[Isms] SSHSM pre-release
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[Isms] SSHSM pre-release
Hi,
We need reviews to both TMSM and SSHSM drafts at this point to make
sure that the big decisions we have made have been applied properly to
the documents, and that we haven't overlooked any important factors
while doing the design. (That's not an open invitation to recommend
unnecessary, nifty-to-have new features). Please read the minutes of
the last IETF meeting if you don't know what big decisions we have
reached consensus on; we reviewed them at that meeting.
To make progress, I had to "interpret" how some of the decisions would
apply, so you should definitely look at how I interpreted some of the
"ripples" from the WG decisions.
The TMSM I-D was published a week or two ago. I believe it is in
fairly good shape.
I'm still working on the SSHSM draft. It's heavy going; I've gotten
most of the expository text updated. I'll post it here for the WG to
be able to review it at this stage while I keep working on it. I'll
try to get an official I-D published by the end of next week. The
attached file doesn't pass id-nits because of some references and
miscellaneous stuff that will be fixed in the I-D.
We've addressed the big issues in the WG; now we need to get into the
details.
I am now working on the elements of procedure for SSHSM, where we need
to define the sequence of necessary steps, and where the data to
perform each step comes from, including the MIB module definitions. I
am finding that the tmsmSessionTable needs to be used by the EOP to
lookup info on the session at a point where the EOP has no knowledge
of a sessionID. We will almost certainly need to change the indices
for the table. The transportDomain and transportAddress need to be
added to generateRequest() and generateResponse() so the MPSP has the
address when finding an existing session to use. Knowing who an
existing session is connected to is somewhat important ;-)
Enjoy reading.
David Harrington
dharrington at huawei.com
dbharrington at comcast.net
ietfdbh at comcast.net
Network Working Group D. Harrington
Internet-Draft Futurewei Technologies
Expires: November 16, 2006 J. Salowey
Cisco Systems
May 15, 2006
Secure Shell Security Model for SNMP
draft-ietf-isms-secshell-02.9.txt
Status of This Memo
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This Internet-Draft will expire on November 16, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
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. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Modularity . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Conventions . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. The Secure Shell Protocol . . . . . . . . . . . . . . . . 7
1.6. Constraints . . . . . . . . . . . . . . . . . . . . . . . 7
2. How SSHSM Fits into the TMSM Architecture . . . . . . . . . . 8
2.1. Security Capabilities of this Model . . . . . . . . . . . 9
2.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.2. SSHSM Sessions . . . . . . . . . . . . . . . . . . . . 11
2.1.3. Authentication Protocol . . . . . . . . . . . . . . . 12
2.1.4. Privacy Protocol . . . . . . . . . . . . . . . . . . . 12
2.1.5. Protection against Message Replay, Delay and
Redirection . . . . . . . . . . . . . . . . . . . . . 12
2.1.6. Security Protocol Requirements . . . . . . . . . . . . 13
2.2. Security Parameter Passing Requirement . . . . . . . . . . 14
2.3. Requirements for Notifications and Proxy . . . . . . . . . 14
3. RFC 3411 Abstract Service Interfaces . . . . . . . . . . . . . 15
3.1. Public Abstract Service Interfaces . . . . . . . . . . . . 15
3.1.1. Public ASIs for Outgoing Messages . . . . . . . . . . 15
3.1.2. Public ASIs for Incoming Messages . . . . . . . . . . 18
4. SNMP Messages Using this Security Model . . . . . . . . . . . 19
4.1. SNMPv3 Message Fields . . . . . . . . . . . . . . . . . . 19
4.1.1. msgGlobalData . . . . . . . . . . . . . . . . . . . . 21
4.1.2. msgSecurityParameters . . . . . . . . . . . . . . . . 21
4.2. Passing Security Parameters . . . . . . . . . . . . . . . 21
4.2.1. tmStateReference . . . . . . . . . . . . . . . . . . . 21
4.2.2. securityStateReference . . . . . . . . . . . . . . . . 22
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 23
5.1. Generating an Outgoing SNMP Message . . . . . . . . . . . 23
5.2. MPSP . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.3. Sending an Outgoing SNMP Message to the Network . . . . . 25
5.3.1. TMSP . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.4. [todo] Prepare Data Elements from an Incoming SNMP
Message . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.5. Processing an Incoming SNMP Message . . . . . . . . . . . 26
5.6. Establishing a Session . . . . . . . . . . . . . . . . . . 28
5.7. Closing a Session . . . . . . . . . . . . . . . . . . . . 30
5.8. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 31
6. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7. Structure of the MIB Module . . . . . . . . . . . . . . . . . 31
7.1. Textual Conventions . . . . . . . . . . . . . . . . . . . 31
7.2. The sshsmStats Subtree . . . . . . . . . . . . . . . . . . 32
7.3. The sshsmsSession Subtree . . . . . . . . . . . . . . . . 32
7.4. Relationship to Other MIB Modules . . . . . . . . . . . . 32
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7.4.1. Relationship to the SNMPv2-MIB . . . . . . . . . . . . 32
7.4.2. Relationship to the SNMP-FRAMEWORK-MIB . . . . . . . . 32
7.4.3. Relationship to the TMSM-MIB . . . . . . . . . . . . . 32
7.4.4. MIB Modules Required for IMPORTS . . . . . . . . . . . 33
8. MIB module definition . . . . . . . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . . 37
9.1. noAuthPriv . . . . . . . . . . . . . . . . . . . . . . . . 37
9.2. skipping public key verification . . . . . . . . . . . . . 38
9.3. the 'none' MAC algorithm . . . . . . . . . . . . . . . . . 38
9.4. MIB module security . . . . . . . . . . . . . . . . . . . 38
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
12.1. Normative References . . . . . . . . . . . . . . . . . . . 40
12.2. Informative References . . . . . . . . . . . . . . . . . . 41
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 42
A.1. Closed Issues . . . . . . . . . . . . . . . . . . . . . . 42
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 48
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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 Security Model extension [I-D.ietf-isms-tmsm]. The
security model specified in this memo is referred to as the Secure
Shell Security Model (SSHSM).
This memo also 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.
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.
1.1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [RFC3410].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. MIB objects are generally
accessed through the Simple Network Management Protocol (SNMP).
Objects in the MIB are defined using the mechanisms defined in the
Structure of Management Information (SMI). This memo specifies a MIB
module that is compliant to the SMIv2, which is described in STD 58,
RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
[RFC2580].
1.2. Modularity
The reader is expected to have read and understood the description of
the SNMP architecture, as defined in [RFC3411],and the TMSM
architecture extension specified in "Transport Mapping Security Model
(TMSM) Architectural Extension for the Simple Network Management
Protocol" [I-D.ietf-isms-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 [RFC4251].
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.
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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.
1.3. 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
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 usability in operational environments to
achieve high deployment success and at the same time minimize
implementation and deployment costs to minimize the time until
deployment is possible.
The work will address the requirement for the SSH client to
authenticate the SSH server, for the SSH server to authenticate the
SSH client, and how SNMP can make use of the authenticated identities
in message authentication and access control.
The work will include the ability to use any of the client
authentication methods described in "SSH Authentication Protocol"
[RFC4252] - 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]. SSHSM SHOULD be able to take
advantage of other defined authentication mechanism such as those
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defined in [RFC4462] and future mechanisms such as those that make
use of X.509 certificate credentials. This will allow SSHSM to
utilize client authentication and key exchange mechanisms which
support different security infrastructures and provide different
security properties.
It is desirable to use mechanisms that could unify the approach for
administrative security for SNMPv3 and Command Line interfaces (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 [I-D.ietf-
netconf-ssh]. This memo describes a method for invoking and running
the SNMP protocol within a Secure Shell (SSH) session as an SSH
subsystem.
This memo describes how SNMP can be used within a Secure Shell (SSH)
session, using the SSH connection protocol [RFC4254] over the SSH
transport protocol , using SSH user-auth [RFC4252]for authentication.
There are a number of challenges to be addressed to map Secure Shell
authentication method parameters into the SNMP architecture so that
SNMP continues to work without any surprises. These are discussed in
detail below.
1.4. Conventions
The terms "manager" and "agent" are not used in this document,
because in the RFC 3411 architecture, all SNMP entities have the
capability of acting as 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 Responder, 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 SNMP entity may act as
client or as server, as discussed further below.
While SSH and USM frequently refer to a user, the terminology used in
RFC3411 [RFC3411] and in this memo is "principal". A principal is
the "who" on whose behalf services are provided or processing takes
place. A principal can be, among other things, an individual acting
in a particular role; a set of individuals, with each acting in a
particular role; an application or a set of applications; and
combinations thereof.
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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]
Sections requiring further editing are identified by [todo] markers
in the text. Points requiring further WG research and discussion are
identified by [discuss] markers in the text.
1.5. 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 [[RFC4253] provides server
authentication, and message 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 [RFC4252] authenticates the
client-side principal to the server. It runs over the transport
layer protocol.
o The Connection Protocol [RFC4254] multiplexes the encrypted tunnel
into several logical channels. It runs over the transport after
successfully authenticating the principal.
The client sends a service request once a secure transport layer
connection has been established. A second service request is sent
after client 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.
1.6. 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 effective management.
2. In times of network stress, the security protocol and its
underlying security mechanisms SHOULD NOT depend upon the ready
availability of other network services (e.g., Network Time
Protocol (NTP) or AAA protocols).
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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 model should require no changes to the SNMP
architecture.
6. A security model should require no changes to the underlying
security protocol.
2. How SSHSM Fits into the TMSM Architecture
SSH is a security layer which is plugged into the TMSM architecture
extension between the underlying transport layer and the message
dispatcher [RFC3411].
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
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 follows the TMSM approach, in which the security-model has two
separate areas of security processing - transport-mapping-related
security processing (TMSP) within the transport mapping section of
the dispatcher, and message processor security processing (MPSP)
which happens within the security model subsystem of the message
processor.
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.
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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 - SSHSM provides for verification
that each received SNMP message has not been modified during its
transmission through the network.
2. Information modification - SSHSM provides 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 - SSHSM provides for both verification of the identity
of the SSH server and verification of the identity of the SSH
client - the principal on whose behalf a received SNMP message
claims to have been generated. It is not possible to assure the
specific principal that originated a received SNMP message;
rather, it is the principal on whose behalf the message was
originated that is authenticated. SSH provides verification of
the identity of the SSH server through the SSH Transport Protocol
server authentication [RFC4253]
4. Verification of principal identity is important for use with the
SNMP access control subsystem, to ensure that only authorized
principals 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 both the SSH server and the SSH client ensures the
authenticity of the SNMP engine that provides MIB data, whether
that engine resides on the server or client side of the
association. Operators or management applications might 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 provider of MIB data is authentic.
6. Disclosure - SSHSM provides that the contents of each received
SNMP message are protected from disclosure to unauthorized
persons.
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7. Replay - SSH ensures that cryptographic keys established at the
beginning of the SSH session and stored in the SSH session state
are fresh new session keys generated for each session. These are
used to authenticate and encrypt data, and to prevent replay
across sessions. SSH uses sequence information to prevent the
replay and reordering of messages within a session.
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)
The Secure Shell protocol provides support for encryption and data
integrity. While it is technically possible to support no
authentication and no encryption in SSH it is NOT RECOMMENDED by
[RFC4253].
SSHSM extracts from SSH the identity of the authenticated principal,
and the type and address associated with an incoming message, and
SSHSM provides this information to SSH for an outgoing message. The
transport layer algorithms used to provide authentication, data
integrity and encryption SHOULD NOT be exposed to the SSHSM layer.
In SNMPv3, we deliberately avoided this and settled for an assertion,
using msgFlags, that auth and priv were applied according to the
rules of the security model. However, SSHSM has no mechanisms by
which it can test whether an underlying SSH connection provides auth
or priv to meet a desired msgFlags setting, so the SSHSM trusts that
the underlying SSH connection has been properly configured to support
security characteristics at least as strong as requested in msgFlags.
SSH does not understand msgFlags, and SSHSM does not know about the
algorithms or options for the SSH session to open SSH sessions that
match different securityLevels. For interoperability of the trust
assumptions between SNMP engines, an SSHSM-compliant implementation
MUST use an SSH connection that provides authentication, data
integrity and encryption that meets the highest level of SNMP
security (authPriv). Outgoing messages requested by SNMP
applications and specified with a lesser securityLevel (noAuthNoPriv
or authNoPriv) are sent by SSHSM as authPriv securityLevel. Other
security models, where the actual securityLevel applied to the
connection can be determined or controlled, can be used when a lesser
level of security is desired.
Implementations SHOULD support whatever authentications are provided
by SSH. The security protocols used in [RFC4253] are considered
acceptably secure at the time of writing. However, the procedures
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allow for new authentication and privacy methods to be specified at a
future time if the need arises.
2.1.2. SSHSM Sessions
The Secure Shell security model will utilize TMSM sessions, with a
single combination of transportAddress, engineID, securityName,
securityModel, and securityLevel associated with each session. A
TMSM session is associated with state information that is maintained
for its lifetime. All SSHSM sessions will utilize the authPriv
securityLevel, and all incoming SSHSM messages will be treated as
having been delivered through authenticated, integrity-checked, and
encrypted connections.
SSHSM sessions are opened during the elements of procedure for an
outgoing SNMP message, never during the elements of procedure for an
incoming message. Implementations MAY choose to instantiate sessions
in anticipation of outgoing messages.
2.1.2.1. Message security versus session security
As part of session creation, the client and server entities are
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
messages. 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 identities. After a message is received and passes
integrity and authentication checks, the state stored in the session
is used to provide further authorization for the message.
[discuss] this sounds like a discussion of what happens at the SSH
layer, not the SSHSM layer; Isn't the SSH state maintained by the SSH
layer? Isn't all this invisible to the SSHSM security model? If an
incoming message fails SSH authentication, integrity, or decryption
then it will not be passed to the SSHSM layer, right?
[discuss] to make it possible for SSHSM to send an outgoing message
via an SSH connection, what state needs to be known/preserved by
SSHSM to tell SSH which session to use?
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2.1.3. Authentication Protocol
SSHSM should support any client authentication mechanism supported by
SSH. This includes the three authentication methods described in the
SSH Authentication Protocol document [RFC4252] - publickey, password,
and host-based.
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 [RFC3588]
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 protocol such as CHAP [RFC1994] or digest
authentication [RFC 2617, draft-ietf-radext-digest-auth-04] to
integrate with RADIUS or Diameter. These mechanisms leave the
password in the clear on the device that is authenticating the
password which introduces threats to the authentication
infrastructure.
GSSKeyex [RFC4462] provides a framework for the addition of client
authentication mechanisms which support different security
infrastructures and provide different security properties.
Additional authentication mechanisms, such as one that supports X.509
certificates, may be added to SSH in the future.
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 delay the
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transmission of all packets from a particular point in a session
since SSH protects the ordering of packets.
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
usages. This document will describe the use of an SSH subsystem for
SNMP to make SNMP usage distinct from other usages.
2.1.6.1. Troubleshooting
SSHSM will likely not work in conditions where access to the CLI has
stopped working. In situations where SNMP access has to work when
the CLI has stopped working, the use of USM should be considered
instead of SSHSM.
2.1.6.2. Coexistence
The Secure Shell security model can coexist with the USM security
model, the only other currently defined security model. [discuss] #6:
Are there are any wrinkles to coexistence with SNMPv1/v2c/USM?
Note that RFC3584 discusses how to transfer fields between SNMPv3 and
SNMPv1 messages. [todo] this area needs detailed analysis.
2.1.6.3. 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. A Command Responder application
uses a contextEngineID equal to the snmpEngineID of its associated
SNMP engine, and the contextEngineID is included in a scopedPDU to
identify the engine associated with the data contained in the PDU.
securityEngineID - 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
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and authentication are handled outside the SNMP engine. The
RFC3411 architecture defines ASIs that include a securityEngineID;
SSHSM should always set the securityEngineID equal to the
snmpEngineID to satisfy the elements of procedure for
generateRequestMsg() defined in RFC3412.
2.2. Security Parameter Passing Requirement
Security-model-specific parameters for an incoming message are
determined from the SSH layer by the transport mapping security
processor (TMSP), before the message processing begins. The TMSP
accepts (decrypted) messages from the SSH subsystem, and records the
transport-related information and the security-related information,
including authenticated identity, in a cache referenced by
tmStateReference, and passes the WholeMsg and the tmStateReference to
the MPSP (via the dispatcher).
For outgoing messages, the security-model-specific parameters are
gathered by the messaging-security-processor (MPSP) and passed with
the outgoing message to the transport mapping. The MPSP portion of
the security model creates the WholeMsg from its component parts. In
the SSHSM model, an SNMPv3 message is built without any content in
the SecurityParameters field of the message, and the WholeMsg is
passed unencrypted back to the Message Processing Model for
forwarding to the Transport Mapping. The MPSP takes input provided
by the SNMP application, converts that information into suitable
security parameters for SSHSM, and passes these in a cache referenced
by tmStateReference to the TMSP (via the dispatcher). The TMSP
establishes sessions as needed and passes messages to the SSH
subsystem for processing.
The cache reference is an additional parameter in the ASIs between
the transport mapping and the messaging security model.
This approach does create dependencies between a model-specific TMSP
and a corresponding specific MPSP. Passing a model-independent cache
reference as a parameter in an ASI is consistent with the
securityStateReference cache already being passed around in the ASI.
2.3. Requirements for Notifications and Proxy
SSH connections may be initiated by command generators or by
notification originators. Command generators are frequently operated
by a human, but notification originators frequently are unmanned
automated processes. As a result, it usually will be necessary to
provision authentication credentials on the SNMP engine containing
the notification originator, or use a third party key provider such
as Kerberos, so the engine can successfully authenticate to an engine
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containing a notification receiver.
The SNMP-TARGET-MIB module [RFC3413] contains objects for defining
management targets, including transport domains and addresses and
security parameters, for applications such as notifications and
proxy.
For SSHSM, transport type and address are configured in the
snmpTargetAddrTable, and the securityModel, securityName, and
securityLevel parameters are configured in the snmpTargetParamsTable.
The default approach is for an administrator to statically
preconfigure this information to identify the targets authorized to
receive notifications or perform proxy.
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 between subsystems, such as
the dispatcher and the Message Processing Subsystem. These RFC 3411-
defined data flows are referred to here as public interfaces.
[todo] Do we need these in a separate section like this? Can we move
these to the EOP section?
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:
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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:
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:
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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
)
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
o statusInformation - An indication of whether the encoding and
securing of the message was successful. If not it contains an
indication of the problem.
o messageProcessingModel - The SNMP version number for the message
to be generated.
o globalData - The message header (i.e., its administrative
information). This data is opaque to SSHSM.
o maxMessageSize - The maximum message size as included in the
message. This data is not used by SSHSM.
o securityModel - The securityModel in use. Should be the SSH
Security Model.
o securityEngineID - SSHSM always sets this to the snmpEngineID of
the sending SNMP engine.
o 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 securityStateReference
cache is used.
o securityLevel - Ignored by SSHSM, which always uses an authPriv
securityLevel.
o scopedPDU - The message payload. The data is opaque to SSHSM.
o securityStateReference - A handle/reference to cachedSecurityData
to be used when securing an outgoing Response message. This is
the exact same securityStateReference as was generated by the SSH
Security module when processing the incoming Request message to
which this is the Response message.
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o securityParameters - Always set to empty by SSHSM.
o wholeMsg - The fully encoded SNMP message ready for sending on the
wire.
o 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:
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::
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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
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. SNMPv1 and SNMPv2c have been declared Historic, so this memo
only deals with SNMPv3 messages.
The processing is compatible with the RFC 3412 primitives,
generateRequestMsg() and processIncomingMsg(), that show the data
flow between the Message Processor and the MPSP.
4.1. SNMPv3 Message Fields
The SNMPv3Message SEQUENCE is defined in [RFC3412].
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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
The following describes how SSHSM treats certain fields in the
message:
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4.1.1. msgGlobalData
msgGlobalData is opaque to SSHSM. The values are set by the Message
Processing model (e.g., SNMPv3 Message Processing), and are not
modified by SSHSM.
msgMaxSize is determined by the implementation.
To avoid the need to mess with the ASN.1 encoding, msgGlobalData
contains the value of msgFlags set by the Message Processing model
(e.g., SNMPv3 Message Processing), not the actual (authPriv)
securityLevel applied to the message by SSHSM.
msgSecurityModel is set by the Message Processing model (e.g.,
SNMPv3) to the IANA-assigned value for the Secure Shell Security
Model. See http://www.iana.org/assignments/snmp-number-spaces.
4.1.2. 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.
The field msgSecurityParameters in SNMPv3 messages has a data type of
OCTET STRING. To prevent its being used in a manner that could be
damaging, such as for carrying a virus or worm, when used with SSHSM
its value MUST be the BER serialization of a zero-length OCTET
STRING.
SSHSMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
SSHsmSecurityParameters ::=
SEQUENCE {
OCTET STRING
}
END
4.2. Passing Security Parameters
For SSHSM, there are two levels of state that need to be maintained:
the session state, and the message state.
4.2.1. tmStateReference
For each session, SSHSM stores information about the session in the
Local Configuration Datastore, supplemented with a cache to store
model- and mechanism-specific parameters.
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Upon opening an SSH connection, the TMSP will store the transport
parameters in the tmSessionTable of the TMSM-MIB [I-D.ietf-isms-tmsm]
for subsequent usage.
tmsmSessionID = a unique local identifier
tmsmTransport = transportDomainSSH
tmsmSessionAddress = a TransportAddressSSH
tmsmSessionSecurityModel - SSHSM
tmsmSessionSecurityLevel = "authPriv"
tmsmSessionSecurityName = the principal name authenticated by SSH.
How this data is extracted from the SSH environment and how it is
translated into a securityName is implementation-dependent. By
default, the tmSecurityName is the name that has been successfully
authenticated by SSH, from the user name field of the
SSH_MSG_USERAUTH_REQUEST message.
tmsmSessionEngineID = if known, the value of the remote engine's
snmpEngineID. [discuss] is this the appropriate place to store an
engineID? It isn't used by SSHSM or TMSM.
How the SSH identity is extracted from the SSH layer, and how the SSH
identity is mapped to a securityName for storage in tmsmSessionTable
is implementation-dependent. Additional information may be stored in
a local datastore (such as a preconfigured mapping table) or in a
cache, such as the value of an SSH session identifier (as distinct
from the tmsmSessionID).
The tmStateReference is used to pass references to the appropriate
session information between the TMSP and MPSP through the ASIs.
The SSHSM has the responsibility for explicitly releasing the
complete tmStateReference and deleting the associated
tmsmSessionEntry in the tmsmSessionTable when the session is
destroyed.
4.2.2. securityStateReference
For each message received, SSHSM caches message-specific security
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. The securityStateReference is used to preserve the data
needed to generate a Response message with the same security
information. This information includes the model-independent
parameters (securityName, securityLevel, transport address, and
transport type). The Message Processing Model has the responsibility
for explicitly releasing the securityStateReference when such data is
no longer needed. The securityStateReference cached data may be
implicitly released via the generation of a response, or explicitly
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released by using the stateRelease primitive, as described in RFC
3411 section 4.5.1."
The SSH standard does not require that a session be maintained nor
that it be closed when the keys associated with the host or client
associated with the session are changed. Some SSH implementations
might close an existing session if the keys associated with the
session change. For SSHSM, if the session is closed between the time
a Request is received and a Response message is being prepared, then
the Response should be discarded.
The parameters associated with an incoming request message to be
applied to the outgoing response.
messageProcessingModel = SNMPv3
securityModel = SSHSM
sessionID = tmSessionID
5. Elements of Procedure
5.1. Generating an Outgoing SNMP Message
This section describes the procedure followed by the MPSP portion of
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. The
TMSM extension has added the tmStateReference to these ASIs.
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN 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
)
5.2. MPSP
1) verify securityModel = sshsmSecurityModel
determine whether we need to use the SSH subsystem for Request/
Responses ("SNMP"), or for Notifications ("SNMPNotification") or
Reports. [discuss] #34 - how do we determine this?
2) If there is a securityStateReference (indicating this must be a
Response or Report message), then extract the tmStateReference,
securityModel, securityName, and securityLevel from the
cachedSecurityData. At this point, the cachedSecurityData can now
be discarded.
2b) If the session referenced by tmStateReference does not still
exist then an error indication (noAvailableSession) is returned to
the calling module and the message is discarded. [todo] add an
approrpiate object to TMSM-MIB.
3) If there is no securityStateReference, then lookup the session
in the tmsmSessionTable using {securityModel, securityName,
securityLevel}, and create a tmStateReference.
[discuss] tmsmSessionTable is indexed by SessionID, which we do
not have, and a unique tmsmSessionEntry requires knowing the
transport and address, which we do not have. Even if we could
look up the appropriate entry, are we not likely to need more than
the model-independent sName/Model/Level info, such as the SSH send
and receive channels?
4) If there is no session info for this index, then create an
incomplete tmStateReference indexed by the provided {securityName,
securityLevel, and securityModel}. Store the securityModel and
maxMessageSize information. When the TMSP gets the incomplete
tmStateReference, it will recognize that it needs to establish a
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new session, and fill in the rest of the information for
subsequent use.
5) fill in the securityParameters with the serialization of a
zero-length OCTET STRING.
6) The wholeMsg is now serialized and then represents the
unauthenticated message being prepared.
7) The completed message (wholeMsg) with its length
(wholeMsgLength) and securityParameters (a zero-length octet
string) and tmStateReference is returned to the calling module
with the statusInformation set to success.
The Message Processing Model then passes information to the
disptacher for forwarding to the Transport Mapping.
5.3. Sending an Outgoing SNMP Message to the Network
The Dispatcher passes the information to the Transport Mapping using
the ASI defined in the TMSM extension:
5.3.1. TMSP
The TMSP portion of the Secure Shell Security Model performs the
following tasks:
8) Uses tmStateReference to lookup session information.
10) If the session information is incomplete (i.e, has no
tmTransportAddress), then call openSession() using the
destTransportDomain and destTransportAddress (the output of the
PrepareOutgoingMessage() ASI) and the securityModel, securityName,
securityLevel from the tmStateReference. Store all information in
the tmStateReference for subsequent use.
11) An SSH_MSG_CHANNEL_DATA message is sent, indicating the
recipient channel and encapsulating the wholeMessage.
[discuss] #28: For notification tables, how do we predefine the
dynamic session identifiers?
5.4. [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.
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5.5. 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, and if state
information is available when a session is closed, the session 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, and
the contained OCTET STRING is not empty, then the snmpInASNParseErrs
counter [RFC3418] is incremented, and an error indication
(parseError) is returned to the calling module. 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,
because in this case there is not enough information to generate a
Report PDU.
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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) tmSecurityName - an identifier for the authenticated entity
c) whether authentication is on or off,
d) whether encryption is on or off,
e) integrity-checking options
3) The securityEngineID is set to the local snmpEngineID, to satisfy
the SNMPv3 message processing model in RFC 3412 section 7.2 13a).
[discuss] verify this works.
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.
10) Information about the value of tmSecurityName is extracted from
the Local Configuration Datastore (LCD) to provide conversion from
the SSH authentication-method-specific tmSecurityName to a model-
independent securityName. If no information is available for the
username in the LCD, then the securityName is set to the username
associated with the session.
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: [todo] copy from the "Passing Security
Parameters" section above.
transportDomain, transportAddress
SSH username,
auth options
encryption options
Integrity checking options
12) The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified in
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the processIncomingMsg primitive.
5.6. Establishing a Session
The Secure Shell Security Model provides the following primitive to
pass data back and forth between the Transport Mapping portion of the
Security Model and the SSH service:
statusInformation =
openSession(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN securityModel -- Security Model to use
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 server to run SNMP over SSH. This
process is followed by any SNMP engine establishing a session for
subsequent use.
This will done automatically by an SNMP 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
openSession() 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
encryption, then the message cannot be sent. An error indication
(unsupportedSecurityLevel) is returned to the calling module.
3) Using destTransportDomain and destTransportAddress, the client
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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 transportDomain and transportAddress are
saved. If the attempt to establish a connection is unsuccessful,
then an error indication [todo] will be returned, and [todo]
processing stops.
[discuss] How do we open an SSH connection? Do we need to specify
what type of authentication we want (password, publickey, etc.)? Do
we have to specify that we want encryption?
4) The provided transport address, securityName and securityLevel are
used to lookup the associated entry in the Local Configuration
Datastore (LCD), and the model-specific information concerning the
principal at the destination is extracted. This step allows
preconfiguration of model-specific principals mapped to the
transport/name/level, for example, for sending notifications using
host-only authentication. Set the username in the
SSH_MSG_USERAUTH_REQUEST to the username extracted from the LCD.
If information about the principal 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 [RFC4252].
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 principal has been successfully authenticated, the client
will invoke the "ssh- connection" service, also known as the SSH
connection protocol [RFC4254].
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.
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9) If successful, this will result in an SSH session. The
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) 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. Once the SSH
session has been established, the SNMP engine will invoke SNMP as an
SSH subsystem, as indicated in the "subsystem" parameter.
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.
[discuss] We must perform some type of engineID discovery to provide
the mapping between transport address, SSH session, TMSM session, and
engineID at this point in the session establishment procedure. The
engine cannot perform an SNMP GET command requesting the value of the
remote engine's snmpEngineID object,
11) Create a tmStateReference cache (and/or sshsmSessionEntry)
recording the following information:
the remote engine's snmpEngineID
the transport address
the recipient and sender channels
5.7. Closing a Session
The Secure Shell Security Model provides the following primitive to
pass data back and forth between the Security Model and the SSH
service:
statusInformation =
closeSession(
IN sessionID
)
The following describes the procedure to follow to close a session
between a client and sever to run SNMP over SSH. This process is
followed by any SNMP engine closing the corresponding SNMP session.
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The Secure Shell Security Model identifies which session should be
closed to the SSH client code, using the closeSession() ASI.
[discuss] #23: We need to discuss the circumstances under which a
session should be closed, and how an SNMP engine should determine if,
and respond if the SSH session is closed by other means.
5.8. 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).
[discuss] #24: How should we enable auto-discovery? 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 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)? ** The problem is that the agent won't respond to a GET if you
do not ** have the proper engineID. So I think we need a discovery
procedure ** which is based on reports.
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.
6. Overview
7. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree
is organized as a set of related objects. The overall structure and
assignment of objects to their subtrees, and the intended purpose of
each subtree, is shown below.
7.1. Textual Conventions
Generic and Common Textual Conventions used in this document can be
found summarized at http://www.ops.ietf.org/mib-common-tcs.html
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7.2. The sshsmStats Subtree
This subtree contains SSHSM security-model-dependent counters.
This subtree provides information for identifying fault conditions
and performance degradation.
7.3. The sshsmsSession Subtree
This subtree contains SSHSM security-model-dependent information
about sessions.
7.4. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing SSHSM. In particular, it is assumed that
an entity implementing SSHSM will implement the SNMPv2-MIB [RFC3418],
the SNMP-FRAMEWORK-MIB [RFC3411] and the TMSM-MIB [I-D.ietf-isms-
tmsm].
This MIB module is for managing SSHSM-specific information.
7.4.1. Relationship to the SNMPv2-MIB
The 'system' group in the SNMPv2-MIB [RFC3418] is defined as being
mandatory for all systems, and the objects apply to the entity as a
whole. The 'system' group provides identification of the management
entity and certain other system-wide data. The SSHSM-MIB does not
duplicate those objects. [todo] if the SSHSM-MIB does not actually
have dependencies on SNMPv2-MIB other than imports, then remove this
paragraph.
7.4.2. Relationship to the SNMP-FRAMEWORK-MIB
[todo] if the SSHSM-MIB does not actually have dependencies on SNMP-
FRAMEWORK-MIB other than imports, then remove this paragraph.
7.4.3. Relationship to the TMSM-MIB
The 'tmsmSession' group in the TMSM-MIB [I-D.ietf-isms-tmsm] is
defined as being applicable to all Transport-Mapping Security Models
that use sessions. [todo] if the SSHSM-MIB does not actually have
dependencies on TMSM-MIB other than imports, then remove this
paragraph.
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7.4.4. MIB Modules Required for IMPORTS
The following MIB module imports items from [RFC2578], [RFC2579],
[RFC2580], [RFC3411], [RFC3419], and [I-D.ietf-isms-tmsm]
8. MIB module definition
** Is AES the only officially required to support SSH encryption **
mechanisms? It seems RFC 4344 has much more to offer. BTW, is it **
useful to export all this information in an SSHSM MIB module? Some
** of the stuff seems generic SSH...
SSHSM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, mib-2, Counter32, Integer32
FROM SNMPv2-SMI
TestAndIncr, AutonomousType
FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
SnmpAdminString, SnmpSecurityLevel, SnmpEngineID
FROM SNMP-FRAMEWORK-MIB
TransportAddress, TransportAddressType
FROM TRANSPORT-ADDRESS-MIB
tmsmSessionID
FROM TMSM-MIB
;
sshsmMIB MODULE-IDENTITY
LAST-UPDATED "200509020000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms at lists.ietf.org
Subscribe: isms-request at lists.ietf.org
Chairs:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
+49 6221 90511-15
quittek at netlab.nec.de
Juergen Schoenwaelder
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International University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder at iu-bremen.de
Co-editors:
David Harrington
Effective Software
50 Harding Rd
Portsmouth, New Hampshire 03801
USA
+1 603-436-8634
ietfdbh at comcast.net
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
jsalowey at 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
-- ---------------------------------------------------------- --
-- subtrees in the SSHSM-MIB
-- ---------------------------------------------------------- --
sshsmNotifications OBJECT IDENTIFIER ::= { sshsmMIB 0 }
sshsmObjects OBJECT IDENTIFIER ::= { sshsmMIB 1 }
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sshsmConformance OBJECT IDENTIFIER ::= { sshsmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
TransportAddressSSH ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"[discuss] Represents either a hostname encoded in ASCII
using the IDNA protocol [RFC3490] followed by
a colon ':' (ASCII character 0x3A) and a decimal port number
in ASCII, or an IP address followed by a colon ':'
(ASCII character 0x3A) and a decimal port number in ASCII.
The name SHOULD be fully qualified whenever possible.
Values of this textual convention are not directly useable
as transport-layer addressing information, and require
runtime resolution. As such, applications that write them
must be prepared for handling errors if such values are
not supported, or cannot be resolved (if resolution occurs
at the time of the management operation).
The DESCRIPTION clause of TransportAddress objects that may
have TransportAddressSSH values must fully describe how (and
when) such names are to be resolved to IP addresses and vice
versa.
This textual convention SHOULD NOT be used directly in
object definitions since it restricts addresses to a
specific format. However, if it is used, it MAY be used
either on its own or in conjunction with
TransportAddressType or TransportDomain as a pair.
When this textual convention is used as a syntax of an
index object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2, STD 58. In this case,
the OBJECT-TYPE declaration MUST include a 'SIZE' clause
to limit the number of potential instance sub-identifiers."
SYNTAX OCTET STRING (SIZE (1..255))
transportDomainSSH OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SSH transport domain. The corresponding transport
address is of type TransportAddressSSH.
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When an SNMP entity uses the transportDomainSSH transport
mapping, it must be capable of accepting messages up to
and including 8192 octets in size. Implementation of
larger values is encouraged whenever possible."
::= { snmpDomains xxxx }
-- RFC Ed.: replace xxxx with IANA-assigned number and
-- remove this note
-- Statistics for the Secure Shell Security Model
sshsmStats OBJECT IDENTIFIER ::= { sshsmObjects 1 }
-- [todo] do we need any stats?
-- The sshsmSession Group
-- -------------------------------------------------------------
-- sshsmMIB - Conformance Information
-- -------------------------------------------------------------
sshsmGroups OBJECT IDENTIFIER ::= { sshsmConformance 1 }
sshsmCompliances OBJECT IDENTIFIER ::= { sshsmConformance 2 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
sshsmGroup OBJECT-GROUP
OBJECTS {
}
STATUS current
DESCRIPTION "A collection of objects for maintaining
information of an SNMP engine which implements the
SNMP Secure Shell Security Model.
"
::= { sshsmGroups 2 }
-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------
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sshsmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
SSHSM-MIB"
MODULE
MANDATORY-GROUPS { sshsmGroup }
::= { sshsmCompliances 1 }
END
9. Security Considerations
This document describes a security model that would permit SNMP to
utilize SSH security services. The security threats and how SSHSM
mitigates those threats is covered in detail throughout this memo.
SSHSM relies on SSH mutual authentication, binding of keys,
confidentiality and integrity. Any authentication method that meets
the requirements of the SSH architecture will provide the properties
of mutual authentication and binding of keys. While SSH does support
turning off confidentiality and integrity, they SHOULD NOT be turned
off when used with SSHSM.
SSHv2 provides Perfect Forward Security (PFS) for encryption keys.
PFS is a major design goal of SSH, and any well-designed keyex
algorithm will provide it.
The security implications of using SSH are discussed in [RFC4251].
SSHSM has no way to verify that server authentication was performed,
to learn the host's public key in advance, or verify that the correct
key is being used. SSHSM simply trusts that these are properly
cvonfigured by the implementer and deployer.
9.1. 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.
SSH supports anonymous access. If SSHSM can extract from SSH an
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authenticated principal to map to securityName, then anonymous access
SHOULD be supported. It is possible for SSH to skip entity
authentication of the client through the "none" authentication method
to support anonymous clients, however in this case an implementation
MUST still support data integrity within the SSH transport protocol
and provide an authenticated principal for mapping to securityName
for access control purposes.
The RFC 3411 architecture does not permit noAuthPriv. SSHSM should
not be used with an SSH connection with the "none" userauth method.
9.2. skipping public key verification
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, then authentication of the ssh server to the
ssh client is not done. Data confidentiality and data integrity
protection to the server still exist, 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 (so passwords are not disclosed
to an attacker).
SSH MUST NOT be configured to skip public key verification for use
with the SSHSM security model.
9.3. the 'none' MAC algorithm
SSH provides the "none" MAC algorithm, which would allow you to turn
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.
9.4. MIB module security
There are a number of management objects defined in this MIB module
with a MAX-ACCESS clause of read-write and/or read-create. Such
objects may be considered sensitive or vulnerable in some network
environments. The support for SET operations in a non-secure
environment without proper protection can have a negative effect on
network operations. These are the tables and objects and their
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sensitivity/vulnerability:
o [todo]
There are no management objects defined in this MIB module that have
a MAX-ACCESS clause of read-write and/or read-create. So, if this
MIB module is implemented correctly, then there is no risk that an
intruder can alter or create any management objects of this MIB
module via direct SNMP SET operations.
Some of the readable objects in this MIB module (i.e., objects with a
MAX-ACCESS other than not-accessible) may be considered sensitive or
vulnerable in some network environments. It is thus important to
control even GET and/or NOTIFY access to these objects and possibly
to even encrypt the values of these objects when sending them over
the network via SNMP. These are the tables and objects and their
sensitivity/vulnerability:
o [todo]
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPSec),
even then, there is no control as to who on the secure network is
allowed to access and GET/SET (read/change/create/delete) the objects
in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410] section 8), including
full support for the SNMPv3 cryptographic mechanisms (for
authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
10. IANA Considerations
IANA is requested to assign:
1. a TCP port number in the range 1..1023 in the
http://www.iana.org/assignments/port-numbers registry which will
be the default port for SNMP over SSH sessions as defined in this
document,
2. an SMI number under mib-2, for the MIB module in this document,
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3. an SnmpSecurityModel for the Secure Shell Security Model, as
documented in the MIB module in this document,
4. "snmp" as an SSH Service Name in the
http://www.iana.org/assignments/ssh-parameters registry.
11. Acknowledgements
The editors would like to thank Jeffrey Hutzelman for sharing his SSH
insights.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
December 2002.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
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[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.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC3419] Daniele, M. and J. Schoenwaelder, "Textual Conventions for
Transport Addresses", RFC 3419, December 2002.
[RFC3430] Schoenwaelder, J., "Simple Network Management Protocol
Over Transmission Control Protocol Transport Mapping",
RFC 3430, December 2002.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
[I-D.ietf-isms-tmsm]
Harrington, D. and J. Schoenwaelder, "Transport Mapping
Security Model (TMSM) Architectural Extension for the
Simple Network Management Protocol (SNMP)",
draft-ietf-isms-tmsm-02 (work in progress), May 2006.
12.2. 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.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
[I-D.ietf-netconf-ssh]
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Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure Shell (SSH)",
draft-ietf-netconf-ssh-06 (work in progress), March 2006.
Appendix A. Open Issues
We need to reach consensus on some issues. I numbered the [discuss]
markers in the text for easy correlation to the issue discussions.
*** When discussing these issues, please use the provided # in the
subject line, and please limit the message to one topic at a time.
***
Here is the current list of issues from the SSHSM document where we
need to reach consensus.
#6: Are there are any wrinkles to coexistence with SNMPv1/v2c/USM?
#10: a) which securityparameters must be supported for the SSHSM
model? b) Which services provided in USM are needed in TMSM/SSHSM?
C) How does the Message Processing model provide this information
to the security model via generateRequestMsg() and
processIncomingMsg() primitives?
#15: What data needs to be stored in the tmStateReference, and how
does SSHSM get the information from SSH, for the various
authentication and transport options?
#21: we need to determine what data should be persistent and
stored in the LCD for notification purposes.
#24: How should we enable auto-discovery?
A.1. Closed Issues
#1: is it important to support anonymous user access to SNMP?
Resolution: We should support whatever authorizations are provided by
SSH; if SSH supports anonymous access, and SSHSM can extract a
username, then it should be supported.
#2: a) is server authentication a requirement that SNMP will require
of the client? yes. b) how can we verify that server authentication
was performed, or do we take simply trust the SSH client layer to
perform such authentication? we trust the SSH layer to provide such
auithentication. c) for the common case of DH signed by public keys,
how does the client learn the host's public key in advance, and
verify that the correct key is being used? this is out of scope for
this document
#3: we need some text contributed to discuss the implications of
sessions on SNMP. See TMSM.
#4: Should the SSHSM document include a discussion of the operational
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expectations of this model for use in troubleshooting a broken
network, or can this be covered in the TMSM document? (Either way,
we could use some contributed text on the topic). See TMSM.
#5: Should the SSHSM document include a discussion of ways SNMP could
be extended to better support management/monitoring needs when a
network is running just fine, or can this be covered in the TMSM
document, or in an applicability document? Out of scope for this
document.
#7: is there still a need for an "authoritative SNMP engine"? No.
#8: Do we need a mapping between the SSH key (or other SSH engine
identifier) and SNMP engineID? No. What happens if an agent
"spoofs" another engineID, and an NMS perfoms a SET of sensitive
parameters to the agent? Resolution: we do not need to address this
for local SSH and local snmpEngineID, unless smebody can show a use
case requirement. There is likely to be a need to map, in an
implementation-dependent manner, the remote engineIDs with the
associated SSH host (mapping of engineID/transport address/host key).
#9: Can an existing R/R session be reused for notifications? Yes.
#11: If we eliminate all msgSecurityParameters, should the
msgSecurityParameters field in the SNMPv3 message simply be a zero-
length OCTET STRING, or should it be an ASN.1 NULL? It MUST be a
BER-encoded OCTET STRING
#12: a) how does SSHSM determine whether SSH can provide the security
services requested in msgFlags? It doesn't. B) There were
discussions about whether it was acceptable for a transport-mapping-
model to provide stronger security than requested. Does this need to
be discussed in the SSHSM document, or should we discuss this in the
TMSM document? Both. c) when sending a message into an environment
where encryption is not legal, how do we ensure that encryption is
not provided? The Danvers Doctrine seems to indicate this in not
necessary to discuss.
#13: will SSHSM be impacted by keychanges to the SSH local datastore?
Resolution: if the session is closed while the Response is being
prepared, discard the Response.
#14: MUST the SSHSM model provide mutual authentication of the client
and server, and MUST it authenticate, integrity-check, and encrypt
the messages? Resolution: yes.
#16: The SSH server doesn't necessarily authorize the name carried in
the SSH_MSG_USERAUTH_REQUEST message, but may return a different name
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or list of names that are authorized to be used given the
authentication of the provided username. Resolution: this is
mistaken; the username from the SSH_MSG_USERAUTH_REQUEST SHOULD be
used. A) What should be the source of the SSHSM mechanism-specific
username for mapping to securityname? Resolution: the username from
the SSH_MSG_USERAUTH_REQUEST SHOULD be used.
#16 B) passing a securityName might be useful for passing as a hint
to RADIUS or other authorization mechanism to indicate which identity
we want to use when doing access control, and RADIUS,etc. can tell us
whether the username being authenticated is allowed to be mapped to
that authorization/accounting identity. Should we provide
securityName when establishing a session, so the authentication
machanisms can use it as a hint? SSHSM provides securityName/Model/
Level and tranport; whether SSH passes this to RADIUS is out of scope
for this document.
#17: I believe somebody suggested we require mutual authentication.
I'm not sure I understand the edits. Done.
#18: I currently have multiple sections, one for each known auth
mechanism. We need to discuss the parameters that need to be cached
for each, and determine whether we can collapse this into one
section. a) Using Passwords to Authenticate SNMP Principals B) Using
Public keys to Authenticate SNMP Principals C) Using Host-based
Authentication of SNMP Principals Resolution: I will collapse this
later, after we have verified we have considered all current/likely
scenarios. Done.
#19: RADIUS is just an instance of the password authentication
protocol. The details of RADIUS are within the SSH layer. I don't
think it is a good idea to expose this outside of SSH. Resolution:
If possible, the details of RADIUS should not be exposed in SSHSM.
There may be an issue with receiving authorization without exposing
the details.
#20: How do we get the mapping from model-specific identity to a
model independent securityName?. Resolution: Implementation-
dependent, both in the case of extracting tmSecurityname from SSH for
an incoming message, and for providing an LCD mapping.
#22: Joe: There are a significant number of security problems
associated with mapping to a transport address which may need to be
discussed in the security considerations section. Resolution: add a
transporttype for hostname.
#23: We need to discuss the circumstances under which a session
should be closed, and how an SNMP engine should determine if, and
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respond if the SSH session is closed by other means, See TMSM, and
implementation-dependent.
#25: Where is the best place to call openSession()? Note that the
whole message is completely put together within the message-
processing portion of the security model, in the hopes that a session
will be able to be established when the message gets to the transport
mapping portion of the architecture. It is done this way because the
RFC3411 arcitecture doesn't pass the transport addressing info into
the security model via messaging model. It would seem a much more
efficient approach to verify that the session can be established,
while still in the security model portion of the messaging model. If
we don't establish the session until we get to the transport mapping,
we've done a lot of work for nothing. And thus far, there is no
place to record failed attempts to establish a session, so an engine
doesn't learn to not try to open a session. In an environment where
the SNMP engine might be a daemon used by multiple applications, an
attacker could use this to cause a denial of service attack at the
NMS. This would likely occur on the NMS side. I don't know if
there's any way to cause it to happen on the agent side. I suppose a
rogue agent with callhome functionality might be able to cause a
denial of service for an NMS by repeatedly requesting callhome and
then refusing the connections. Resolution: called from TMSP.
#26: 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).Resolution: applications do not know about
sessions.
#27: 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. Do we need to
discuss these issues? Where? in the security considerations? See
TMSM.
#28: For notification tables, how do we predefine the dynamic session
identifiers? 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).. Resolution:
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applications do not know about sessions, only transport and
securityN/M/L; if separate sessions are desired, then they can be
differentiated by transport and securityN/M/L parameters.
#29: do we need to support reports? For what purpose? Yes, reports
are used from application processing and for contextEngine discovery.
#30: If we actually do not extract anything from securityParameters,
do we need to check whether this field 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 is
to ensure somebody isn't using the filed for non-standard purposes,
such as passing a virus in the field. If we do check it, do we need
to report it through Reports? Resolution: yes; it won't hurt to
check it.
#32: For an incoming message (Processing an Incoming Message section
10), is using a default securityName mapping the right thing to do?
Resolution: Yes, it is the right thing to do.
#31: Is maxSizeResponseScopedPDU relevant? Can it be calculated once
for the session? Do we need to take into consideration the SSH
window size? Resolution: It can probably be calculated once per
session.
#33: does the mib need to be writable, so sessions can be
preconfigured, such as for callhome, or would it be populated at
creation time by the underlying instrumentation, and not writable by
SNMP? This is about the session table, which has been moved to TMSM.
[discuss] #34 - how do we determine whether a PDU contains a Request
/Response or a Notification? By configuring the securityName or the
transport parameters.
[discuss] #35 - which subsystem is used for Reports? ** Reports are a
reaction to a previously received message and thus they go wherever
the previous message triggering the report came from.
Appendix B. Change Log
"From -01- to -02-"
Added TransportDomainSSH and Address
Removed implementation considerations
Changed all "user auth" to "client auth"
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Removed unnecessary MIB module objects
updated references
improved consistency of references to TMSM as architecural
extension
updated conventions
updated threats to be more consistent with RFC3552
discussion of specific SSH mechanism configurations moved to
security considerations
modified session discussions to reference TMSM sessions
expanded discussion of engineIDs
wrote text to clarify the roles of MPSP and TMSP
clarified how snmpv3 message parts are ised by SSHSM
modified nesting of subsections as needed
securityLevel used by SSHSM always equals authpriv
removed discussion of using SSHSM with SNMPv1/v2c
started updating Elements of Procedure, but realized missing info
needs discussion.
updated MIB module relationship to other MIB modules
"From -00- to -01-"
-00- initial draft as ISMS work product:
updated references to SecSH RFCs
Modified text related to issues# 1, 2, 8, 11, 13, 14, 16, 18, 19,
20, 29, 30, and 32.
updated security considerations
removed Juergen Schoenwaelder from authors, at his request
ran the mib module through smilint
Authors' Addresses
David Harrington
Futurewei Technologies
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: dharrington at huawei.com
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Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
EMail: jsalowey at cisco.com
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