Network Working Group D. Harrington
Internet-Draft Independent
Expires: April 1, 2005 J. Schoenwaelder
International University Bremen
October 2004
Transport Mapping Security Model (TMSM) for the Simple Network
Management Protocol version 3 (SNMPv3)
draft-schoenw-snmp-tlsm-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a Transport Mapping Security Model (TMSM) for
the Simple Network Management Protocol (SNMP) architecture defined in
RFC3411. At this stage, this document does not provide a complete
solution - it rather identifies and discusses some key aspects that
need discussion and future work.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements of a Transport Mapping Security Model . . . . . . 4
3.1 Security Requirements . . . . . . . . . . . . . . . . . . 4
3.2 Architectural Modularity Requirements . . . . . . . . . . 5
3.3 Passing messages between Dispatchers . .
. . . . . . . . . 5
3.4 Security Parameter Passing Requirement . . . . . . . . . . 6
3.4.1 Using an ASI . . . . . . . . . . . . . . . . . . . . . 6
3.4.2 Using a cache . . . . . . . . . . . . . . . . . . . . 6
3.4.3 Using an encapsulating header . . . . . . . . . . . . 7
3.4.4 Using existing fields in a message . . . . . . . . . . 7
3.5 Access Control Requirements . . . . . . . . . . . . . . . 8
3.5.1 Architectural securityName Binding Requirement . . . . 8
4. Fields in the SNMPv3 message . . . . . . . . . . . . . . . . . 8
4.1 msgVersion . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 msgGlobalData . . . . . . . . . . . . . . . . . . . . . . 9
4.3 securityLevel and msgFlags . . . . . . . . . . . . . . . . 9
4.4 The tmStateReference for Passing Security Parameters . . . 11
4.5 securityStateReference Cached Security Data . . . . . . . 11
4.5.1 Prepare an Outgoing SNMP Message . . . . . . . . . . . 12
4.5.2 Prepare Data Elements from an Incoming SNMP Message . 12
4.6 Notifications . . . . . . . . . . . . . . . . . . . . . . 13
5. Transport Mapping Security Model Samples . . . . . . . . . . . 13
5.1 TLS/TCP Transport Mapping Security Model . . . . . . . . . 13
5.1.1 tmStateReference for TLS . . . . . . . . . . . . . . . 13
5.1.2 MP portion for TLS TM-Security Model . . . . . . . . . 14
5.1.3 MIB Module for TLS Security . . . . . . . . . . . . . 14
5.2 DTLS/UDP Transport Mapping Security Model . . . . . . . . 14
5.2.1 tmStateReference for DTLS . . . . . . . . . . . . . . 15
5.3 SASL Transport Mapping Security Model . . . . . . . . . . 16
5.3.1 tmStateReference for SASL DIGEST-MD5 . . . . . . . . 16
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 17
7.2 Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
A. Message security versus session security . . . . . . . . . . . 19
A.1 msgFlags versus actual security . . . . . . . . . . . . . 19
A.2 Message security versus session security . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . 21
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1. Introduction
There are multiple ways to secure one's home or business, but they
largely boil down to a continuum of alternatives. Let's consider
three general approaches. In the first approach, an individual/
company could buy a gun, learn to use it, and sit on your front porch
waiting for intruders. In the second approach, one could hire an
employee with a gun, schedule the employee, position the employee to
guard what you want protected, hire a second guard to cover if the
first gets sick, and so on. In the third approach, you could hire a
security company, tell them what you want protected, and they could
hire employees, train them, buy the guns, position the guards,
schedule the guards, send a replacement when a guard cannot make it,
etc., thus providing the security you want, with no significant
effort on your part other than identifying requirements and verifying
the quality of the service being provided.
The User-based Security Model (USM) as defined in [RFC3414] largely
uses the first approach - it provides its own security. It utilizes
existing mechanisms (MD5=the gun), but provides all the coordination.
USM provides for the authentication of a principal, message
encryption, data integrity checking, timeliness checking, etc.
USM was designed to be independent of other existing security
infrastructures. USM therefore requires a separate user and key
management infrastructure. Operators have reported that deploying
another user and key management infrastructure in order to use SNMPv3
is a reason for not deploying SNMPv3 at this point in time. It is
possible but difficult to define external mechanisms that handle the
distribution of keys for use by the USM approach.
A solution based on the second approach might use a USM-compliant
architecture, but replace the authentication mechanism with an
external mechanism, such as RADIUS, to provide the authentication
service. It might be possible to utilize an external protocol to
encrypt a message, to check timeliness, to check data integrity, etc.
It is difficult to cobble together a number of subcontracted services
and coordinate them however, because it is difficult to build solid
security bindings between the various services, and potential for
gaps in the security is significant.
A solution based on the third approach might utilize one or more
lower-layer security mechanisms to provide the message-oriented
security services required. These would include authentication of
the sender, encryption, timeliness checking, and data integrity
checking. There are a number of IETF standards available or in
development to address these problems at lower layers, frequently at
the transport layer. A solution based on this approach might also
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utilize a "transport" that is actually another application operating
at the application layer, such as SSH [SSHauth]
This document proposes a Transport Mapping Security Model (TMSM), as
an extension of the SNMPv3 architecture, that would allow security to
be provided an external protocol connected to the SNMP engine through
an SNMP transport-mapping. Such a TMSM would then enable the use of
existing security mechanisms such as (TLS) [RFC2246], Kerberos
[RFC1510] or SASL [RFC2222] within the SNMPv3 architecture.
As pointed out in the EUSM proposal [EUSM], it is desirable to use
mechanisms that could "unify the approach for administrative security
for SNMPv3 and CLI" and other management interfaces. The use of
security services provided by lower layers or other applications is
the approach commonly used for the CLI, and is the approach being
proposed for NETCONF
This document provides the motivation for leveraging transport layer
security mechanisms for secure SNMP communication, identifies some
key issues and provides some proposals for design choices that may be
made to provide a workable solution that meets operational
requirements and fits into the SNMP architecture defined in [RFC3411]
2. Motivation
There are a number of Internet security protocols and mechanisms that
are in wide spread use. Many of them try to provide a generic
infrastructure to be used by many different application layer
protocols. The motivation behind TMSM is to leverage these
protocols where it seems useful.
There are a number of challenges to be addressed to map the security
provided by a secure transport into the SNMP architecture so that
SNMP continues to work without any surprises. These are discussed in
detail below.
3. Requirements of a Transport Mapping Security Model
3.1 Security Requirements
Transport mapping security protocols SHOULD ideally provide the
protection against the following message-oriented threats [RFC3411]:
1. modification of information
2. masquerade
3. message stream modification
4. disclosure
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According to [RFC3411], it i
s not required to protect against denial
of service or traffic analysis.
3.2 Architectural Modularity Requirements
[RFC3411] section 3 describes a modular architecture to allow the
evolution of the SNMP protocol standards over time. This
architecture includes a Security Subsystem which is responsible for
realizing security services.
Transport mapping security is by its very nature a security layer
which is plugged in between the transport layer and the dispatcher.
Conceptually, transport mapping security models will be called from
within the Transport Mapping portion of an SNMP engine, or will be
positioned between the transport mapping subsystem and the
dispatcher.
The design of a transport mapping security model must abide the goals
of the RFC3411 architecture, section 1. To that end, this transport
mapping security model proposal focuses on a modular subsystem that
can be advanced through the standards process independently of other
proposals, and independent of other subsystems as much as possible.
This subsystem is designed as an architectural extension that permits
different transport mapping security protocols to be "plugged into"
this subsystem, to support supplemental transport mapping security
models in addition to those described here.
IETF standards typically require one mandatory-to-implement solution,
with the capability of adding new security mechanisms in the future.
Any transport mapping security model should define one
minimum-compliance mechanism, preferably one which is already widely
deployed within the transport layer security protocol used.
This architectural extension is illustrated by the following diagram,
which is a modified version of the diagram taken from the SNMP
architecture document.
TODO: Insert drawing here...
3.3 Passing messages between Dispatchers
Typically, with a TMSM model, the transport mapping will establish an
encrypted tunnel between the transport mappings of two SNMP engines,
without passing anything to the SNMP dispatcher. One transport
mapping security model instance encrypts all messages, and the other
transport mapping security model instance decrypts the messages.
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After the transport layer tunnel is established, then SNMP messages
can conceptually be sent through the tunnel from one SNMP engine
dispatcher to another SNMP engine dispatcher. SNMP messages are
passed unencrypted from the source dispatcher to its own TMSM, and
presented unencrypted to the destination SNMP dispatcher.
Once the tunnel is established, multiple SNMP messages may be able to
be passed through the same tunnel.
3.4 Security Parameter Passing Requirement
[RFC3411] section 4 describes primitives to describe the abstract
service interfaces used to conceptually pass information between the
various subsystems, models and applications within the architecture.
A Transport mapping Security Model must pass information between
subsystems as well.
The RFC3411 architecture has no ASI parameters for passing security
information between the transport mapping and the dispatcher, and
between the dispatcher and the message processing model. Since the
TM portion of the security model and the MP portion of the security
model are co-resident within an implementation, it is assumed there
is a trust relationship that exists within the implementation. There
are four approaches that could be used for passing information
between the TM portion of the securitymodel and the MP portion of the
security model :
we could define an ASI to supplement the existing ASIs, or
the TMSM could pass the information in an implementation-specific
cache, or
the TMSM could add a header to encapsulate the SNMP message, or
the TMSM could utilize fields already defined in the existing
SNMPv3 message.
3.4.1 Using an ASI
RFC3411 discusses the purpose, and an explicit non-purpose, of the
ASI approach: "This modularity of specification is not meant to be
interpreted as imposing any specific requirements on implementation."
An ASI is not an API, and following a defined ASI is not required for
interoperability, so implementors are really free to use any method
they choose. However, defining an ASI has the advantage of being
consistent with existing RFC3411/3412 practice.
3.4.2 Using a cache
A cache mechanism could be used, into which the TM portion of the
security model puts information about the security applied to an
incoming message, and an MP portion of the security model extracts
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that information from the cache. The cache is not passed via an
explicit ASI. Given that there may be multiple TM-security caches, a
cache ID probably needs to be passed in the message in the ASI so the
MP portion of the security model knows which cache to consult. This
approach would be consistent with the securityStateReference cache
already being passed around in the ASI.
The cache could be thought of as an additional parameter in the ASI.
The ASI would not need to be changed since the SNMPv3 WG expected
that additional parameters could be passed for value-add features of
specific implementations.
3.4.3 Using an encapsulating header
A header could encapsulate the SNMP message to pass necessary
information from the TM portion of the security model to the
dispatcher and then to the MP portion of the security model. The
message header would be included in the wholeMessage ASI parameter,
and would be removed by a corresponding messaging model. This would
imply a new messaging model would need to be specified as well. The
other approaches may be able to use the standard SNMPv3 messaging
model, with a new MP-security model.
3.4.4 Using existing fields in a message
[RFC3412] describes the SNMPv3 message, which contains fields to pass
security related parameters. The TMSM could use these fields in an
SNMPv3 message, or comparable fields in other message formats to pass
information between transport mapping security models in different
SNMP engines, and to pass information between a TM security model and
the corresponding MP security model.
It is importnat to understand that SNMP messages are ASN.1 encoded,
and the SNMP architecture places no constraints on how the ASN.1 gets
decoded - it might be decoded in one massive decode, or individual
portions of the message, such as individual varbinds, may be decoded
only as needed. This is an implementation decision.
If the fields in an incoming SNMPv3 message are changed by the TM
portion before passing it to the MP portion, then the TM portion will
need to encode its parameters in ASN.1 or the message model would
need to be modified to permit non-encoded data to be added to the
message in a manner that would not impact the existing ASN.1
encoding/decoding of the message. In addition, the MP portion may
not be able to perform a transport-independent message integrity
check, and transport-independent encryption may not be able to be
performed by the MP portion of the model. While it may be desirable
for most TMSM models to perform those services through the TM portion
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of the model, assuming the use of a cache or an encapsulating header
would not impose such constraints on future models.
This document will describe a cache approac
h, but an encapsulating
header or other mechanisms could also be used if preferred for
specific TM security models.
3.5 Access Control Requirements
3.5.1 Architectural securityName Binding Requirement
For SNMP access control to function properly, the security mechanism
must establish a securityName, which is the security model
independent identifier for a principal, a security model identifier,
and a securityLevel. The SNMPv3 message processing architecture
subsystem relies on a message model based security model, such as
USM, to play an role in security that goes beyond protecting the
message - it ties various security models for the same principal to a
security-model independent securityName which can be used for
subsequent processing, such as for access control.
The TMSM assumes two portions to a security model, one tied to the
transport mapping and another tied to the message processing model.
and will be referred to here as a TM-portion and an MP-portion of the
security model. Depending on the specific design of the security
model, different features might be provided by the TM portion or by
the MP portion. For example, the binding of a mechanism-specific
authenticated identity to a securityName might be done by the TM
portion or by the MP portion.
The SNMP architecture distinguishes between messages with no
authentication and no privacy (noAuthNoPriv), authentication without
privacy (authNoPriv) and authentication with privacy (authPriv).
Hence, the authentication of a transport layer identity plays an
important role and must be considered by any transport layer security
mechanism used. However, it is also possible that a second level of
authentication, one provided by a AAA server, for example, may be
used to provide the authentication identity which is bound to the
securityName, if the type of authentication provided by the transport
layer (e.g. host-based or anonymous) is considered adequate to
secure and/or encrypt the message, but inadequate to provide the
desired granularity of access control (e.g. user-based).
4. Fields in the SNMPv3 message
4.1 msgVersion
For proposals that reuse the SNMPv3 message format, this field should
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contain the value 3.
4.2 msgGlobalData
msgID and msgMaxSize are used identically for the TMSM models as for
the USM model.
msgSecurityModel should be set to a value from the SnmpSecurityModel
enumeration [RFC3410] to identify the specific TMSM model.
msgSecurityParameters is used identically for the TMSM models as for
the USM model.
msgFlags have the same values for the TMSM models as for the USM
model. "The authFlag and privFlag fields indicate the securityLevel
that was applied to the message before it was sent on the wire."
4.3 securityLevel and msgFlags
For an outgoing message, msgFlags is the requested security for the
message; if a TMSM cannot provide the requested securityLevel, the
model MUST describe a standard behavior that is followed for that
situation. If the TMSM cannot provide at least the requested level
of security, the TMSM MUST discard the request and SHOULD notify the
message processing model that the request failed. [dbh: how is yet
to be determined, and may be model-specific or
implementation-specific.]
For an outgoing message, if the TMSM is able to provide stronger
than requested security, that may be acceptable. The transport layer
protocol would need to indicate to the receiver what security has
been applied to the actual message. To avoid the need to mess with
the ASN.1 encoding, the SNMPv3 message carries the requested
msgFlags, not the actual securityLevel applied to the message. If a
message format other than SNMPv3 is used, then the new message may
carry the more accurate securityLevel in the SNMP message.
For an incoming message, the receiving TMSM knows what must be done
to process the message based on the transport layer mechanisms. If
the underlying transport security mechanisms for the receiver cannot
provide the matching securityLevel, then the message should follow
the standard behaviors for the transport security mechanism, or be
discarded silently.
Part of the responsibility of the TMSM is to ensure that the actual
security provided by the underlying transport layer security
mechanisms is configured to meet or exceed the securityLevel required
by the msgFlags in the SNMP message. When the MP portion of the
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security model processes the incoming message, it should compare the
msgFlags field to the securityLevel actually provided for the message
by the transport layer security. If they differ, the MP portion of
the security model should determine whether the changed securityLevel
is acceptable. If not, it should discard the message. Depending on
the model, the MP portion may issue a reportPDU with the XXXXXXX
model-specific counter.
Questions about msgFlags:
Is the securityLevel looked at before the security model gets to
it.? No. the security model has two parts - the TM portion and
the MP portion. The securityLevel is looked at by the TM portion
before it gets to the MP piece, but both are parts of the same
security model.
Would it be legal for the security model to ignore the incoming
flags and change them before passing them back up? If it changed
them, it wouldn't necessarily be ignoring them. The TM portion
should pass both an actual securityLevel applied to the message,
and the msgFlags in the SNMP message to the MP piece for
consideration related to access control.. The msgFlags parameter
in the SNMP message is never changed when processing an incoming
message.
Would it be legal for the security model to ignore the outgoing
flags and change them before passing them out? no; because the two
portions are parts of the same security model, either the MP piece
should recognize that a securityLevel cannot be met or exceeded,
and reject the message during the message-build phase, or the TM
piece should determine if it is possible to honor the request. It
is possible to apply an increased securityLevel for an outgoing
request, but the procedure to do so must be spelled out clearly in
the model design.
The security model would need to (MUST) check the incoming
security level flags to make sure they matched the TLS/whatever
session setup and if not drop the message. Yes, mostly.
Depending on the model, either the TM portion or the MP portion
MUST verify that the actual processing met or exceeded the
securityLevel requested by the msgFlags and that it is acceptable
to the specific-model processing (or operator configuration) for
this different securityLevel to be applied to the message. This
is also true (especially) for outgoing messages.
You might legally be able to have a authNoPriv message that is
actually encrypted via the transport (but not the other way around
of course). Yes, a TMSM could define that as the behavior (or
permit an operator to specify that is acceptable behavior) when a
requested securityLevel cannot be provided, but a stronger
securityLevel can be provided.
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See the Appendix A appendix for further discussion of the msgFlags
field ve
rsus the actual securityLevel provided. [dbh: it may be a
good thing to merge the Question and Answer with the appendix, either
here or there.]
4.4 The tmStateReference for Passing Security Parameters
A tmStateReference is used to pass data between the TM portion and
the MP portion of the security model, similar to the
securityStateReference described in RFC3412. This can be envisioned
as being appended to the ASIs between the TM and the MP or as being
passed in an encapsulating header.
The TM portion of the security model may provide only some aspects of
security, and leave some aspects to the MP portion of the model.
tmStateReference should be used to pass any parameters, in a model-
and mechanism-specific format, that will be needed to coordinate the
activities of the TM and MP portions of the model, and the parameters
subsequently passed in securityStateReference . For example, the TM
portion may provide privacy and data integrity and authentication and
authorization policy retrievals, or some subset of these features,
depending on the features available in the transport mechanisms. A
field in tmStateReference should identify which services were
provided for each received message by the TM portion, the
securityLevel applied to the received message, the model-specific
security identity, the session identifier for session based transport
security, and so on.
4.5 securityStateReference Cached Security Data
From RFC3411: "For each message received, the Security Model caches
the state information such that a Response message can be generated
using the same security information, even if the Local Configuration
Datastore is altered between the time of the incoming request and the
outgoing response.
A Message Processing Model has the responsibility for explicitly
releasing the cached data if such data is no longer needed. To
enable this, an abstract securityStateReference data element is
passed from the Security Model to the Message Processing Model. The
cached security data may be implicitly released via the generation of
a response, or explicitly released by using the stateRelease
primitive, as described in section 4.5.1."
To differentiate what information needs to be provided to the MP
portion by the TM portion, and vice-versa, this document will
differentiate the tmStateReference from the securityStateReference.
An implementation MAY use one cache and one reference to serve both
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functions, but an implementor must be aware of the cache-release
issues to prevent the cache from being released before the TM portion
has had an opportunity to extract the information it needs.
4.5.1 Prepare an Outgoing SNMP Message
According to RFC3412, section 7.1, the SNMPv3 message processing
model calls the MP portion of the TM security model using the
generateResponseMsg() or generateRequestMsg(). The MP portion of the
model may need to put information into the tmStateReference cache for
use by the TM portion of the model, such as:
tmSecurityStateReference - the unique identifier for the cached
information
tmTransportDomain
tmTransportAddress
tmSecurityModel - an indicator of which mechanisms to use
tmSecurityName - a model-specific identifier of the security
principal
tmSecurityLevel - an indicator of which security services are
requested
and may contain additional information such as
tmSessionID
tmSessionKey
tmSessionMsgID
4.5.2 Prepare Data Elements from an Incoming SNMP Message
For an incoming message, the TM portion of a model will need to put
information from the transport mechanisms used into the
tmStateReference so the MP portion of the model can extract the
information and add it conceptually to the securityStateReference.
The tmStateReference cache will likely contain at least the following
information:
tmStateReference - a unique identifier for the cached information
tmSecurityStateReference - the unique identifier for the cached
information
tmTransportDomain
tmTransportAddress
tmSecurityModel - an indicator of which mechanisms to use
tmSecurityName - a model-specific identifier of the security
principal
tmSecurityLevel - an indicator of which security services are
requested
tmAuthProtocol
tmPrivProtocol
and may contain additional information such as
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tmSessionID
tmSessionKey
tmSessionMsgID
4.6 Notifications
For notifications, if the cache has been released and then session
closed, then the MP portion of the security model will request the TM
portion of the security model to establish a session, populate the
cache, and pass the securityStateReference to the MP portion of the
security model.
TODO: We need to determine what state needs to be saved here.
5. Transport Mapping Security Model Samples
5.1 TLS/TCP Transport Mapping Security Model
SNMP supports multiple transports. The preferred transport for SNMP
over IP is UDP [RFC3417]. An experimental transport for SNMP over
TCP is defined in [RFC3430].
TLS/TCP will create an association between the TMSM of one SNMP
entity and the TMSM of another SNMP entity. The created "tunnel" may
provide encryption and data integrity. Both encryption and data
integrity are optional features in TLS. The TLS TM portion of the
security model MUST provide authentication if auth is requested in
the securityLevel of the SNMP message request (RFC3412 4.1.1). The
TLS TM-security model MUST specify that the messages be encrypted if
priv is requested in the securityLevel parameter of the SNMP message
request (RFC3412 4.1.1).
The TLS TM-security model SHOULD use the TLS Handshake Protocol with
mutual authentication.
5.1.1 tmStateReference for TLS
Upon establishment of a TLS session, the TM-security model will cache
the state information. A tmStateReference that is unique within the
SNMP entity will be stored in the cache, and passed to the
corresponding MP portion of the security model, to enable lookup.
The MP security model will pass the securityStateReference to the
Message Processing Model for memory management.
The tmStateReference cache:
tmStateReference
tmSecurityStateReference
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tmTransportDomain = TCP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - TLS TMSM
tmSecurityName = "dbharrington"
tmSecurityLevel = "authPriv"
tmAuthProtocol = Handshake MD5
tmPrivProtocol = Handshake DES
tmSessionID = Handshake session identifier
tmSessionKey = Handshake peer certificate
tmSessionMasterSecret = master secret
tmSessionParameters = compression method, cipher spec,
is-resumable
5.1.2 MP portion for TLS TM-Security Model
messageProcessingModel = SNMPv3
securityModel = TLS TMSM
securityName = tmSecurityName
securityLevel = msgSecurityLevel
5.1.3 MIB Module for TLS Security
Each security model should use its own MIB module, rather than
utilizing the USM MIB, to eliminate dependencies on a model that
could be replaced some day. See RFC3411 section 4.1.1.
The TLS MIB module needs to provide the mapping from model-specific
identity to a model independent securityName.
TO
DO: Module needs to be worked out once things become stable...
5.2 DTLS/UDP Transport Mapping Security Model
DTLS has been proposed as a UDP-based TLS. Transport Layer Security
(TLS) [RFC2246] traditionally requires a connection-oriented
transport and is usually used over TCP. Datagram Transport Layer
Security (DTLS) [DTLS] provides security services equivalent to TLS
for connection-less transports such as UDP.
DTLS provides all the security services needed from an SNMP
architectural point of view. Although it is possible to derive a
securityName from the public key certificates (e.g. the subject
field), this approach requires to install certificates on agents and
as well as managers, leading to a certificate management problem
which again does not integrate well with established AAA systems.
Another option is to run an authentication exchange which is
integrated with TLS, such as Secure Remote Password with TLS
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[SRP-TLS]. A similar option would be to use Kerberos authentication
with TLS as defined in [RFC2712].
It is important to stress that the authentication exchange must be
integrated into the TLS mechanism to prevent man-in-the-middle
attacks. While SASL [RFC2222] is often used on top of a TLS
encrypted channel to authenticate users, this choice seems to be
problematic until the mechanism to cryptographically bind SASL into
the TLS mechanism has been defined.
DTLS will create an association between the TMSM of one SNMP entity
and the TMSM of another SNMP entity. The created "tunnel" may
provide encryption and data integrity. Both encryption and data
integrity are optional features in DTLS. The DTLS TM-security model
MUST provide authentication if auth is requested in the securityLevel
of the SNMP message request (RFC3412 4.1.1). The TLS TM-security
model MUST specify that the messages be encrypted if priv is
requested in the securityLevel parameter of the SNMP message request
(RFC3412 4.1.1).
The DTLS TM-security model SHOULD use the TLS Handshake Protocol with
mutual authentication.
5.2.1 tmStateReference for DTLS
Upon establishment of a DTLS session, the TM-security model will
cache the state information. A tmStateReference that is unique
within the SNMP entity will be stored in the cache, and passed to the
corresponding MP portion of the security model, to enable lookup.
The MP security model will pass the securityStateReference to the
Message Processing Model for memory management.
The tmStateReference cache:
tmStateReference
tmSecurityStateReference
tmTransportDomain = UDP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - DTLS TMSM
tmSecurityName = "dbharrington"
tmSecurityLevel = "authPriv"
tmAuthProtocol = Handshake MD5
tmPrivProtocol = Handshake DES
tmSessionID = Handshake session identifier
tmSessionKey = Handshake peer certificate
tmSessionMasterSecret = master secret
tmSessionParameters = compression method, cipher spec,
is-resumable
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tmSessionSequence = epoch, sequence
TODO:
Need to discuss to what extent DTLS is a reasonable choice for
SNMP interactions.
What is the status of the work to cryptographically bind SASL to
DTLS?
More details need to be worked out...
5.3 SASL Transport Mapping Security Model
The Simple Authentication and Security Layer (SASL) [RFC2222]
provides a hook for authentication and security mechanisms to be used
in application protocols. SASL supports a number of authentication
and security mechanisms, among them Kerberos via the GSSAPI
mechanism.
This sample will use DIGEST-MD5 because it supports authentication,
integrity checking, and confidentiality.
DIGEST-MD5 supports auth, auth with integrity, and auth with
confidentiality. Since SNMPv3 assumes integrity checking is part of
authentication, if msgFlags is set to authNoPriv, the qop-value
should be set to auth-int; if msgFlags is authPriv, then qop-value
should be auth-conf.
Realm is optional, but can be utilized by the securityModel if
desired. SNMP does not use this value, but a TMSM could map the
realm into SNMP processing in various ways. For example, realm and
username could be concatenated to be the securityName value, e.g.
helpdesk::username", or the realm could be used to specify a
groupname to use in the VACM access control. This would be similar
to the EUSM's approach to having the securityName-to-group mapping
done by the external AAA server.
5.3.1 tmStateReference for SASL DIGEST-MD5
The tmStateReference cache:
tmStateReference
tmSecurityStateReference
tmTransportDomain = TCP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - SASL TMSM
tmSecurityName = username
tmSecurityLevel = [auth-conf]
tmAuthProtocol = md5-sess
tmPrivProtocol = 3des
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tmServicesProvided =
mutual authentication,
reauthentication,
integrity,
encryption
tmParameters = "realm=helpdesk, serv-type=SNMP
6. Acknowledgments
The authors would like to thank Ira McDonald, Ken Hornstein, and
Nagendra Modadugu for their comments and suggestions.
7. References
7.1 Normative References
[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 SNMP", STD 62, RFC
3412, 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.
[RFC3417] Presuhn (Editor), R., "Transport Mappings for the Simple
Network Management Protocol (SNMP)", STD 62, RFC 3417,
December 2002.
[RFC3430] Schoenwaelder, J., "Simple Network Management Protocol
(SNMP) over Transmission Control Protocol (TCP) Transport
Mapping", RFC 3430, December 2002.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[RFC2222] Myers, J., "Simple Authentication and Security Layer
(SASL)", STD 62, RFC RFC2222, October 1997.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", ID draft-rescorla-dtls-01.txt, July 2004.
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7.2 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.
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.
[SRP-TLS] Taylor, D., Wu, T., Mavroyanopoulos, M. and T. Perrin,
"Using SRP for TLS Authentication", ID
draft-ietf-tls-srp-08.txt, August 2004.
[EUSM] Narayan, D., McCloghrie, K., Salowey, J. and C. Elliot,
"External USM for SNMPv3", ID
draft-kaushik-snmp-external-usm-00.txt, July 2004.
[NETCONF] Enns, R., "NETCONF Configuration Protocol", ID
draft-ietf-netconf-prot-04.txt, October 2004.
[SSHauth] Lonvick, C., "SSH Authentication Protocol", ID
draft-ietf-secsh-userauth-21.txt, June 2004.
Authors' Addresses
David Harrington
Independent
Harding Rd
Portsmouth NH
USA
Phone: +1 603 436 8634
EMail: dbharrington@comcast.net
Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
EMail: j.schoenwaelder@iu-bremen.de
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Appendix A. Message security versus session security
A.1 msgFlags versus actual security
Using IPSEC, SSH, or SSL/TLS to provide security services "below" the
SNMP message, the use of securityName and securityLevel will differ
from the USM/VACM approach to SNMP access control. VACM uses the
"securityName" and the "securityLevel" to determine if access is
allowed. With the SNMPv3 message and USM security model, both
securityLevel and securityName are contained in every SNMPv3 message.
Any proposal for a security model using IPSEC, SSH, or SSL/TLS needs
to specify how this info is made available to the SNMPv3 message
processing, and how it is used.
One specific case to consider is the relationship between the
msgFlags of an SNMPv3 message, and the actual services provided by
the lower layer security. For example, if a session is set up with
encryption, is the priv bit always (or never) set in the msgFlags
field, and is the PDU never (or always) encrypted? Do msgFlags have
to match the security services provided by the lower layer, or are
the msgFlags ignored and the values from the lower layer used?
A.2 Message security versus session security
For SBSM, and for many TMSM models, securityName is specified during
session setup, and associated with the session identifier. Is it
possible for the request (and notification) originator to specify per
message auth and encryption services, or are they are "fixed" by the
transport/session model?
If a session is created as 'authPriv', then keys for encryption would
still be negotiated once at the beginning of the session. But if a
message is presented to the session with a security level of
authNoPriv, then that message could simply be authenticated and not
encrypted. Wouldn't that also have some security benefit, in that it
reduces the encrypted data available to an attacker gathering packets
to try and discover the encryption keys?
Agents are often resource-constrained. Adding sessions increases the
need for resources, we shouldn't require two sessions when one can
suffice. 2 bytes per session structure and a compare or two is much
less of a resource burden on an agent than two separate sessions.
It's not just about CPU power of the device but the percentage of CPU
cycles that are spent on network management. There isn't much value
in using encryption for a performance management system polling PEs
for performance data on thousands of interfaces every ten minutes,it
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just adds significant overhead to processing of the packet. Using an
encrypted TLS channel for everything may not work for use cases in
performance management wherein we collect massive amounts of non
sensitive data at periodic intervals. Each SNMP "session" would have
to negotiate two separate protection channels (authPriv and
authNoPriv) and for every packet the SNMP engine will use the
appropriate channel based on the desired securityLevel.
If the underlying transport layer security was configurable on a
per-message basis, a TMSM could have a MIB module with configurable
maxSecurityLevel and a minSecurityLevel objects to identify the range
of possible levels, and not all messages sent via that session are of
the same level. A session's maxSecurityLevel would identify the
maximum security it could provide, and a session created with a
minSecurityLevel of authPriv would reject an attempt to send an
authNoPriv message.
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