over a secure transport like
the DTLS Transport Model, the securityName and securityLevel used for
access control decisions are then derived from the community string,
not the authenticated identity and securityLevel provided by the DTLS
Transport Model.
9.3. MIB Module Security
The MIB objects in this document must be protected with an adequate
level of at least integrity protection, especially those objects
which are writable. 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.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPSec or
DTLS) 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 section 8 of [RFC3410]),
including full support for the USM (see [RFC3414]) and the DTLS
Transport Model cryptographic mechanisms (for authentication and
privacy).
10. IANA Considerations
IANA is requested to assign:
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1. a UDP port number in the range 1..1023 in the
http://www.iana.org/assignments/port-numbers registry which will
be the default port for SNMP over a DTLS Transport Model as
defined in this document,
2. a UDP port number in the range 1..1023 in the
http://www.iana.org/assignments/port-numbers registry which will
be the default port for SNMPTRAP over a DTLS Transport Model as
defined in this document,
3. an SMI number under snmpDomains for the snmpDTLSDomain object
identifier,
4. a SMI number under snmpModules, for the MIB module in this
document,
5. "dtls" as the corresponding prefix for the snmpDTLSDomain in the
SNMP Transport Model registry;
11. Acknowledgements
This document closely follows and copies the Secure Shell Transport
Model for SNMP defined by David Harrington and Joseph Salowey in
[I-D.ietf-isms-secshell].
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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, March 1999.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
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[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.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[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.
[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.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[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
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(CRL) Profile", RFC 5280, May 2008.
[I-D.ietf-isms-transport-security-model]
Harington, D., "Transport Security Model for SNMP".
[I-D.ietf-isms-tmsm]
Harington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)".
[X509] Rivest, R., Shamir, A., and L. M. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems".
[AES] National Institute of Standards, "Specification for the
Advanced Encryption Standard (AES)".
[DES] National Institute of Standards, "American National
Standard for Information Systems-Data Link Encryption".
[DSS] National Institute of Standards, "Digital Signature
Standard".
[RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems".
12.2. Informative References
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[I-D.ietf-isms-secshell]
Harington, D. and J. Salowey, "Secure Shell Transport
Model for SNMP".
[RFC5343] Schoenwaelder, J., "Simple Network Management Protocol
(SNMP) Context EngineID Discovery".
[I-D.hodges-server-ident-check]
Hodges, J. and B. Morgan, "Generic Server Identity Check".
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Appendix A. Target and Notificaton Configuration Example
Configuring the SNMP-TARGET-MIB and NOTIFICATION-MIB along with
access control settings for the SNMP-VIEW-BASED-ACM-MIB can be a
daunting task without an example to follow. The following section
describes an example of what pieces must be in place to accomplish
this configuration.
The isAccessAllowed() ASI requires configuration to exist in the
following SNMP-VIEW-BASED-ACM-MIB tables:
vacmSecurityToGroupTable
vacmAccessTable
vacmViewTreeFamilyTable
The only table that needs to be discussed as particularly different
here is the vacmSecurityToGroupTable. This table is indexed by both
the SNMPv3 security model and the security name. The security model,
when DTLSTM is in use, should be set to the value of XXX
corresponding to the TSM [I-D.ietf-isms-transport-security-model].
An example vacmSecurityToGroupTable row might be filled out as
follows (using a single SNMP SET request):
Note to RFC editor: replace XXX in the previous paragraph above with
the actual IANA-assigned number for the TSM security model and remove
this note.
vacmSecurityModel = XXX (TSM)
vacmSecurityName = "blueberry"
vacmGroupaName = "administrators"
vacmSecurityToGroupStorageType = 3 (nonVolatile)
vacmSecurityToGroupStatus = 4 (createAndGo)
Note to RFC editor: replace XXX in the vacmSecurityModel line above
with the actual IANA-assigned number for the TSM security model and
remove this note.
This example will assume that the "administrators" group has been
given proper permissions via rows in the vacmAccessTable and
vacmViewTreeFamilyTable.
Depending on whether this VACM configuration is for a Command
Responder or a Command Generator the security name "blueberry" will
come from a few different locations.
For Notification Generator's performing authorization checks, the
server's certificate must be verified against the expected
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certificate before proceeding to send the notification. The
securityName be set by the SNMP-TARGET-MIB's
snmpTargetParamsSecurityName column or other configuration mechanism
and the certificate to use would be taken from the appropriate entry
in the dtlstmParamsTable. The dtlstmParamsTable augments the SNMP-
TARGET-MIB's snmpTargetParamsTable with client-side certificate
information.
For Command Responder applications, the vacmSecurityName "blueberry"
value is a value that needs to come from an incoming DTLS session.
The mapping from a recevied DTLS client certificate to a securityName
is done with the dtlstmCertificateToSNTable. The certificates must
be loaded into the device so that a dtlstmCertificateToSNEntry may
refer to it. As an example, consider the following entry which will
provide a mapping from a X.509's hash fingerprint directly to the
"blueberry" securityName:
dtlstmCertID = 1 (arbitrarily chosen)
dtlstmCertHashType = sha256
dtlstmCertHashValue = (appropriate sha256 fingerprint)
dtlstmCertMapType = specified(1)
dtlstmCertSecurityName = "blueberry"
dtlstmCertStorageType = 3 (nonVolatile)
dtlstmCertRowStatus = 4 (createAndGo)
The above is an example of how to map a particular certificate to a
particular securityName. It is recommended that users make use of
direct subjectAltName or CommonName mappings where possible since it
will provide a more scalable approach to certificate management.
This entry provides an example of using a subjectAltName mapping:
dtlstmCertID = 1 (arbitrarily chosen)
dtlstmCertHashType = sha256
dtlstmCertHashValue = (appropriate sha256 fingerprint)
dtlstmCertMapType = bySubjectAltName(2)
dtlstmCertStorageType = 3 (nonVolatile)
dtlstmCertRowStatus = 4 (createAndGo)
The above entry indicates the subjectAltName field for certificates
created by an Issuing certificate with a corresponding hash type and
value will be trusted to always produce common names that are
directly 1 to 1 mappable into SNMPv3 securityNames. This type of
configuration should only be used when the certificate authorities
naming conventions are carefully controlled.
For the example, if the incoming DTLS client provided certificate
contained a subjectAltName of "blueberry" and the certificate was
signed by a certificate matching the dtlstmCertHashType and
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dtlstmCertHashValue values above and the CA's certificate was
properly installed on the device then the CommonName of "blueberry"
would be used as the securityName for the session.
Author's Address
Wes Hardaker
Sparta, Inc.
P.O. Box 382
Davis, CA 95617
US
Phone: +1 530 792 1913
Email: ietf@hardakers.net
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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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