RFC 5591 INTERNET STANDARD SNMP · SECURITY · MIB
SNMP transport security: Transport Security Model for the Simple Network Management Protocol (SNMP)
An SNMP security model that takes the sender's identity and protection level from a secure transport instead of doing its own cryptography.
RFC EDITOR
Harrington, D. and W. Hardaker, "Transport Security Model for the Simple Network Management Protocol (SNMP)", STD 78, RFC 5591, DOI 10.17487/RFC5591, June 2009, <https://www.rfc-editor.org/info/rfc5591>.
BIBTEX
@misc{rfc5591,
series = {Request for Comments},
number = 5591,
howpublished = {RFC 5591},
publisher = {RFC Editor},
doi = {10.17487/RFC5591},
url = {https://www.rfc-editor.org/info/rfc5591},
author = {D. Harrington and W. Hardaker},
title = {{Transport Security Model for the Simple Network Management Protocol (SNMP)}},
pagetotal = 28,
year = 2009,
month = jun,
} RFC text
Contents
Network Working Group D. Harrington
Request for Comments: 5591 Huawei Technologies (USA)
Category: Standards Track W. Hardaker
Cobham Analytic Solutions
June 2009
Transport Security Model for the
Simple Network Management Protocol (SNMP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Abstract
This memo describes a Transport Security Model for the Simple Network
Management Protocol (SNMP).
This memo also defines a portion of the Management Information Base
(MIB) for monitoring and managing the Transport Security Model for
SNMP.
Table of Contents
1. Introduction ....................................................3
1.1. The Internet-Standard Management Framework .................3
1.2. Conventions ................................................3
1.3. Modularity .................................................4
1.4. Motivation .................................................5
1.5. Constraints ................................................5
2. How the Transport Security Model Fits in the Architecture .......6
2.1. Security Capabilities of this Model ........................6
2.1.1. Threats .............................................6
2.1.2. Security Levels .....................................7
2.2. Transport Sessions .........................................7
2.3. Coexistence ................................................7
2.3.1. Coexistence with Message Processing Models ..........7
2.3.2. Coexistence with Other Security Models ..............8
2.3.3. Coexistence with Transport Models ...................8
3. Cached Information and References ...............................8
3.1. Transport Security Model Cached Information ................9
3.1.1. securityStateReference ..............................9
3.1.2. tmStateReference ....................................9
3.1.3. Prefixes and securityNames ..........................9
4. Processing an Outgoing Message .................................10
4.1. Security Processing for an Outgoing Message ...............10
4.2. Elements of Procedure for Outgoing Messages ...............11
5. Processing an Incoming SNMP Message ............................12
5.1. Security Processing for an Incoming Message ...............12
5.2. Elements of Procedure for Incoming Messages ...............13
6. MIB Module Overview ............................................14
6.1. Structure of the MIB Module ...............................14
6.1.1. The snmpTsmStats Subtree ...........................14
6.1.2. The snmpTsmConfiguration Subtree ...................14
6.2. Relationship to Other MIB Modules .........................14
6.2.1. MIB Modules Required for IMPORTS ...................15
7. MIB Module Definition ..........................................15
8. Security Considerations ........................................20
8.1. MIB Module Security .......................................20
9. IANA Considerations ............................................21
10. Acknowledgments ...............................................22
11. References ....................................................22
11.1. Normative References .....................................22
11.2. Informative References ...................................23
Appendix A. Notification Tables Configuration ....................24
A.1. Transport Security Model Processing for Notifications .....25
Appendix B. Processing Differences between USM and Secure
Transport ............................................26
B.1. USM and the RFC 3411 Architecture .........................26
B.2. Transport Subsystem and the RFC 3411 Architecture .........27
1. Introduction
This memo describes a Transport Security Model for the Simple Network
Management Protocol for use with secure Transport Models in the
Transport Subsystem [RFC5590].
This memo also defines a portion of the Management Information Base
(MIB) for monitoring and managing the Transport Security Model for
SNMP.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the Transport
Security Model described in this memo fits into the architecture and
interacts with other subsystems and models within the architecture.
It is expected that readers will have also read and understood
[RFC3411], [RFC3412], [RFC3413], and [RFC3418].
1.1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [RFC3410].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. MIB objects are generally
accessed through the Simple Network Management Protocol (SNMP).
Objects in the MIB are defined using the mechanisms defined in the
Structure of Management Information (SMI). This memo specifies a MIB
module that is compliant to the SMIv2, which is described in STD 58,
RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
[RFC2580].
1.2. Conventions
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].
Lowercase versions of the keywords should be read as in normal
English. They will usually, but not always, be used in a context
that relates to compatibility with the RFC 3411 architecture or the
subsystem defined here but that might have no impact on on-the-wire
compatibility. These terms are used as guidance for designers of
proposed IETF models to make the designs compatible with RFC 3411
subsystems and Abstract Service Interfaces (ASIs). Implementers are
free to implement differently. Some usages of these lowercase terms
are simply normal English usage.
For consistency with SNMP-related specifications, this document
favors terminology as defined in STD 62, rather than favoring
terminology that is consistent with non-SNMP specifications 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.
Authentication in this document typically refers to the English
meaning of "serving to prove the authenticity of" the message, not
data source authentication or peer identity authentication.
The terms "manager" and "agent" are not used in this document
because, in the RFC 3411 architecture, all SNMP entities have the
capability of acting as manager, agent, or both depending on the SNMP
applications included in the engine. Where distinction is needed,
the application names of command generator, command responder,
notification originator, notification receiver, and proxy forwarder
are used. See "Simple Network Management Protocol (SNMP)
Applications" [RFC3413] for further information.
While security protocols frequently refer to a user, the terminology
used in [RFC3411] and in this memo is "principal". A principal is
the "who" on whose behalf services are provided or processing takes
place. A principal can be, among other things, an individual acting
in a particular role, a set of individuals each acting in a
particular role, an application or a set of applications, or a
combination of these within an administrative domain.
1.3. Modularity
The reader is expected to have read and understood the description of
the SNMP architecture, as defined in [RFC3411], and the architecture
extension specified in "Transport Subsystem for the Simple Network
Management Protocol (SNMP)" [RFC5590], which enables the use of
external "lower-layer transport" protocols to provide message
security. Transport Models are tied into the SNMP architecture
through the Transport Subsystem. The Transport Security Model is
designed to work with such lower-layer, secure Transport Models.
In keeping with the RFC 3411 design decisions to use self-contained
documents, this memo includes the elements of procedure plus
associated MIB objects that are needed for processing the Transport
Security Model for SNMP. These MIB objects SHOULD NOT be referenced
in other documents. This allows the Transport Security Model to be
designed and documented as independent and self-contained, having no
direct impact on other modules. It also allows this module to be
upgraded and supplemented as the need arises, and to move along the
standards track on different time-lines from other modules.
This modularity of specification is not meant to be interpreted as
imposing any specific requirements on implementation.
1.4. Motivation
This memo describes a Security Model to make use of Transport Models
that use lower-layer, secure transports and existing and commonly
deployed security infrastructures. This Security Model is designed
to meet the security and operational needs of network administrators,
maximize usability in operational environments to achieve high
deployment success, and at the same time minimize implementation and
deployment costs to minimize the time until deployment is possible.
1.5. Constraints
The design of this SNMP Security Model is also influenced by the
following constraints:
1. In times of network stress, the security protocol and its
underlying security mechanisms SHOULD NOT depend solely upon the
ready availability of other network services (e.g., Network Time
Protocol (NTP) or Authentication, Authorization, and Accounting
(AAA) protocols).
2. When the network is not under stress, the Security Model and its
underlying security mechanisms MAY depend upon the ready
availability of other network services.
3. It might not be possible for the Security Model to determine when
the network is under stress.
4. A Security Model SHOULD NOT require changes to the SNMP
architecture.
5. A Security Model SHOULD NOT require changes to the underlying
security protocol.
2. How the Transport Security Model Fits in the Architecture
The Transport Security Model is designed to fit into the RFC 3411
architecture as a Security Model in the Security Subsystem and to
utilize the services of a secure Transport Model.
For incoming messages, a secure Transport Model will pass a
tmStateReference cache, described in [RFC5590]. To maintain RFC 3411
modularity, the Transport Model will not know which securityModel
will process the incoming message; the Message Processing Model will
determine this. If the Transport Security Model is used with a non-
secure Transport Model, then the cache will not exist or will not be
populated with security parameters, which will cause the Transport
Security Model to return an error (see Section 5.2).
The Transport Security Model will create the securityName and
securityLevel to be passed to applications, and will verify that the
tmTransportSecurityLevel reported by the Transport Model is at least
as strong as the securityLevel requested by the Message Processing
Model.
For outgoing messages, the Transport Security Model will create a
tmStateReference cache (or use an existing one), and will pass the
tmStateReference to the specified Transport Model.
2.1. Security Capabilities of this Model
2.1.1. Threats
The Transport Security Model is compatible with the RFC 3411
architecture and provides protection against the threats identified
by the RFC 3411 architecture. However, the Transport Security Model
does not provide security mechanisms such as authentication and
encryption itself. Which threats are addressed and how they are
mitigated depends on the Transport Model used. To avoid creating
potential security vulnerabilities, operators should configure their
system so this Security Model is always used with a Transport Model
that provides appropriate security, where "appropriate" for a
particular deployment is an administrative decision.
2.1.2. Security Levels
The RFC 3411 architecture recognizes three levels of security:
- without authentication and without privacy (noAuthNoPriv)
- with authentication but without privacy (authNoPriv)
- with authentication and with privacy (authPriv)
The model-independent securityLevel parameter is used to request
specific levels of security for outgoing messages and to assert that
specific levels of security were applied during the transport and
processing of incoming messages.
The transport-layer algorithms used to provide security should not be
exposed to the Transport Security Model, as the Transport Security
Model has no mechanisms by which it can test whether an assertion
made by a Transport Model is accurate.
The Transport Security Model trusts that the underlying secure
transport connection has been properly configured to support security
characteristics at least as strong as reported in
tmTransportSecurityLevel.
2.2. Transport Sessions
The Transport Security Model does not work with transport sessions
directly. Instead the transport-related state is associated with a
unique combination of transportDomain, transportAddress,
securityName, and securityLevel, and is referenced via the
tmStateReference parameter. How and if this is mapped to a
particular transport or channel is the responsibility of the
Transport Subsystem.
2.3. Coexistence
In the RFC 3411 architecture, a Message Processing Model determines
which Security Model SHALL be called. As of this writing, IANA has
registered four Message Processing Models (SNMPv1, SNMPv2c, SNMPv2u/
SNMPv2*, and SNMPv3) and three other Security Models (SNMPv1,
SNMPv2c, and the User-based Security Model).
2.3.1. Coexistence with Message Processing Models
The SNMPv1 and SNMPv2c message processing described in BCP 74
[RFC3584] always selects the SNMPv1(1) and SNMPv2c(2) Security
Models. Since there is no mechanism defined in RFC 3584 to select an
alternative Security Model, SNMPv1 and SNMPv2c messages cannot use
the Transport Security Model. Messages might still be able to be
conveyed over a secure transport protocol, but the Transport Security
Model will not be invoked.
The SNMPv2u/SNMPv2* Message Processing Model is an historic artifact
for which there is no existing IETF specification.
The SNMPv3 message processing defined in [RFC3412] extracts the
securityModel from the msgSecurityModel field of an incoming
SNMPv3Message. When this value is transportSecurityModel(4),
security processing is directed to the Transport Security Model. For
an outgoing message to be secured using the Transport Security Model,
the application MUST specify a securityModel parameter value of
transportSecurityModel(4) in the sendPdu Abstract Service Interface
(ASI).
2.3.2. Coexistence with Other Security Models
The Transport Security Model uses its own MIB module for processing
to maintain independence from other Security Models. This allows the
Transport Security Model to coexist with other Security Models, such
as the User-based Security Model (USM) [RFC3414].
2.3.3. Coexistence with Transport Models
The Transport Security Model (TSM) MAY work with multiple Transport
Models, but the RFC 3411 Abstract Service Interfaces (ASIs) do not
carry a value for the Transport Model. The MIB module defined in
this memo allows an administrator to configure whether or not TSM
prepends a Transport Model prefix to the securityName. This will
allow SNMP applications to consider Transport Model as a factor when
making decisions, such as access control, notification generation,
and proxy forwarding.
To have SNMP properly utilize the security services coordinated by
the Transport Security Model, this Security Model MUST only be used
with Transport Models that know how to process a tmStateReference,
such as the Secure Shell Transport Model [RFC5592].
3. Cached Information and References
When performing SNMP processing, there are two levels of state
information that might need to be retained: the immediate state
linking a request-response pair and a potentially longer-term state
relating to transport and security. "Transport Subsystem for the
Simple Network Management Protocol (SNMP)" [RFC5590] defines general
requirements for caches and references.
This document defines additional cache requirements related to the
Transport Security Model.
3.1. Transport Security Model Cached Information
The Transport Security Model has specific responsibilities regarding
the cached information.
3.1.1. securityStateReference
The Transport Security Model adds the tmStateReference received from
the processIncomingMsg ASI to the securityStateReference. This
tmStateReference can then be retrieved during the generateResponseMsg
ASI so that it can be passed back to the Transport Model.
3.1.2. tmStateReference
For outgoing messages, the Transport Security Model uses parameters
provided by the SNMP application to look up or create a
tmStateReference.
For the Transport Security Model, the security parameters used for a
response MUST be the same as those used for the corresponding
request. This Security Model uses the tmStateReference stored as
part of the securityStateReference when appropriate. For responses
and reports, this Security Model sets the tmSameSecurity flag to true
in the tmStateReference before passing it to a Transport Model.
For incoming messages, the Transport Security Model uses parameters
provided in the tmStateReference cache to establish a securityName,
and to verify adequate security levels.
3.1.3. Prefixes and securityNames
The SNMP-VIEW-BASED-ACM-MIB module [RFC3415], the SNMP-TARGET-MIB
module [RFC3413], and other MIB modules contain objects to configure
security parameters for use by applications such as access control,
notification generation, and proxy forwarding.
Transport domains and their corresponding prefixes are coordinated
via the IANA registry "SNMP Transport Domains".
If snmpTsmConfigurationUsePrefix is set to true, then all
securityNames provided by, or provided to, the Transport Security
Model MUST include a valid transport domain prefix.
If snmpTsmConfigurationUsePrefix is set to false, then all
securityNames provided by, or provided to, the Transport Security
Model MUST NOT include a transport domain prefix.
The tmSecurityName in the tmStateReference stored as part of the
securityStateReference does not contain a prefix.
4. Processing an Outgoing Message
An error indication might return an Object Identifier (OID) and value
for an incremented counter, a value for securityLevel, values for
contextEngineID and contextName for the counter, and the
securityStateReference, if this information is available at the point
where the error is detected.
4.1. Security Processing for an Outgoing Message
This section describes the procedure followed by the Transport
Security Model.
The parameters needed for generating a message are supplied to the
Security Model by the Message Processing Model via the
generateRequestMsg() or the generateResponseMsg() ASI. The Transport
Subsystem architectural extension has added the transportDomain,
transportAddress, and tmStateReference parameters to the original RFC
3411 ASIs.
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- (NEW) specified by application
IN transportAddress -- (NEW) specified by application
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) transport info
)
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- (NEW) specified by application
IN transportAddress -- (NEW) specified by application
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
IN securityStateReference -- reference to security state
-- information from original
-- request
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) transport info
)
4.2. Elements of Procedure for Outgoing Messages
1. If there is a securityStateReference (Response or Report
message), then this Security Model uses the cached information
rather than the information provided by the ASI. Extract the
tmStateReference from the securityStateReference cache. Set the
tmRequestedSecurityLevel to the value of the extracted
tmTransportSecurityLevel. Set the tmSameSecurity parameter in
the tmStateReference cache to true. The cachedSecurityData for
this message can now be discarded.
2. If there is no securityStateReference (e.g., a Request-type or
Notification message), then create a tmStateReference cache. Set
tmTransportDomain to the value of transportDomain,
tmTransportAddress to the value of transportAddress, and
tmRequestedSecurityLevel to the value of securityLevel.
(Implementers might optimize by pointing to saved copies of these
session-specific values.) Set the transaction-specific
tmSameSecurity parameter to false.
If the snmpTsmConfigurationUsePrefix object is set to false, then
set tmSecurityName to the value of securityName.
If the snmpTsmConfigurationUsePrefix object is set to true, then
use the transportDomain to look up the corresponding prefix.
(Since the securityStateReference stores the tmStateReference
with the tmSecurityName for the incoming message, and since
tmSecurityName never has a prefix, the prefix-stripping step only
occurs when we are not using the securityStateReference).
If the prefix lookup fails for any reason, then the
snmpTsmUnknownPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
If the lookup succeeds, but there is no prefix in the
securityName, or the prefix returned does not match the prefix
in the securityName, or the length of the prefix is less than
1 or greater than 4 US-ASCII alpha-numeric characters, then
the snmpTsmInvalidPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
Strip the transport-specific prefix and trailing ':' character
(US-ASCII 0x3a) from the securityName. Set tmSecurityName to
the value of securityName.
3. Set securityParameters to a zero-length OCTET STRING ('0400').
4. Combine the message parts into a wholeMsg and calculate
wholeMsgLength.
5. The wholeMsg, wholeMsgLength, securityParameters, and
tmStateReference are returned to the calling Message Processing
Model with the statusInformation set to success.
5. Processing an Incoming SNMP Message
An error indication might return an OID and value for an incremented
counter, a value for securityLevel, values for contextEngineID and
contextName for the counter, and the securityStateReference, if this
information is available at the point where the error is detected.
5.1. Security Processing for an Incoming Message
This section describes the procedure followed by the Transport
Security Model whenever it receives an incoming message from a
Message Processing Model. The ASI from a Message Processing Model to
the Security Subsystem for a received message is:
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- from the received message
IN securityParameters -- from the received message
IN securityModel -- from the received message
IN securityLevel -- from the received message
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
IN tmStateReference -- (NEW) from the Transport Model
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
) -- information, needed for response
5.2. Elements of Procedure for Incoming Messages
1. Set the securityEngineID to the local snmpEngineID.
2. If tmStateReference does not refer to a cache containing values
for tmTransportDomain, tmTransportAddress, tmSecurityName, and
tmTransportSecurityLevel, then the snmpTsmInvalidCaches counter
is incremented, an error indication is returned to the calling
module, and Security Model processing stops for this message.
3. Copy the tmSecurityName to securityName.
If the snmpTsmConfigurationUsePrefix object is set to true, then
use the tmTransportDomain to look up the corresponding prefix.
If the prefix lookup fails for any reason, then the
snmpTsmUnknownPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
If the lookup succeeds but the prefix length is less than 1 or
greater than 4 octets, then the snmpTsmInvalidPrefixes counter
is incremented, an error indication is returned to the calling
module, and message processing stops.
Set the securityName to be the concatenation of the prefix, a
':' character (US-ASCII 0x3a), and the tmSecurityName.
4. Compare the value of tmTransportSecurityLevel in the
tmStateReference cache to the value of the securityLevel
parameter passed in the processIncomingMsg ASI. If securityLevel
specifies privacy (Priv) and tmTransportSecurityLevel specifies
no privacy (noPriv), or if securityLevel specifies authentication
(auth) and tmTransportSecurityLevel specifies no authentication
(noAuth) was provided by the Transport Model, then the
snmpTsmInadequateSecurityLevels counter is incremented, an error
indication (unsupportedSecurityLevel) together with the OID and
value of the incremented counter is returned to the calling
module, and Transport Security Model processing stops for this
message.
5. The tmStateReference is cached as cachedSecurityData so that a
possible response to this message will use the same security
parameters. Then securityStateReference is set for subsequent
references to this cached data.
6. The scopedPDU component is extracted from the wholeMsg.
7. The maxSizeResponseScopedPDU is calculated. This is the maximum
size allowed for a scopedPDU for a possible Response message.
8. The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified
in the processIncomingMsg ASI.
6. MIB Module Overview
This MIB module provides objects for use only by the Transport
Security Model. It defines a configuration scalar and related error
counters.
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.
6.1.1. The snmpTsmStats Subtree
This subtree contains error counters specific to the Transport
Security Model.
6.1.2. The snmpTsmConfiguration Subtree
This subtree contains a configuration object that enables
administrators to specify if they want a transport domain prefix
prepended to securityNames for use by applications.
6.2. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing the Transport Security Model. In
particular, it is assumed that an entity implementing the Transport
Security Model will implement the SNMP-FRAMEWORK-MIB [RFC3411], the
SNMP-TARGET-MIB [RFC3413], the SNMP-VIEW-BASED-ACM-MIB [RFC3415], and
the SNMPv2-MIB [RFC3418]. These are not needed to implement the
SNMP-TSM-MIB.
6.2.1. MIB Modules Required for IMPORTS
The following MIB module imports items from [RFC2578], [RFC2579], and
[RFC2580].
7. MIB Module Definition
SNMP-TSM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
mib-2, Counter32
FROM SNMPv2-SMI -- RFC2578
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF -- RFC2580
TruthValue
FROM SNMPv2-TC -- RFC2579
;
snmpTsmMIB MODULE-IDENTITY
LAST-UPDATED "200906090000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
+49 6221 90511-15
quittek@netlab.nec.de
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@jacobs-university.de
Editor:
David Harrington
Huawei Technologies USA
1700 Alma Dr.
Plano TX 75075
USA
+1 603-436-8634
ietfdbh@comcast.net
Wes Hardaker
Cobham Analytic Solutions
P.O. Box 382
Davis, CA 95617
USA
+1 530 792 1913
ietf@hardakers.net
"
DESCRIPTION
"The Transport Security Model MIB.
In keeping with the RFC 3411 design decisions to use
self-contained documents, the RFC that contains the definition
of this MIB module also includes the elements of procedure
that are needed for processing the Transport Security Model
for SNMP. These MIB objects SHOULD NOT be modified via other
subsystems or models defined in other documents. This allows
the Transport Security Model for SNMP to be designed and
documented as independent and self-contained, having no direct
impact on other modules, and this allows this module to be
upgraded and supplemented as the need arises, and to move
along the standards track on different time-lines from other
modules.
Copyright (c) 2009 IETF Trust and the persons
identified as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, are permitted provided that the
following conditions are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials
provided with the distribution.
- Neither the name of Internet Society, IETF or IETF Trust,
nor the names of specific contributors, may be used to endorse
or promote products derived from this software without
specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
This version of this MIB module is part of RFC 5591;
see the RFC itself for full legal notices."
REVISION "200906090000Z"
DESCRIPTION "The initial version, published in RFC 5591."
::= { mib-2 190 }
-- ---------------------------------------------------------- --
-- subtrees in the SNMP-TSM-MIB
-- ---------------------------------------------------------- --
snmpTsmNotifications OBJECT IDENTIFIER ::= { snmpTsmMIB 0 }
snmpTsmMIBObjects OBJECT IDENTIFIER ::= { snmpTsmMIB 1 }
snmpTsmConformance OBJECT IDENTIFIER ::= { snmpTsmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
-- Statistics for the Transport Security Model
snmpTsmStats OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 1 }
snmpTsmInvalidCaches OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of incoming messages dropped because the
tmStateReference referred to an invalid cache.
"
::= { snmpTsmStats 1 }
snmpTsmInadequateSecurityLevels OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of incoming messages dropped because
the securityLevel asserted by the Transport Model was
less than the securityLevel requested by the
application.
"
::= { snmpTsmStats 2 }
snmpTsmUnknownPrefixes OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of messages dropped because
snmpTsmConfigurationUsePrefix was set to true and
there is no known prefix for the specified transport
domain.
"
::= { snmpTsmStats 3 }
snmpTsmInvalidPrefixes OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of messages dropped because
the securityName associated with an outgoing message
did not contain a valid transport domain prefix.
"
::= { snmpTsmStats 4 }
-- -------------------------------------------------------------
-- Configuration
-- -------------------------------------------------------------
-- Configuration for the Transport Security Model
snmpTsmConfiguration OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 2 }
snmpTsmConfigurationUsePrefix OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRIPTION "If this object is set to true, then securityNames
passing to and from the application are expected to
contain a transport-domain-specific prefix. If this
object is set to true, then a domain-specific prefix
will be added by the TSM to the securityName for
incoming messages and removed from the securityName
when processing outgoing messages. Transport domains
and prefixes are maintained in a registry by IANA.
This object SHOULD persist across system reboots.
"
DEFVAL { false }
::= { snmpTsmConfiguration 1 }
-- -------------------------------------------------------------
-- snmpTsmMIB - Conformance Information
-- -------------------------------------------------------------
snmpTsmCompliances OBJECT IDENTIFIER ::= { snmpTsmConformance 1 }
snmpTsmGroups OBJECT IDENTIFIER ::= { snmpTsmConformance 2 }
-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------
snmpTsmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines that support
the SNMP-TSM-MIB.
"
MODULE
MANDATORY-GROUPS { snmpTsmGroup }
::= { snmpTsmCompliances 1 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
snmpTsmGroup OBJECT-GROUP
OBJECTS {
snmpTsmInvalidCaches,
snmpTsmInadequateSecurityLevels,
snmpTsmUnknownPrefixes,
snmpTsmInvalidPrefixes,
snmpTsmConfigurationUsePrefix
}
STATUS current
DESCRIPTION "A collection of objects for maintaining
information of an SNMP engine that implements
the SNMP Transport Security Model.
"
::= { snmpTsmGroups 2 }
END
8. Security Considerations
This document describes a Security Model, compatible with the RFC
3411 architecture, that permits SNMP to utilize security services
provided through an SNMP Transport Model. The Transport Security
Model relies on Transport Models for mutual authentication, binding
of keys, confidentiality, and integrity.
The Transport Security Model relies on secure Transport Models to
provide an authenticated principal identifier and an assertion of
whether authentication and privacy are used during transport. This
Security Model SHOULD always be used with Transport Models that
provide adequate security, but "adequate security" is a configuration
and/or run-time decision of the operator or management application.
The security threats and how these threats are mitigated should be
covered in detail in the specifications of the Transport Models and
the underlying secure transports.
An authenticated principal identifier (securityName) is used in SNMP
applications for purposes such as access control, notification
generation, and proxy forwarding. This Security Model supports
multiple Transport Models. Operators might judge some transports to
be more secure than others, so this Security Model can be configured
to prepend a prefix to the securityName to indicate the Transport
Model used to authenticate the principal. Operators can use the
prefixed securityName when making application decisions about levels
of access.
8.1. MIB Module Security
There are a number of management objects defined in this MIB module
with a MAX-ACCESS clause of read-write and/or read-create. Such
objects may be considered sensitive or vulnerable in some network
environments. The support for SET operations in a non-secure
environment without proper protection can have a negative effect on
network operations. These are the tables and objects and their
sensitivity/vulnerability:
o The snmpTsmConfigurationUsePrefix object could be modified,
creating a denial of service or authorizing SNMP messages that
would not have previously been authorized by an Access Control
Model (e.g., the View-based Access Control Model (VACM)).
Some of the readable objects in this MIB module (i.e., objects with a
MAX-ACCESS other than not-accessible) may be considered sensitive or
vulnerable in some network environments. It is thus important to
control even GET and/or NOTIFY access to these objects and possibly
to even encrypt the values of these objects when sending them over
the network via SNMP. These are the tables and objects and their
sensitivity/vulnerability:
o All the counters in this module refer to configuration errors and
do not expose sensitive information.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPsec),
even then, there is no control as to who on the secure network is
allowed to access and GET/SET (read/change/create/delete) the objects
in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410], section 8),
including full support for the USM and Transport Security Model
cryptographic mechanisms (for authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
9. IANA Considerations
IANA has assigned:
1. An SMI number (190) with a prefix of mib-2 in the MIB module
registry for the MIB module in this document.
2. A value (4) to identify the Transport Security Model, in the
Security Models registry of the SNMP Number Spaces registry.
This results in the following table of values:
Value Description References
----- ----------- ----------
0 reserved for 'any' [RFC3411]
1 reserved for SNMPv1 [RFC3411]
2 reserved for SNMPv2c [RFC3411]
3 User-Based Security Model (USM) [RFC3411]
4 Transport Security Model (TSM) [RFC5591]
10. Acknowledgments
The editors would like to thank Jeffrey Hutzelman for sharing his SSH
insights and Dave Shield for an outstanding job wordsmithing the
existing document to improve organization and clarity.
Additionally, helpful document reviews were received from Juergen
Schoenwaelder.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
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.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)",
RFC 5590, June 2009.
11.2. Informative References
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[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.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
Appendix A. Notification Tables Configuration
The SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are used to
configure notification originators with the destinations to which
notifications should be sent.
Most of the configuration is Security-Model-independent and
Transport-Model-independent.
The values we will use in the examples for the five model-independent
security and transport parameters are:
transportDomain = snmpSSHDomain
transportAddress = 192.0.2.1:5162
securityModel = Transport Security Model
securityName = alice
securityLevel = authPriv
The following example will configure the notification originator to
send informs to a notification receiver at 192.0.2.1:5162 using the
securityName "alice". "alice" is the name for the recipient from the
standpoint of the notification originator and is used for processing
access controls before sending a notification.
The columns marked with an "*" are the items that are Security-Model-
specific or Transport-Model-specific.
The configuration for the "alice" settings in the SNMP-VIEW-BASED-
ACM-MIB objects are not shown here for brevity. First, we configure
which type of notification will be sent for this taglist (toCRTag).
In this example, we choose to send an Inform.
snmpNotifyTable row:
snmpNotifyName CRNotif
snmpNotifyTag toCRTag
snmpNotifyType inform
snmpNotifyStorageType nonVolatile
snmpNotifyColumnStatus createAndGo
Then we configure a transport address to which notifications
associated with this taglist will be sent, and we specify which
snmpTargetParamsEntry will be used (toCR) when sending to this
transport address.
snmpTargetAddrTable row:
snmpTargetAddrName toCRAddr
* snmpTargetAddrTDomain snmpSSHDomain
* snmpTargetAddrTAddress 192.0.2.1:5162
snmpTargetAddrTimeout 1500
snmpTargetAddrRetryCount 3
snmpTargetAddrTagList toCRTag
snmpTargetAddrParams toCR (MUST match below)
snmpTargetAddrStorageType nonVolatile
snmpTargetAddrColumnStatus createAndGo
Then we configure which principal at the host will receive the
notifications associated with this taglist. Here, we choose "alice",
who uses the Transport Security Model.
snmpTargetParamsTable row:
snmpTargetParamsName toCR
snmpTargetParamsMPModel SNMPv3
* snmpTargetParamsSecurityModel TransportSecurityModel
snmpTargetParamsSecurityName "alice"
snmpTargetParamsSecurityLevel authPriv
snmpTargetParamsStorageType nonVolatile
snmpTargetParamsRowStatus createAndGo
A.1. Transport Security Model Processing for Notifications
The Transport Security Model is called using the generateRequestMsg()
ASI, with the following parameters (those with an * are from the
above tables):
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- *snmpTargetParamsMPModel
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- *snmpTargetAddrTDomain
IN transportAddress -- *snmpTargetAddrTAddress
IN securityModel -- *snmpTargetParamsSecurityModel
IN securityEngineID -- immaterial; TSM will ignore.
IN securityName -- snmpTargetParamsSecurityName
IN securityLevel -- *snmpTargetParamsSecurityLevel
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- reference to transport info
)
The Transport Security Model will determine the Transport Model based
on the snmpTargetAddrTDomain. The selected Transport Model will
select the appropriate transport connection using the
tmStateReference cache created from the values of
snmpTargetAddrTAddress, snmpTargetParamsSecurityName, and
snmpTargetParamsSecurityLevel.
Appendix B. Processing Differences between USM and Secure Transport
USM and secure transports differ in the processing order and
responsibilities within the RFC 3411 architecture. While the steps
are the same, they occur in a different order and might be done by
different subsystems. The following lists illustrate the difference
in the flow and the responsibility for different processing steps for
incoming messages when using USM and when using a secure transport.
(These lists are simplified for illustrative purposes, and do not
represent all details of processing. Transport Models MUST provide
the detailed elements of procedure.)
With USM, SNMPv1, and SNMPv2c Security Models, security processing
starts when the Message Processing Model decodes portions of the
ASN.1 message to extract header fields that are used to determine
which Security Model will process the message to perform
authentication, decryption, timeliness checking, integrity checking,
and translation of parameters to model-independent parameters. By
comparison, a secure transport performs those security functions on
the message, before the ASN.1 is decoded.
Step 6 cannot occur until after decryption occurs. Steps 6 and
beyond are the same for USM and a secure transport.
B.1. USM and the RFC 3411 Architecture
1) Decode the ASN.1 header (Message Processing Model).
2) Determine the SNMP Security Model and parameters (Message
Processing Model).
3) Verify securityLevel (Security Model).
4) Translate parameters to model-independent parameters (Security
Model).
5) Authenticate the principal, check message integrity and
timeliness, and decrypt the message (Security Model).
6) Determine the pduType in the decrypted portions (Message
Processing Model).
7) Pass on the decrypted portions with model-independent parameters.
B.2. Transport Subsystem and the RFC 3411 Architecture
1) Authenticate the principal, check integrity and timeliness of the
message, and decrypt the message (Transport Model).
2) Translate parameters to model-independent parameters (Transport
Model).
3) Decode the ASN.1 header (Message Processing Model).
4) Determine the SNMP Security Model and parameters (Message
Processing Model).
5) Verify securityLevel (Security Model).
6) Determine the pduType in the decrypted portions (Message
Processing Model).
7) Pass on the decrypted portions with model-independent security
parameters.
If a message is secured using a secure transport layer, then the
Transport Model will provide the translation from the authenticated
identity (e.g., an SSH user name) to a human-friendly identifier
(tmSecurityName) in step 2. The Security Model will provide a
mapping from that identifier to a model-independent securityName.
Authors' Addresses
David Harrington
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: ietfdbh@comcast.net
Wes Hardaker
Cobham Analytic Solutions
P.O. Box 382
Davis, CA 95617
US
Phone: +1 530 792 1913
EMail: ietf@hardakers.net
AI ELI5
On this page
PROBLEMSNMP needed a security model that works with secure transports such as SSH, reusing existing security infrastructure, without changing the SNMP architecture or the underlying security protocol.
MECHANISMThe Transport Security Model (TSM, security model number 4) reads the authenticated identity and security level from the tmStateReference cache, builds the securityName, checks the level, and defines a small MIB module with a prefix switch and four error counters.
WHY IT MATTERSSNMPv3 messages can be secured by the transport while SNMP applications still see a normal securityName and securityLevel. The RFC is an Internet Standard and TSM works only with Transport Models that understand tmStateReference.
§1Introduction and Constraints
RFC 5591 describes a Transport Security Model (TSM) for SNMP, for use with secure Transport Models in the Transport Subsystem defined in RFC 5590. It also defines a portion of the Management Information Base (MIB) for monitoring and managing the model. The aim is to use existing, commonly deployed security infrastructures and to minimize implementation and deployment cost.
The design follows five constraints. In times of network stress, the security protocol SHOULD NOT depend solely on services such as NTP or AAA. When the network is not under stress it MAY depend on them. It might not be possible to tell when the network is stressed. A Security Model SHOULD NOT require changes to the SNMP architecture, and SHOULD NOT require changes to the underlying security protocol.
This is a rulebook for a new kind of name-checker at the SNMP mail room. It has to work with locks that already exist, and keep working when the network is having a bad day.
§2How TSM Fits the Architecture
TSM sits in the Security Subsystem and uses a secure Transport Model. For incoming messages the Transport Model passes a tmStateReference cache. TSM creates the securityName and securityLevel for applications and checks that the tmTransportSecurityLevel reported by the transport is at least as strong as the securityLevel requested by the Message Processing Model. For outgoing messages TSM creates (or reuses) a tmStateReference and passes it to the Transport Model.
TSM provides no authentication or encryption of its own. Which threats are covered depends on the Transport Model, and it trusts that the transport was configured to be at least as strong as it reports. It cannot test those claims. The three levels it deals with are noAuthNoPriv, authNoPriv and authPriv.
On coexistence, SNMPv1 and SNMPv2c message processing always selects the SNMPv1 and SNMPv2c Security Models, so those messages cannot use TSM. For SNMPv3, a msgSecurityModel value of transportSecurityModel(4) directs processing to TSM, and an application sending a message MUST specify that value. TSM keeps its own MIB module, so it can coexist with USM, and it MUST only be used with Transport Models that know how to process a tmStateReference, such as the Secure Shell Transport Model of RFC 5592.
The name-checker does not make locks. It reads the name tag and lock rating the tunnel reports, and trusts that the tunnel was set up properly.
§4Processing Outgoing Messages
When TSM builds an outgoing message there are two cases. For a Response or Report, a securityStateReference exists, and TSM extracts the cached tmStateReference. It sets tmRequestedSecurityLevel to the cached tmTransportSecurityLevel and sets tmSameSecurity to true, so the response uses the same security as the request.
For a request or notification there is no such reference, so TSM creates a cache with the transport domain, transport address and requested level, and sets tmSameSecurity to false. If snmpTsmConfigurationUsePrefix is false, tmSecurityName is the securityName. If it is true, TSM looks up the prefix for the transport domain. A failed lookup increments snmpTsmUnknownPrefixes. A missing, mismatched or wrong-length prefix (not 1 to 4 characters) increments snmpTsmInvalidPrefixes. In both cases processing stops. Otherwise the prefix and the trailing : are stripped to get tmSecurityName.
Finally securityParameters is set to a zero-length OCTET STRING (0400), the message parts are combined, and the message and cache reference are returned with success.
When sending, the checker writes the sticky note: who the message is for and how locked the truck must be. For a reply, it copies the note from the question, so the answer goes back the same safe way.
§5Processing Incoming Messages
For an incoming message TSM sets securityEngineID to the local snmpEngineID. If the tmStateReference does not point to a cache with a transport domain, transport address, tmSecurityName and tmTransportSecurityLevel, it increments snmpTsmInvalidCaches, returns an error and stops. This is what happens if TSM is used with a non-secure Transport Model.
TSM copies tmSecurityName to securityName. If prefixes are enabled, it looks up the prefix for the transport domain (errors increment the unknown or invalid prefix counters) and sets securityName to the prefix, a : and the tmSecurityName.
Next it compares levels. If the requested securityLevel needs privacy but the transport gave none, or needs authentication but the transport gave none, snmpTsmInadequateSecurityLevels is incremented and an unsupportedSecurityLevel error is returned. Otherwise the cache is kept as cachedSecurityData for the response, the scopedPDU is extracted from the message, maxSizeResponseScopedPDU is calculated, and success is returned.
When a message arrives, the checker reads the name tag, adds the truck-type nickname if the rules say so, and checks the lock is at least as strong as needed. If not, it drops the message and adds a tally mark.
§6The SNMP-TSM-MIB Module
The MIB module is registered at mib-2 190 and is for use only by TSM. It has two subtrees. snmpTsmStats holds four read-only Counter32 error counters: snmpTsmInvalidCaches, snmpTsmInadequateSecurityLevels, snmpTsmUnknownPrefixes and snmpTsmInvalidPrefixes.
snmpTsmConfiguration holds one read-write object, snmpTsmConfigurationUsePrefix. When true, securityNames going to and from the application carry a transport-domain prefix, added for incoming messages and removed for outgoing ones. It defaults to false and SHOULD persist across reboots. The module follows the RFC 3411 habit of self-contained documents, so other documents SHOULD NOT reference these objects.
An entity implementing TSM is assumed to implement the SNMP-FRAMEWORK-MIB, SNMP-TARGET-MIB, SNMP-VIEW-BASED-ACM-MIB and SNMPv2-MIB, though these are not needed to implement the SNMP-TSM-MIB.
The MIB is a small dashboard. It has one switch for the nickname feature and four tally counters for the kinds of mistakes the checker can spot.
- 1snmpTsmInvalidCaches — Incoming messages dropped because tmStateReference referred to an invalid cache.
- 2snmpTsmInadequateSecurityLevels — Incoming messages dropped because the transport's securityLevel was less than requested.
- 3snmpTsmUnknownPrefixes — Messages dropped because prefixes were enabled but none is known for the transport domain.
- 4snmpTsmInvalidPrefixes — Messages dropped because an outgoing securityName lacked a valid prefix.
- configsnmpTsmConfigurationUsePrefix — Read-write switch for transport-domain prefixes; defaults to false.
§8Security Considerations
TSM relies on Transport Models for mutual authentication, binding of keys, confidentiality and integrity. It SHOULD always be used with Transport Models that provide adequate security, but what counts as adequate is a configuration or run-time decision by the operator or management application.
Because Transport Models differ, TSM can be configured to prepend a prefix to the securityName so access decisions can take the authenticating Transport Model into account. For the MIB, the one sensitive object is snmpTsmConfigurationUsePrefix. Changing it could create a denial of service or authorize messages that an access control model such as VACM would not have authorized before. The counters refer to configuration errors and do not expose sensitive information.
The RFC recommends SNMPv3 with cryptographic security and says deploying versions prior to SNMPv3 is NOT RECOMMENDED.
The checker is only as good as the tunnel it trusts. The nickname switch is sensitive, because flipping it could let in people who were not allowed before.
§7Notification Example and USM Comparison
Appendix A configures a notification originator to send informs to 192.0.2.1:5162 over snmpSSHDomain using the securityName "alice", the Transport Security Model and authPriv. The Transport Security Model finds the Transport Model from snmpTargetAddrTDomain, and that model selects a connection using the tmStateReference built from the address, security name and level.
Appendix B contrasts the order of work. With USM, the Message Processing Model decodes the ASN.1 header first and the Security Model then authenticates, checks integrity and timeliness, and decrypts. With a secure transport, the Transport Model does the authentication, integrity, timeliness and decryption first and translates the identity (for example an SSH user name) into a tmSecurityName, and only then is the ASN.1 header decoded. The Security Model maps that identifier to a securityName.
With the old way the mail room opens the envelope to see who it is from. With the new way the armored truck checks the sender at the door before the envelope is opened.
| Step | USM | Secure transport |
|---|---|---|
| Authenticate, check integrity and timeliness, decrypt | Security Model, after the header is decoded | Transport Model, before the ASN.1 is decoded |
| Translate to model-independent parameters | Security Model | Transport Model gives tmSecurityName; Security Model maps it to securityName |
| Decode the ASN.1 header THIS RFC | Message Processing Model, first | Message Processing Model, after the transport's work |
§8References
[RFC3411]RFC 3411Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, December 2002.December 2002[RFC3412]RFC 3412Case, J., Harrington, D., Presuhn, R., and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3412, December 2002.December 2002[RFC3414]RFC 3414Blumenthal, 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.December 2002[RFC5590]RFC 5590Harrington, D. and J. Schoenwaelder, "Transport Subsystem for the Simple Network Management Protocol (SNMP)", RFC 5590, June 2009.June 2009[RFC5592]RFC 5592Harrington, D., Salowey, J., and W. Hardaker, "Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)", RFC 5592, June 2009.June 2009