Network Working Group D. Harrington
Internet-Draft Effective Software
Expires: March 20, 2006 J. Schoenwaelder
International University Bremen
J. Salowey
Cisco Systems
September 16, 2005
Secure Shell Security Model for SNMP
draft-harrington-isms-secshell-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo describes a Security Model for the Simple Network
Management Protocol, using the Secure Shell protocol within a
Transport Mapping.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 The Internet-Standard Management Framework . . . . . . . . 6
1.3 The Secure Shell Protocol . . . . . . . . . . . . . . . . 6
1.4 Constraints . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Conventions . . . . . . . . . . . . . . . . . . . . . . . 7
2. How SSHSM Fits into the TMSM Architecture . . . . . . . . . . 7
2.1 Security Capabilities of this Model . . . . . . . . . . . 8
2.1.1 Threats . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.2 Sessions . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.3 Authentication Protocol . . . . . . . . . . . . . . . 11
2.1.4 Privacy Protocol . . . . . . . . . . . . . . . . . . . 12
2.1.5 Protection against Message Replay, Delay and
Redirection . . . . . . . . . . . . . . . . . . . . . 12
2.1.6 Security Protocol Requirements . . . . . . . . . . . . 12
2.2 Security Parameter Passing Requirement . . . . . . . . . . 14
2.3 Requirements for Notifications . . . . . . . . . . . . . . 15
2.4 Scenario Diagrams . . . . . . . . . . . . . . . . . . . . 15
2.4.1 Command Generator or Notification Originator . . . . . 15
2.4.2 Command Responder . . . . . . . . . . . . . . . . . . 16
2.5 Abstract Service Interfaces . . . . . . . . . . . . . . . 17
3. RFC 3411 Abstract Service Interfaces . . . . . . . . . . . . . 18
3.1 Public Abstract Service Interfaces . . . . . . . . . . . . 18
3.1.1 Public ASIs for Outgoing Messages . . . . . . . . . . 18
3.1.2 Public ASIs for Incoming Messages . . . . . . . . . . 20
3.2 Private Abstract Service Interfaces . . . . . . . . . . . 21
4. SNMP Messages Using this Security Model . . . . . . . . . . . 22
4.1 SNMPv1 and SNMPv2c Messages Using this Security Model . . 22
4.2 SNMPv3 Messages Using this Security Model . . . . . . . . 23
4.2.1 msgGlobalData . . . . . . . . . . . . . . . . . . . . 25
4.3 Passing Security Parameters . . . . . . . . . . . . . . . 26
4.3.1 Transport Session Parameters . . . . . . . . . . . . . 27
4.3.2 [todo] Using Local Accounts to Authenticate Users . . 28
4.3.3 [todo] Using RADIUS Accounts to Authenticate Users . . 28
4.3.4 securityStateReference for SSHSM . . . . . . . . . . . 28
4.4 MIB Module for SSH Security Model . . . . . . . . . . . . 28
4.5 [todo] Notifications . . . . . . . . . . . . . . . . . . . 28
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 29
5.1 Establishing a Session . . . . . . . . . . . . . . . . . . 29
5.2 Discovery . . . . . . . . . . . . . . . . . . . . . . . . 31
5.3 Generating an Outgoing SNMP Message . . . . . . . . . . . 32
5.4 Sending an Outgoing SNMP Message to the Network . . . . . 34
5.5 [todo] Prepare Data Elements from an Incoming SNMP
Message . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.6 Processing an Incoming SNMP Message . . . . . . . . . . . 35
6. MIB module definition . . . . . . . . . . . . . . . . . . . . 37
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7. Security Considerations . . . . . . . . . . . . . . . . . . . 40
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 40
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
10.1 Normative References . . . . . . . . . . . . . . . . . . . 40
10.2 Informative References . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 42
A. Change Log from the first revision of -00- . . . . . . . . . . 43
Intellectual Property and Copyright Statements . . . . . . . . 44
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1. Introduction
This memo describes a Security Model for the Simple Network
Management Protocol, using the Secure Shell protocol within a
Transport Mapping.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the Security
Model described in this memo fits into the architecture and interacts
with other subsystems within the architecture. The reader is
expected to have read and understood the description of the SNMP
architecture, as defined in [RFC3411],and the "Transport Mapping
Security Model (TMSM) for the Simple Network Management Protocol"
architecture extension defined in [TMSM], which enables the use of
external "lower layer" protocols to provide message security, tied
into the SNMP architecture through the transport mapping subsystem.
One such external protocol is the Secure Shell protocol [SSHArch].
This memo describes the Secure Shell Security Model for SNMP, a
specific SNMP security model to be used within the SNMP Architecture,
to provide authentication, encryption, and integrity checking of SNMP
messages.
This memo defines a portion of the Management Information Base (MIB)
for use with network management protocols in TCP/IP based internets.
In particular it defines objects for monitoring and managing the
Secure Shell Security Model for SNMP.
In keeping with the RFC 3411 design decisions to use self-contained
documents, this memo includes the elements of procedure plus
associated MIB objects which are needed for processing the Secure
Shell Security Model for SNMP. These MIB objects should not be
referenced in other documents. This allows the Secure Shell Security
Model for SNMP to be designed and documented as independent and self-
contained, having no direct impact on other modules, and allowing
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.
1.1 Motivation
Version 3 of the Simple Network Management Protocol (SNMPv3) added
security to the previous versions of the protocol. The User Security
Model (USM) [RFC3414] was designed to be independent of other
existing security infrastructures, to ensure it could function when
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third party authentication services were not available, such as in a
broken network. As a result, USM typically utilizes 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.
This memo describes a security model that will make use of the
existing and commonly deployed Secure Shell security infrastructure.
It is designed to meet the security and operational needs of network
administrators, maximize useability 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.
The work will address the requirement for the SSH client to
authenticate the SSH server, for the SSH server to authenticate the
SSH client (the user), and how SNMP can make use of the authenticated
identities in authentication and auditing. .
The work will include the ability to use any of the user
authentication methods described in "SSH Authentication Protocol"
[SSHAuth] - public key, password, and host-based. Local accounts may
be supported through the use of the public key, host-based or
password based mechanisms. The password based mechanism allows for
integration with deployed password infrastructure such as AAA servers
using the RADIUS protocol [RFC2865]. In the future it should also be
able to take advantage of other defined authentication mechanism such
as those defined in [gsskeyex] which will allow for user
authentication mechanisms which support different security
infrastructures and provide security properties.
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 Secure Shell is
the approach commonly used for the CLI, and is the approach being
adopted for use with NETCONF [Netconf]. Similar to NETCONF over SSH
[NetconfSSH], this memo describes a method for invoking and running
the SNMP protocol within a Secure Shell (SSH) session as an SSH
subsystem.
This memo defines how SNMP can be used within a Secure Shell (SSH)
session, using the SSH connection protocol [SSHConnect] over the SSH
transport protocol [SSHTrans], using SSH user-auth [SSHAuth]for
authentication.
There are a number of challenges to be addressed to map Secure Shell
authentication method parameters into the SNMP architecture so that
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SNMP continues to work without any surprises. These are discussed in
detail below. Some points requiring further WG research and
discussion are identified by [todo] markers in the text.
1.2 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].
1.3 The Secure Shell Protocol
SSH is a protocol for secure remote login and other secure network
services over an insecure network. It consists of three major
components:
o The Transport Layer Protocol [[SSHTrans] provides server
authentication, confidentiality, and integrity. It may optionally
also provide compression. The transport layer will typically be
run over a TCP/IP connection, but might also be used on top of any
other reliable data stream.
o The User Authentication Protocol [SSHAuth] authenticates the
client-side user to the server. It runs over the transport layer
protocol.
o The Connection Protocol [SSHConnect] multiplexes the encrypted
tunnel into several logical channels. It runs over the user
authentication protocol.
The client sends a service request once a secure transport layer
connection has been established. A second service request is sent
after user authentication is complete. This allows new protocols to
be defined and coexist with the protocols listed above.
The connection protocol provides channels that can be used for a wide
range of purposes. Standard methods are provided for setting up
secure interactive shell sessions and for forwarding ("tunneling")
arbitrary TCP/IP ports and X11 connections.
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1.4 Constraints
The design of this SNMP Security Model is also influenced by the
following constraints:
1. When the requirements of effective management in times of network
stress are inconsistent with those of security, the design of
this model gives preference to the former.
2. In times of network stress, neither the security protocol nor its
underlying security mechanisms should depend upon the ready
availability of other network services (e.g., Network Time
Protocol (NTP) or AAA protocols).
3. 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.
4. It may not be possible for the security model to determine when
the network is under stress.
5. A security mechanism should entail no changes to the basic SNMP
network management philosophy.
1.5 Conventions
The terms "manager" and "agent" are not used in this document,
because in the RFC 3411 architecture, all entities have the
capability of acting as either manager or agent or both depending on
the SNMP applications included in the engine. Where distinction is
required, the application names of Command Generator, Command
Responder, Notification Generator, Notification Rewsponder, and Proxy
Forwarder are used. See "SNMP Applications" [RFC3413] for further
information.
Throughout this document, the terms "client" and "server" are used to
refer to the two ends of the SSH transport connection. The client
actively opens the SSH connection, and the server passively listens
for the incoming SSH connection. Either entity may act as client or
as server, as discussed further below.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. How SSHSM Fits into the TMSM Architecture
SSH is a security layer which is plugged into the TMSM architecture
between the underlying transport layer and the message dispatcher.
The SSHSM model will establish an encrypted tunnel between the
transport mappings of two SNMP engines. The sending transport
mapping security model instance encrypts outgoing messages, and the
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receiving transport mapping security model instance decrypts the
messages.
After the transport layer tunnel is established, then SNMP messages
can conceptually be sent through the tunnel from one SNMP message
dispatcher to another SNMP message dispatcher. Once the tunnel is
established, multiple SNMP messages may be able to be passed through
the same tunnel.
Within an engine, outgoing SNMP messages are passed unencrypted from
the message dispatcher to the transport mapping, and incoming
messages are passed unencrypted from the transport mapping to the
message dispatcher. SSHSM security processing will be called from
within the Transport Mapping functionality of an SNMP engine
dispatcher to perform the translation of transport security
parameters to/from security-model-independent parameters. Some SSHSM
security processing will also be performed within a message
processing portion of the model, for compatibility with the ASIs
between the RFC 3411 Security Subsystem and the Message Processing
Subsystem.
2.1 Security Capabilities of this Model
2.1.1 Threats
The security protocols used in this memo are considered acceptably
secure at the time of writing. However, the procedures allow for new
authentication and privacy methods to be specified at a future time
if the need arises.
The Secure Shell Security Model provides protection against the
threats identified by the RFC 3411 architecture [RFC3411]:
1. Message stream modification - Provide for verification that each
received SNMP message has not been modified during its
transmission through the network. .
2. Information modification - Provide for verification that the
contents of each received SNMP message has not been modified
during its transmission through the network. Data has not been
altered or destroyed in an unauthorized manner, nor have data
sequences been altered to an extent greater than can occur non-
maliciously
3. Masquerade - Provide for both verification of the identity of the
user on whose behalf a received SNMP message claims to have been
generated, and the verification of the identity of the MIB owner.
For the protocols specified in this memo, it is not possible to
assure the specific user that originated a received SNMP message;
rather, it is the user on whose behalf the message was originated
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that is authenticated. SSH provides verification of the identity
of the MIB owner through the SSH Transport Protocol server
authentication [SSHTrans].
4. Verification of user identity is important for use with the SNMP
access control subsystem, to ensure that only authorized users
have access to potentially sensitive data. The SSH user identity
will be used to map to an SNMP model-independent securityname for
use with SNMP access control.
5. Authenticating the server ensures the authenticity of the SSH
server that is associated with the SNMP engine that provides MIB
data. Operators or management applications could act upon the
data they receive (e.g. raise an alarm for an operator, modify
the configuration of the device that sent the notification,
modify the configuration of other devices in the network as the
result of the notification, and so on) , so it is important to
know that the data is authentic. SSH allows for authenticaiton
of the SSH server using the SSH public key credentials described
in [SSHTrans] and mechanisms such as those described in
[gsskeyex].
6. Disclosure - Provide, when necessary, that the contents of each
received SNMP message are protected from disclosure to
unauthorized persons..
7. Replay - Provide for detection of received SNMP messages, which
request or contain management information, whose time of
generation was not recent. A message whose generation time is
outside of a time window is not accepted. Note that message
reordering is not dealt with and can occur in normal conditions
2.1.1.1 Data Origin Authentication Issues
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)
SSH provides support for encryption and data integrity. While it is
technically possible to support noAuthNoPriv and authNoPriv in SSH it
is NOT RECOMMENDED by [SSHTrans]. This means that an SSH connection
should support authPriv, which is the highest level of security
defined in RFC 3411. It is possible for SSH to skip entity
authenticaiton of the client through the "none" authentication method
to support anonymous clients, however in the this case an
implementation MUST still support data integrity within the SSH
transport protocol. The security protocols used in [SSHTrans] are
considered acceptably secure at the time of writing. However, the
procedures allow for new authentication and privacy methods to be
specified at a future time if the need arises.
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authNoPriv may be important to accommodate governmental regulation
(e.g. export laws) regarding encryption technologies.
[todo] is it important to support anonymous user access to SNMP?
Should the transport layer provide what data integrity and encryption
algorithms were negotiated to the SSHSM layer? In SNMP, we
deliberately avoided this, and settled for an assertion that auth and
priv were applied accoridng to the rules of the security model.
SSH should also provide the identity of the authenticated parties.
From this information it should be possible for the SNMP subsystem to
determine if the session is allowed access to the subsystem.
2.1.1.1.1 noAuthPriv
SSH provides the "none" userauth method, which is normally rejected
by servers and used only to find out what userauth methods are
supported. However, it is legal for a server to accept this method,
which has the effect of not authenticating the ssh client to the ssh
server. Doing this does not compromise authentication of the ssh
server to the ssh client, nor does it compromise data confidentiality
or data integrity.
The RFC 3411 architecture does not permit noAuthPriv. SSHSM should
refuse a noAuthPriv session [todo] If we do not allow some of these
options, how do we determine the option was used, so we can reject
it? How does an SNMP engine reject a session?
2.1.1.1.2 skipping public key verification
[todo] Most key exchange algorithms are able to authenticate the SSH
server's identity to the client. However, for the common case of DH
signed by public keys, this requires the client to know the host's
public key a priori and to verify that the correct key is being used.
If this step is skipped, you no longer have authentication of the ssh
server to the ssh client. You do still get data confidentiality and
data integrity protection to whatever server you're talking to, but
these are of dubious value when an attacker can insert himself
between the client and the real ssh server. Note that some userauth
methods may defend against this situation, but many of the common
ones (including password and keyboard-interactive) do not, and in
fact depend on the fact that the server's identity has been verified
(otherwise you may be giving your password to an attacker).
2.1.1.1.3 the 'none' MAC algorithm
SSH provides the "none" MAC algorithm, which would allow you to turn
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off data integrity while maintaining confidentiality. However, if
you do this, then an attacker may be able to modify the data in
flight, which means you effectively have no authentication.
SSH must not be configured using the "none" MAC algorithm for use
with the SSHSM security model.
2.1.2 Sessions
Sessions are not part of RFC 3411 architecture, but are considered
desirable because the cost of authentication can be amortized over
potentially many transactions. The Secure Shell security model will
utilize sessions, with a single user and security level associated
with each session. If an exchange with another engine would require
a different security level or would be on behalf of a different user,
then another session would be needed. An immediate consequence of
this is that implementations should be able to maintain some
reasonable number of concurrent sessions. This document will discuss
the impact of sessions on SNMP usage. [todo]
2.1.2.1 Message security versus session security
As part of session creation the client and server entities are
typically authenticated and authorized access to the session. In
addition as part of session establishment cryptographic key material
is exchanged and is then used to control access to the session on a
message by message basis. Messages that fail the basic data origin
authenticaiton/ data integrity checks will be rejected. Entities
receiving the messages that do not have the correct encryption keys
established during session creation will not be able to read the
messgaes. In order for an entity to process messages it must
maintain certain state associated with the session. This includes,
but is not limited to cryptographic encryption and data integrity
keys, entity identities and authorization information associated with
the authenticated identites. After a message is received and passes
integrity and authentication checks then the state stored in the
session is used to provide further authorization for the message.
2.1.3 Authentication Protocol
SSHSM should support any user authentication mechanism supported by
SSH.
The SSH Authentication Protocol document describes three
authentication methods - publickey, password, and host-based. All
three authentication methods are supported by the Secure Shell
Security Model for SNMP.
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The password authentication mechanism allows for integration with
deployed password based infrastructure. It is possible to hand a
password to a service such as RADIUS [RFC2865] or Diameter [RFC 3588]
for validation. The validation could be done using the user-name and
user-password attributes. It is also possible to use a different
password validation protcol such as CHAP [RFC1994] or digest
authentication [RFC 2617, draft-ietf-radext-digest-auth-04] to
integrate with RADIUS. Any of these mechanism leave the password in
the clear on the device that is authenticating the password which
introduces threats on the authentication infrastructure which is less
than ideal. It is possible that new mechanism will be developed
using authentication mechanisms defined in [gsskeyex] which will
allow for user authentication mechanisms which support different
security infrastructures and provide security properties."
2.1.4 Privacy Protocol
The Secure Shell Security Model uses the SSH transport layer
protocol, which provides strong encryption, server authentication,
and integrity protection.
2.1.5 Protection against Message Replay, Delay and Redirection
The Secure Shell Security Model uses the SSH transport layer
protocol. SSH uses sequence numbers and integrity checks to protect
against replay and reordering of messages within a connection.
SSH also provides protection against replay of entire sessions. In a
properly-implemented DH exchange, both sides will generate new random
numbers for each exchange, which means the exchange hash and thus the
encryption and integrity keys will be distinct for every session.
This would prevent capturing an SNMP message and redirecting it to
another SNMP engine.
Message delay is not as important an issue with SSH as it is with
USM. USM checks the timeliness of messages because it does not
provide session protection or message sequence ordering. The only
delay that would seem to be possible would be to capture the next
sequence numbered packet and hold it to play within the same session
later.
2.1.6 Security Protocol Requirements
Modifying the Secure Shell protocol, or configuring it in a
particular manner, may change its security characteristics in ways
that would impact other existing usages. If a change is necessary,
the change should be an extension that has no impact on the existing
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usages. This document will describe the use of an SSH subsytem for
SNMP.
It has been a long-standing requirement that SNMP be able to work
when the network is unstable, to enable network troubleshooting and
repair. The UDP approach has been considered to meet that need well,
with an assumption that getting small messages through, even if out
of order, is better than gettting no messages through. There has
been a long debate about whether UDP actually offers better support
than TCP when the underlying IP or lower layers are unstable. There
has been recent discussion of whether operators actually use SNMP to
troubleshoot and repair unstable networks. This document will
include a discussion of the operational expectations of this model
for use in troubleshooting a broken network.[todo] This may belong
in the TMSM document.
There has been discussion of ways SNMP could be extended to better
support management/monitoring needs when a network is running just
fine. Use of a TCP transport, for example, could enable larger
message sizes and more efficient table retrievals. Secure Shell runs
over TCP. This document will discuss the expected ramifications of
using a TCP transport for SNMP, and the coexistence of UDP and TCP
transport for SNMP. [todo] Should this be discussed in the TMSM
document?
The Secure Shell security model can coexist with the USM security
model, the only other currently defined security model. [todo]
compare to RFC3584 to see if there are any wrinkles to coexistence
with SNMPv1/v2c.
2.1.6.1 Mapping SSH to EngineID
In the RFC3411 architecture, there are three use cases for an
engineID:
snmpEngineID - RFC3411 includes the SNMP-FRAMEWORK-MIB, which
defines a snmpEngineID object. An snmpEngineID is the unique and
unambiguous identifier of an SNMP engine. Since there is a one-
to-one association between SNMP engines and SNMP entities, it also
uniquely and unambiguously identifies the SNMP entity within an
administrative domain.
contextEngineID - Management information resides at an SNMP entity
where a Command Responder Application has local access to
potentially multiple contexts. This application uses a
contextEngineID equal to the snmpEngineID of its associated SNMP
engine.
securityEngineID - The RFC3411 architecture defines ASIs that
include a securityEngineID - the authoritative SNMP entity - which
is either the local snmpEngineID or the target snmpEngineID,
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depending on the type of operation. Since a security model might
utilize shared credentials and integrity-checking parameters, and
the datastores of the two endpoints could get out of sync, the
"authoritative" engineID indicates which end has the values to be
used.
The securityEngineID is used by USM when performing integrity
checking and authentication, to look up values in the USM tables, and
to synchronize "clocks". The securityEngineID is not needed by
SSHSM, since integrity checking and authentication are handled
outside the SNMP engine.
[todo] is there still a need for an "authoritative SNMP engine"?
Does authoritative have any meaning in a TMSM/SSHSM environment? In
SNMPv3, the authoritative engine is usually the engine with the
command responder, i.e. the agent; in non-proxy situations,
securityEngineID equals contextEngineID. in client-server terms, the
authoritative engine is usually the server. So, should the SNMP
engine associated with the SSH server be authoritative? Would Infoms
change that? Would bidirectional messaging change that? Would call-
home change that? Do we need to set the securityEngineID to indicate
which side is the SSH server?
2.2 Security Parameter Passing Requirement
[SSHSM follows the TMSM approach, in which the security -model
specific parameters can be determined from the transport layer by the
transport mapping, before the message processing begins.
[todo] For outgoing messages, it is necessary to have an MPSP portion
of the security model because it is the MPSP that actually creates
the WholeMsg from its component parts. In the SSHSM model, the MPSP
does not apply encryption, integrity-checking, or authentication.
For example, an SNMPv3 message is built without any content in the
SecurityParameters field, and the WholeMsg is passed unencrypted back
to the Message Processing Model for forwarding to the Transport
Mapping.
A cache mechanism will be used, into which the TMSP puts information
about the security applied to an incoming message, and an MPSP
extracts that information from the cache. Given that there may be
multiple TM-security caches, a cache ID will need to be passed
through an ASI so the MPSP knows which cache of information to
consult. [todo]
The cache reference could be thought of as an additional parameter in
the ASIs between the transport mapping and the messaging security
model. The RFC 3411 ASIs would not need to be changed since the
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SNMPv3 WG expected that additional parameters could be passed for
value-add features of specific implementations.
This approach does create dependencies between a model-specific TPSP
and a corresponding specific MPSP. If a TMSM-model-independent ASI
parameter is passed, this approach would be consistent with the
securityStateReference cache already being passed around in the ASI.
2.3 Requirements for Notifications
[todo] cleanup this section
RFC 3430 (SNMP over TCP) suggests that TCP connections are initiated
by notification originators in case there is no currently established
connection that can be used to send the notification. Following this
approach with ssh would require to provision authentication
credentials on the agent so that agents can successfully authenticate
to a notification receiver. There might be other approaches, like
the reuse of manager initiated secure transport connections for
notifications. There is some text in Appendix A in RFC 3430 which
captures some of these discussions when RFC 3430 was written.
2.4 Scenario Diagrams
RFC 3411 section 4.6 provides scenario diagrams to illustrate how an
outgoing message is created, and how an incoming message is
processed. Both diagrams are incomplete, however.In section 4.61,
the diagram doesn't show the ASI for sending an SNMP request to the
network or receiving an SNMP response message from the network. In
section 4.6.2, the diagram doesn't illustrate the interfaces required
to receive an SNMP message from the network, or to send an SNMP
message to the network.
2.4.1 Command Generator or Notification Originator
This diagram from RFC 3411 4.6.1 shows how a Command Generator or
Notification Originator application requests that a PDU be sent, and
how the response is returned (asynchronously) to that application.
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Command Dispatcher Message Security
Generator | Processing Model
| | Model |
| sendPdu | | |
|------------------->| | |
| | prepareOutgoingMessage | |
: |----------------------->| |
: | | generateRequestMsg |
: | |-------------------->|
: | | |
: | |<--------------------|
: | | |
: |<-----------------------| |
: | | |
: |------------------+ | |
: | Send SNMP | | |
: | Request Message | | |
: | to Network | | |
: | v | |
: : : : :
: : : : :
: : : : :
: | | | |
: | Receive SNMP | | |
: | Response Message | | |
: | from Network | | |
: |<-----------------+ | |
: | | |
: | prepareDataElements | |
: |----------------------->| |
: | | processIncomingMsg |
: | |-------------------->|
: | | |
: | |<--------------------|
: | | |
: |<-----------------------| |
| processResponsePdu | | |
|<-------------------| | |
| | | |
2.4.2 Command Responder
This diagram shows how a Command Responder or Notification Receiver
application registers for handling a pduType, how a PDU is dispatched
to the application after an SNMP message is received, and how the
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Response is (asynchronously) send back to the network.
Command Dispatcher Message Security
Responder | Processing Model
| | Model |
| | | |
| registerContextEngineID | | |
|------------------------>| | |
|<------------------------| | | |
| | Receive SNMP | | |
: | Message | | |
: | from Network | | |
: |<-------------+ | |
: | | |
: |prepareDataElements | |
: |------------------->| |
: | | processIncomingMsg |
: | |------------------->|
: | | |
: | |<-------------------|
: | | |
: |<-------------------| |
| processPdu | | |
|<------------------------| | |
| | | |
: : : :
: : : :
| returnResponsePdu | | |
|------------------------>| | |
: | prepareResponseMsg | |
: |------------------->| |
: | |generateResponseMsg |
: | |------------------->|
: | | |
: | |<-------------------|
: | | |
: |<-------------------| |
: | | |
: |--------------+ | |
: | Send SNMP | | |
: | Message | | |
: | to Network | | |
: | v | |
2.5 Abstract Service Interfaces
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3. RFC 3411 Abstract Service Interfaces
Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an
SNMP entity. The Secure Shell Security Model uses some of these
conceptual data flows when communicating with other subsystems, such
as the Message Processing Subsystem. These RFC 3411-defined data
flows are referred to here as public interfaces.
3.1 Public Abstract Service Interfaces
3.1.1 Public ASIs for Outgoing Messages
The IN parameters of the prepareOutgoingMessage() ASI are used to
pass information from the dispatcher (application subsystem) to the
message processing subsystem. The OUT parameters are used to pass
information from the message processing subsystem to the dispatcher
and on to the transport mapping:
statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
IN sendPduHandle -- the handle for matching
-- incoming responses
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)
The abstract service primitive from a Message Processing Model to a
Security Model to generate the components of a Request message is:
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statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
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
)
The abstract service primitive from a Message Processing Model to a
Security Model to generate the components of a Response message is:
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
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
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows: [todo] check each parameter and
determine if it is necessary for SSHSM and whether the description is
accurate
statusInformation - An indication of whether the encoding and
securing of the message was successful. If not it is an
indication of the problem.
messageProcessingModel - The SNMP version number for the message
to be generated. This data is not used by the User-based Security
module.
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globalData - The message header (i.e., its administrative
information). This data is not used by the User-based Security
module.
maxMessageSize - The maximum message size as included in the
message. This data is not used by the User-based Security module.
securityParameters - These are the security parameters. They will
be filled in by the SSH Security module.
securityModel - The securityModel in use. Should be SSH Security
Model.
securityName - identifies a principal to be used for securing an
outgoing message. The securityName has a format that is
independent of the Security Model. In case of a response this
parameter is ignored and the value from the cache is used.
securityLevel - The Level of Security from which the SSH Security
module determines if the message needs to be protected from
disclosure and if the message needs to be authenticated.
securityEngineID - The snmpEngineID of the authoritatvie SNMP
engine to which a dateRequest message is to be sent. In case of a
response it is implied to be the processing SNMP engine's
snmpEngineID and so if it is specified, then it is ignored.
scopedPDU - The message payload. The data is opaque as far as the
SSH Security Model is concerned.
securityStateReference - A handle/reference to cachedSecurityData
to be used when securing an outgoing Response message. This is
the exact same handle/reference as it was generated by the SSH
Security module when processing the incoming Request message to
which this is the Response message.
wholeMsg - The fully encoded SNMP message ready for sending on the
wire.
wholeMsgLength - The length of the encoded SNMP message
(wholeMsg).
Upon completion of the process, the SSH Security module returns
statusInformation. If the process was successful, the completed
message is returned, without the privacy and authentication
applied yet. If the process was not successful, then an
errorIndication is returned.
3.1.2 Public ASIs for Incoming Messages
The abstract service primitive from a Transport Mapping (in the
dispatcher) to a Message Processing Model for a received message is::
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result = -- SUCCESS or errorIndication
prepareDataElements(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN wholeMsg -- as received from the network
IN wholeMsgLength -- as received from the network
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- the version of the PDU
OUT PDU -- SNMP Protocol Data Unit
OUT pduType -- SNMP PDU type
OUT sendPduHandle -- handle for matched request
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for possible Response
)
The abstract service primitive 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 -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
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
3.2 Private Abstract Service Interfaces
A set of abstract service interfaces have been defined within this
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document to describe the conceptual data flows between the Secure
Shell Security Model (SSHSM) and the self-contained transport mapping
services.These apply only to the Secure Shell Security Model (SSHSM),
and are referred to here as private interfaces.
The Secure Shell Security Model provides the following internal
primitives to pass data back and forth between the Security Model
itself and the SSH authentication service:
statusInformation =
establishSession(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN securityModel -- Security Model to use
IN securityEngineID -- SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
OUT sessionID
)
4. SNMP Messages Using this Security Model
The syntax of an SNMP message using this Security Model adheres to
the message format defined in the version-specific Message Processing
Model document (for example [RFC3412]). At the time of this writing,
there are three defined message formats - SNMPv1, SNMPv2c, and
SNMPv3.
4.1 SNMPv1 and SNMPv2c Messages Using this Security Model
Since message security is provided by a "lower layer", the message
does not need to carry message security parameters within the PDU.
The securityModel and securityName parameters are determined by the
Secure Shell Security Model from the SSH service. SSHSM requires
that transport always be authenticated and integrity-checked, and
encrypted, so all SSHSM messages are authpriv. Since an incoming
SNMPv1 or SNMPv2c message lacks a msgFlags field, the msgFlags is
always treated as authPriv. [todo]
The communitystring is not used as an authentication mechansism,
since user authentication is provided by SSH userauth. The community
string is still used to provide context information.
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The SNMPv1 and SNMPv2c message formats do not contain a
contextEngineID, but do contain an IP Address field that can be used
to perform proxy. [todo] determine the proxy forwarding mechanism, if
any.
4.2 SNMPv3 Messages Using this Security Model
RFC 3412 defines two primitives, generateRequestMsg() and
processIncomingMsg() which require the specification of an
authoritative SNMP entity. [todo] We need to discuss what the meaning
of authoritative would be in a TMSM environment, whether the specific
services provided in USM security from msgSecurityParameters are
needed in TMSM/SSHSM, and how the Message Processing model provides
this information to the security model via generateRequestMsg() and
processIncomingMsg() primitives.
The SNMPv3Message SEQUENCE is defined in [RFC3412]. The following
fields are specific to the Secure Shell Security Model:
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SNMPv3MessageSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
SNMPv3Message ::= SEQUENCE {
-- identify the layout of the SNMPv3Message
-- this element is in same position as in SNMPv1
-- and SNMPv2c, allowing recognition
-- the value 3 is used for snmpv3
msgVersion INTEGER ( 0 .. 2147483647 ),
-- administrative parameters
msgGlobalData HeaderData,
-- security model-specific parameters
-- format defined by Security Model
msgSecurityParameters OCTET STRING,
msgData ScopedPduData
}
HeaderData ::= SEQUENCE {
msgID INTEGER (0..2147483647),
msgMaxSize INTEGER (484..2147483647),
msgFlags OCTET STRING (SIZE(1)),
-- .... ...1 authFlag
-- .... ..1. privFlag
-- .... .1.. reportableFlag
-- Please observe:
-- .... ..00 is OK, means noAuthNoPriv
-- .... ..01 is OK, means authNoPriv
-- .... ..10 reserved, MUST NOT be used.
-- .... ..11 is OK, means authPriv
msgSecurityModel INTEGER (1..2147483647)
}
ScopedPduData ::= CHOICE {
plaintext ScopedPDU,
encryptedPDU OCTET STRING -- encrypted scopedPDU value
}
ScopedPDU ::= SEQUENCE {
contextEngineID OCTET STRING,
contextName OCTET STRING,
data ANY -- e.g., PDUs as defined in [RFC3416]
}
END
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4.2.1 msgGlobalData
SSHSM requires that transport always be authenticated, integrity-
checked, and encrypted, so all SSHSM messages are authpriv. The
msgFlags MUST always be set to authPriv.
msgSecurityModel is set to the IANA-assigned value for the Secure
Shell Security Model. See
http://www.iana.org/assignments/snmp-number-spaces.
4.2.1.1 msgSecurityParameters
Since message security is provided by a "lower layer", and the
securityName parameter is always determined from the SSH
authentication method, the SNMP message does not need to carry
message security parameters within the msgSecurityParameters field.
To prevent its being used in a manner that could be damaging, such as
for carrying a virus or worm, when used with SSHSM, it is an empty
field. [todo] should this be an ASN.1 NULL within the OCTET STRING,
or just a zero-length OCTET STRING?
The field msgSecurityParameters in SNMPv3 messages has a data type of
OCTET STRING. Its value MUST be the BER serialization of the
following ASN.1 sequence:
SSHSMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
SSHsmSecurityParameters ::=
SEQUENCE {
NULL
}
END
4.2.1.2 msgFlags
[todo] For an outgoing message, msgFlags is the requested security
for the message; if a SSHSM cannot provide the requested
securityLevel, the request MUST be discarded and SHOULD notify the
message processing model that the request failed. [todo: how does
this apply in the SSHSM model, especially if the msgFlags MUST always
be set to authpriv?]
[todo] do we need do discuss the rest of this, or is this applicable
to all TMSM models?
For an outgoing message, it is acceptable for the SSHSM to provide
stronger than requested security. To avoid the need to mess with the
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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 SSHSM 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 SSHSM 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 MPSP 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 MPSP should determine whether
the changed securityLevel is acceptable. If not, it should discard
the message.
4.3 Passing Security Parameters
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 Configuration Datastore is
altered between the time of the incoming request and the outgoing
response. For SSHSM, there are three levels of state that need to be
maintained: the session, the message, and the model-independent
translations.
[todo]ensuring consistent security for responses even if the
datastore changes is important for USM because USM handles
keychanges; will SSHSM allow keychanges to the SSH local datastore?
if not, this cache of message-pair-state may be an unnecessary
constraint. is it important to ensure responses use the same security
as the request for secureity reasons?
tmSessionReference is used to pass model- and mechanism-specific
parameters to coordinate the session-related activities of the TMSP
and MPSP. The SSHSM has the responsibility for explicitly releasing
the tmSessionReference when the session is destroyed.
tmStateReference is used to pass model- and mechanism-specific
parameters to coordinate the activities of the TMSP and MPSP related
to a specific pair of messages. The SSHSM has the responsibility for
explicitly releasing the tmStateReference once a response message has
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been sent, or the data is no longer needed.
The MPSP translates select parameters from the tmSessionReference
cache into model-independent parameters subsequently passed in the
securityStateReference cache to a Message Processing Model. The
Message Processing Model has the responsibility for explicitly
releasing the securityStateReference if such data is no longer
needed. The securityStateReference cached data may be implicitly
released via the generation of a response, or explicitly released by
using the stateRelease primitive, as described in RFC 3411 section
4.5.1."
4.3.1 Transport Session Parameters
[todo] SSHSM will create a session between the TMSM of one SNMP
entity and the TMSM of another SNMP entity. The created "tunnel"
will provide encryption and data integrity. The SSHSM model MUST
provide mutual authentication of the client and server, and MUST
authenticate, integrity-check, and encrypt the messages.
Upon establishment of a SSH session, the TMSP will cache the state
information about the transport parameters. The tmSessionReference
will be passed to the corresponding MPSP.
[todo] The tmSessionReference cache for use with the SSH
Authentication Protocol [SSHAuth]:
tmStateReference
tmSecurityStateReference
tmTransportDomain = TCP/IPv4
tmTransportAddress = x.x.x.x:y
tmSecurityModel - SSHSM
tmSecurityLevel = "authPriv"
tmSessionID = Handshake session identifier
tmSessionKey = Handshake peer certificate
tmSessionMasterSecret = master secret
tmSessionParameters = compression method, cipher spec, is-
resumable
tmSecurityName = "dbharrington"
tmAuthMechanism = "[todo]"
tmAuthProtocol = ""
tmRadiusServer = ""
tmPrivProtocol = ""
Additional information will be added to the tmStateReference by the
authentication portion of the SSHSM.
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4.3.1.1 Authenticating Servers and Clients
[todo] can we mandate mutual authentication?
4.3.2 [todo] Using Local Accounts to Authenticate Users
Upon creation of a SSH session leveraging SSH Local Accounts, the
TMSP will cache the session authentication information in the
tmSessionReference:
tmSecurityName = "dbharrington"
tmAuthMechanism = "public key"
tmAuthProtocol = LocalAccounts
4.3.3 [todo] Using RADIUS Accounts to Authenticate Users
Upon creation of a SSH session leveraging RADIUS Accounts, the TMSP
will cache the session authentication information in the
tmSessionReference:
tmSecurityName is the name used in username field of the RADIUS
ACCESS-REQUEST message.
tmAuthMechanism = "[todo]"
tmAuthProtocol = RADIUS
tmRadiusServer = x.x.x.x:y
4.3.4 securityStateReference for SSHSM
[todo]
messageProcessingModel = SNMPv3
securityModel = SSHSM
securityName = tmSecurityName
securityLevel = msgSecurityLevel
4.4 MIB Module for SSH Security Model
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 RFC 3411 section 4.1.1.
The SSHSM-MIB module needs to provide the mapping from model-specific
identity to a model independent securityName, and possibly a mapping
to a groupname.
[todo] Module needs to be worked out once things become stable...
4.5 [todo] Notifications
For notifications, if the cache has been released and then session
closed, then the MPSP will request the TMSP to establish a session,
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populate the cache, and pass the securityStateReference to the MPSP.
[todo] We need to determine what state needs to be saved here.
5. Elements of Procedure
5.1 Establishing a Session
The Secure Shell Security Model provides the following internal
primitive to pass data back and forth between the Security Model
itself and the SSH authentication service:
statusInformation =
establishSession(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN securityModel -- Security Model to use
IN securityEngineID -- SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
OUT sessionID
)
The following describes the procedure to follow to establish a
session between a client and sever to run SNMP over SSH. This
process is followed by any SNMP engine establishing a session for
subsequent use. In practice, this is done by an application that
initiates a transaction, such as a Command Generator or a
Notification Originator or a Proxy Forwarder. It is never triggered
by an application preparing a response message, such as a Command
Responder or Notification Receiver, because securityStatereference
will always have session information for a response message
The parameters necessary to establish a session are provided by the
Secure Shell Security Model to the SSH client code, using the
establishSession() ASI.
1) If the securityLevel specifies that the message is to be
authenticated, but the SSH implementation does not support an
authentication protocol, then the message cannot be sent. An error
indication (unsupportedSecurityLevel) is returned to the calling
module.
2) If the securityLevel specifies that the message is to be protected
from disclosure, but the SSH implementation does not support
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encryption, then the message cannot be sent. An error indication
(unsupportedSecurityLevel) is returned to the calling module.
3) Using destTransportDomain and destTransportAddress, the client
will establish an SSH transport connection using the SSH transport
protocol, and the client and server will mutually authenticate, and
exchange keys for message integrity and encryption. if the attempt to
establish a connection is successful, then tmStateReference is
created, and the values of destTransportDomain and
destTransportAddress are saved. If the attempt to establish a
connection is unsuccessful, then an error indication [todo] will be
returned, and [todo] processing stops.
4) The provided securityEngineID is used to lookup the associated
entry in the Local Configuration Datastore (LCD), and the
securityName, securityModel, and securityLevel, information
concerning the user at the destination is extracted. This step
allows preconfiguration of model-specific user identities mapped to a
securityName. Set the username in the SSH_MSG_USERAUTH_REQUEST to
the username extracted from the LCD.
If information about the user is absent from the LCD, then set the
username in the SSH_MSG_USERAUTH_REQUEST to the value of
securityName. This allows a deployment without preconfigured
mappings between model-specific and model-independent names, but the
securityName will need to contain a username recognized by the
authentication mechanism.
5)The client will then invoke the "ssh-userauth" service to
authenticate the user, as described in the SSH authentication
protocol [SSHAuth].
6) If the authentication is unsuccessful, then the transport
connection should be closed, tmStateReference is discarded, the
message is discarded, an error indication (unknownSecurityName) is
returned to the calling module, and processing stops for this
message.
7) Once the user has been successfully authenticated, the client will
invoke the "ssh- connection" service, also known as the SSH
connection protocol [SSHConnect].
8) After the ssh-connection service is established, the client will
use an SSH_MSG_CHANNEL_OPEN message to open a channel of type
"session", providing a selected sender channel number, and a maximum
packet size based on maxMessageSize.
9) If successful, this will result in an SSH session. The
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destTransportDomain nd the destTransportAddress, plus the "recipient
channel" and "sender channel" and other relevant data from the
SSH_MSG_CHANNEL_OPEN_CONFIRMATION are added to the tmStateReference
for subsequent use.
10) Once the SSH session has been established, the SNMP engine will
invoke SNMP as an SSH subsystem called "SNMP". Running SNMP as an
SSH subsystem avoids the need for the script to recognize shell
prompts or skip over extraneous information, such as a system message
that is printed at shell start-up.
In order to allow SNMP traffic to be easily identified and filtered
by firewalls and other network devices, servers associated with SNMP
entities using the Secure Shell Security Model MUST default to
providing access to the "SNMP" SSH subsystem only when the SSH
session is established using the IANA-assigned TCP port (TBD).
Servers SHOULD be configurable to allow access to the SNMP SSH
subsystem over other ports.
[todo] check whether there is a better way to establish a tunnel for
SNMP messages.
[todo] Should we perform some type of engineID discovery to provide
the mapping between transport address, session, and engineID at this
point in the session establishment procedure? We have an established
channel; can we simply send a GET of snmpEngineID and record the
value i the tmStateReference?
11) [todo] the engine will perform an SNMP GET command requesting the
value of the remote engine's snmpEngineID object, and create a
tmSessionReference cache recording the following information:
the remote engine's snmpEngineID
the transport address
the recipient and sender channels
5.2 Discovery
Since snmpEngineID isn't really needed for authentication and
integrity checking, it becomes useful primarily for contextEngineID.
contextEngineID is useful for proxy, and for a management application
to uniquely identify an SNMP entity. Since snmpEngineID is an object
in the SNMP-FRAMEWORK-MIB, the mapping between engineID and transport
address could be established after a tunnel is established, or could
be determined using noAuthNoPriv (with suitable caveats).
[todo] Auto-discovery of SNMP devices is an important feature of many
NMS platforms. Should we simply use a noAuthNoPriv request, and
recommend an associated access control configuration that only makes
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accessible relatively benign data such as sysOID, sysDescription, and
snmpEngineID? Should we standardize this approach for all TMSM
models, including a "named policy" for what can be discovered (a
policy to be configured within whatever access control system is
used)?
Alternatively, can we let USM perform discovery so we don't have to
attenpt to establish an SSH connection first? USM is the mandatory-
to-implement security model, so this could make sense.
5.3 Generating an Outgoing SNMP Message
This section describes the procedure followed by the Secure Shell
Security Model whenever it generates a message containing a
management operation (like a request, a response, a notification, or
a report) on behalf of a user.
The parameters needed are supplied by the Message Processing Model
via the generateRequestMsg() or the generateResponseMsg() ASI
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
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 -- reference to session info
)
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statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
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 -- reference to session info
)
verify securityModel = sshsmSecurityModel
If there is a securityStateReference, extract the tmStateReference
information from the cachedSecurityData from the Request message.
[todo] With USM, at the point the cachedSecurityData can now be
discarded. Since we now have persistent sessions, this is no
longer true, but can some of the cached data be discarded, such as
message pair information?
If there is no securityStateReference, then lookup the session
info indexed by {securityEngineID, securityName, securityLevel},
and set tmStateReference.
If there is no session info for this index, then create an
incomplete tmStateReference indexed by the provided
{securityEngineID, securityName, securityLevel}. Store the
securityModel and maxMessageSize information. When the TMSP gets
the incomplete tmStateReference, it will recognize that it needs
to establish a new session, and fill in the rest of the
information for subsequent use.
fill in securityParameters [todo] a NULL octet string since
[todo] we don't need to send securityEngineID, unless it is
needed for a discovery mechanism..
[todo] we don't need to send Boots and Time values
[todo] we don't need to send a username, since we use the one
from SSH authentication
[todo] we don't need to call authenticateOutgoingMsg()
The wholeMsg is now serialized and then represents the
unauthenticated message being prepared.
The completed message (wholeMsg) with its length (wholeMsgLength)
and securityParameters (a NULL octet string) and tmStateReference
is returned to the calling module with the statusInformation set
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to success.
The Message Processing Model then passes information to the
disptacher for forwarding to the Transport Mapping.
5.4 Sending an Outgoing SNMP Message to the Network
The TMSP portion of the Secure Shell Security Model performs the
following tasks:
Uses tmStateReference to lookup session information.
[todo] verifies that auth and priv can be provided, as requested,
and error-out if not.
If the session information is incomplete (i.e, has no
tmTransportAddress), then call establishSession() using the
destTransportDomain and destTransportAddress (the output of the
PrepareOutgoingMessage() ASI) and the securityModel,
securityEngineID, securityName, securityLevel from the
tmStateReference. Store all information in the tmStateReference
for subsequent use.
An SSH_MSG_CHANNEL_DATA message is sent, indicating the recipient
channel and encapsulating the wholeMessage.
[todo] According to RFC 3411, section 4.1.1, the application provides
the transportDomain and transportAddress to the PDU dispatcher via
the sendPDU() primitive. If we permit multiple sessions per
transportAddress, then we would need to define how session
identifiers get passed from the application to the PDU dispatcher
(and then to the MP model).
[todo] The SNMP over TCP Transport Mapping document [RFC3430] says
that TCP connections can be recreated dynamically or kept for future
use and actually leaves all that to the transport mapping.
[todo] We might have a MIB module that records the session
information for subsequent use by the applications and other
subsytems, or it might be passed in the tmStateReference cache. For
notifications, I assume the SNMPv3 notification tables would be a
place to find the address, but I'm not sure how to identify the
presumably-dynamic session identifiers. The MIB module could
identify whether the session was initiated by the remote engine or
initiated by the current engine, and possibly assigned a purpose
(incoming request/response or outgoing notifications). First we need
to decide whether to handle notifications and requests in one or two
(or more) sessions. Do we use an established session bi-
directionally, or do we establish two separate sessions, one for each
direction as needed?
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5.5 [todo] Prepare Data Elements from an Incoming SNMP Message
For an incoming message, the TMSP will need to put information from
the transport mechanisms used into the tmStateReference so the MPSP
can extract the information and add it conceptually to the
securityStateReference.
5.6 Processing an Incoming SNMP Message
This section describes the procedure followed by an SNMP engine
whenever it receives a message containing a management operation on
behalf of a user.
To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule,
if state information is available when a message gets discarded, the
message-state information should also be released. Also, an error
indication can return an OID and value for an incremented counter and
optionally a value for securityLevel, and values for contextEngineID
or contextName for the counter. In addition, the
securityStateReference data is returned if any such information is
available at the point where the error is detected. [todo] this
paragraph may no longer be accurate because of persistent session
state information.
The abstract service primitive 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 -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
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
1) If the received securityParameters is not the serialization of an
OCTET STRING formatted according to the SSHsmSecurityParameters ,
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then the snmpInASNParseErrs counter [RFC3418] is incremented, and an
error indication (parseError) is returned to the calling module.
[todo] Note that we return without the OID and value of the
incremented counter, which may be important if this security model
supports generating a Report PDU (which SSHSM doesn't so far),
because in this case there is not enough information to generate a
Report PDU.
[todo] If we actually do not extract anything from
securityParameters, do we need to check whether this parses
correctly? It apparently parsed well enough to pass the parse test
in the messaging model. Could we simply ignore the
securityParameters being passed in? The only argument I see for
checking to ensure this is empty/null is to ensure somebody isn't
using the filed for non-standard purposes, such as passing a virus in
the field.
2) The SSHSM queries the associated SSH engine, in an
implementation-dependent manner, to determine the transport and
security parameters for the received message:
a) the transportDomain and transportAddress
b) the authentication parameters, including the authenticated
username
c) the encryption options,
d) the integrity-checking options
3) The securityEngineID to be returned to the caller is determined in
an implementation-dependent manner, such as by using the transport
address to perform a lookup in its Local Configuration Datastore
(LCD). If the securityEngineID is unknown, then an SNMP engine may
perform discovery to create a new entry in its LCD and continue
processing. Note that securityEngineID is required by the SNMPv3
message processing model in RFC 3412 section 7.2 13a)
4) If the information about the message security indicates that the
security options do not match the securityLevel requested by the
caller, then the SSHsmStatsUnsupportedSecLevels counter is
incremented and an error indication (unsupportedSecurityLevel)
together with the OID and value of the incremented counter is
returned to the calling module.
5) The scopedPDU component is assumed to be in plain text and is the
message payload to be returned to the calling module.
7) The maxSizeResponseScopedPDU is calculated. This is the maximum
size allowed for a scopedPDU for a possible Response message.
Provision is made for a message header that allows the same
securityLevel as the received Request.
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[todo] Is this relevant? Can it be calculated once for the session?
Do we need to take into consideration the SSH window size?
10) Information about the value of the SSH username is extracted from
the Local Configuration Datastore (LCD) to provide conversion from
the SSHSM model-specific username to a model-independent
securityName. If no information is available for the username, then
the securityName is set to the username used in the SSH-USER-AUTH-
REQUEST. [todo] Note that USM at this point would flag an
unknownSecurityName error.
11) The security data is cached as cachedSecurityData, so that a
possible response to this message can and will use the same
authentication and privacy parameters. Information to be saved/
cached is as follows:
transportDomain, transportAddress
securityEngineID
SSH username,
auth options [todo]
encryption options [todo]
Integrity checking options [todo]
12) 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 primitive.
6. MIB module definition
SSHSM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, mib-2 FROM SNMPv2-SMI
TEXTUAL-CONVENTION FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
SnmpSecurityModel FROM SNMP-FRAMEWORK-MIB;
sshsmMIB MODULE-IDENTITY
LAST-UPDATED "200509020000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chair:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
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69115 Heidelberg
Germany
+49 6221 90511-15
quittek@netlab.nec.de
Co-editors:
David Harrington
Effective Software
50 Harding Rd
Portsmouth, New Hampshire 03801
USA
+1 603-436-8634
ietfdbh@comcast.net
Juergen Schoenwaelder
International University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@iu-bremen.de
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
jsalowey@cisco.com
"
DESCRIPTION "The Secure Shell Security Model MIB
Copyright (C) The Internet Society (2005). This
version of this MIB module is part of RFC XXXX;
see the RFC itself for full legal notices.
-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
"
REVISION "200509020000Z" -- 02 September 2005
DESCRIPTION "The initial version, published in RFC XXXX.
-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and remove this note
"
::= { mib-2 xxxx }
-- RFC Ed.: replace xxxx with IANA-assigned number and
-- remove this note
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-- ---------------------------------------------------------- --
-- subtrees in the SSHSM-MIB
-- ---------------------------------------------------------- --
sshsmNotifications OBJECT IDENTIFIER ::= { sshsmMIB 0 }
sshsmObjects OBJECT IDENTIFIER ::= { sshsmMIB 1 }
sshsmConformance OBJECT IDENTIFIER ::= { sshsmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
-- -------------------------------------------------------------
-- sshsmMIB - Conformance Information
-- -------------------------------------------------------------
sshsmGroups OBJECT IDENTIFIER ::= { sshsmConformance 1 }
sshsmCompliances OBJECT IDENTIFIER ::= { sshsmConformance 2 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
sshsmGroup OBJECT-GROUP
OBJECTS {
}
STATUS current
DESCRIPTION
"Secure Shell Security Model objects"
::= { sshsmGroups 2 }
-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------
sshsmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for support of the
Secure Shell Security Model"
MODULE
MANDATORY-GROUPS {
}
::= { sshsmCompliances 1 }
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END
7. Security Considerations
This document describes a security model that would permit SNMP to
utilize SSH security services. [todo] expand as needed.
SSHv2 provides PFS for encryption keys. PFS is a major design goal
of SSH, and any well-designed keyex algorithm will provide it.
[todo] We will probably need to discuss the security implications of
password based authentication methods.
8. IANA Considerations
IANA is requested to assign:
a TCP port number which will be the default port for SNMP over SSH
sessions as defined in this document,
an SMI number under mib-2, for the MIB module in this document,
an SNMP SecurityModel for the Secure Shell Security Model, as
documented in the MIB module in this document,
9. Acknowledgments
The editors would like to thank Jeffrey Hutzelman and Nicholas
Williams for sharing their SSH insights.
10. References
10.1 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,
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December 2002.
[RFC3430] Schoenwaelder, J., "Simple Network Management Protocol
(SNMP) over Transmission Control Protocol (TCP) Transport
Mapping", RFC 3430, December 2002.
[TMSM] Harrington, D. and J. Schoenwaelder, "Transport Mapping
Security Model (TMSM) for the Simple Network Management
Protocol version 3 (SNMPv3)",
draft-schoenw-snmp-tlsm-04.txt (work in progress),
August 2005.
[SSHArch] Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-22 (work in progress),
March 2005.
[SSHTrans]
Lonvick, C., "SSH Transport Layer Protocol",
draft-ietf-secsh-transport-24 (work in progress),
March 2005.
[SSHAuth] Lonvick, C. and T. Ylonen, "SSH Authentication Protocol",
draft-ietf-secsh-userauth-27 (work in progress),
March 2005.
[SSHConnect]
Lonvick, C. and T. Ylonen, "SSH Connection Protocol",
draft-ietf-secsh-connect-25 (work in progress),
March 2005.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[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.
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10.2 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.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[Netconf] Enns, R., "NETCONF Configuration Protocol",
ID draft-ietf-netconf-prot-04.txt, October 2004.
[NetconfSSH]
Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure Shell (SSH)",
draft-ietf-netconf-ssh-04 (work in progress), April 2005.
[gsskeyex]
Hutzelman, J., "GSSAPI Authentication and Key Exchange for
the Secure Shell Protocol", draft-ietf-secsh-gsskeyex-09
(work in progress), May 2005.
Authors' Addresses
David Harrington
Effective Software
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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Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
Email: jsalowey@cisco.com
Appendix A. Change Log from the first revision of -00-
-00-b draft:
re-ordered the sections from abstract to concrete.
worked on how the SSHSM fitsinto the RFC3411 and TMSM
architectures.
added goals tot he Motivation section.
worked on Security Capabilities based on input from Joe, Nick, and
JHutz. Added Joe to the authors list based on contributed text.
created Data origin Authentication section, to separate this
discussion.
expanded "Authentication Protocol" section
Updated Message replay section.
--00-c draft
worked on security information cacheing, including breaking caches
into session, message, and model-independent (which we probably
want to remerge later)
eliminated a lot of TMSM-carryover stuff and modified to be SSHSM-
specific.
updated references.
filled in Elements of Procedure for Outgoing Messages
created shell for SSHSM-MIB
-01- draft
added Processing an Incoming Message
updated change log
modified masquerade discussion to differentiate server vs user
authentication
added [todos] for Data origin Authentication Issues section,
trying to nail down whether we even need this section.
disallow the 'none' MAC algorithm
added text to Message secuirty versus session security
wordsmithed "authentication protocol" section
rewrote "Mapping SSH to EngineID", eliminating most [todos]
modified "Establishing a Session"
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