Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Intended status: Standards Track J. Salowey
Expires: August 28, 2008 Cisco Systems
February 25, 2008
Secure Shell Transport Model for SNMP
draft-ietf-isms-secshell-10
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Copyright (C) The IETF Trust (2008).
Abstract
This memo describes a Transport Model for the Simple Network
Management Protocol, using the Secure Shell protocol (SSH).
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 Transport Model for SNMP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Modularity . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Constraints . . . . . . . . . . . . . . . . . . . . . . . 7
2. The Secure Shell Protocol . . . . . . . . . . . . . . . . . . 7
3. How SSHTM Fits into the Transport Subsystem . . . . . . . . . 8
3.1. Security Capabilities of this Model . . . . . . . . . . . 9
3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Message Authentication Issues . . . . . . . . . . . . 10
3.1.3. Authentication Protocol . . . . . . . . . . . . . . . 10
3.1.4. Privacy Protocol . . . . . . . . . . . . . . . . . . . 11
3.1.5. Protection against Message Replay, Delay and
Redirection . . . . . . . . . . . . . . . . . . . . . 11
3.1.6. SSH Subsystem . . . . . . . . . . . . . . . . . . . . 11
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 12
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 12
4. Passing Security Parameters . . . . . . . . . . . . . . . . . 13
4.1. tmStateReference . . . . . . . . . . . . . . . . . . . . . 13
4.2. tmSecurityName . . . . . . . . . . . . . . . . . . . . . . 14
4.3. tmSameSecurity . . . . . . . . . . . . . . . . . . . . . . 15
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 15
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 16
5.2. Procedures for an Outgoing Message . . . . . . . . . . . . 16
5.3. Establishing a Session . . . . . . . . . . . . . . . . . . 17
5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 19
6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 20
6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 20
6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 20
6.3. Relationship to Other MIB Modules . . . . . . . . . . . . 20
6.3.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 20
7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 21
8. Operational Considerations . . . . . . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
9.1. noAuthPriv . . . . . . . . . . . . . . . . . . . . . . . . 27
9.2. Use with SNMPv1/v2c Messages . . . . . . . . . . . . . . . 28
9.3. Skipping Public Key Verification . . . . . . . . . . . . . 28
9.4. The 'none' MAC Algorithm . . . . . . . . . . . . . . . . . 29
9.5. MIB Module Security . . . . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 32
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Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
This memo describes a Transport Model for the Simple Network
Management Protocol, using the Secure Shell protocol (SSH) [RFC4251]
within a transport subsystem [I-D.ietf-isms-tmsm]. The transport
model specified in this memo is referred to as the Secure Shell
Transport Model (SSHTM).
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 Transport Model for SNMP.
It is important to understand the SNMP architecture [RFC3411] and the
terminology of the architecture to understand where the Transport
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. Conventions
For consistency with SNMP-related specifications, this document
favors terminology as defined in STD62 rather than favoring
terminology that is consistent with non-SNMP specifications. This is
consistent with the IESG decision to not require the SNMPv3
terminology be modified to match the usage of other non-SNMP
specifications when SNMPv3 was advanced to Full Standard.
Authentication in this document typically refers to the English
meaning of "serving to prove the authenticity of" the message, not
data source authentication or peer identity authentication.
The terms "manager" and "agent" are not used in this document,
because in the RFC 3411 architecture [RFC3411], all SNMP entities
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have the capability of acting in either manager or agent or in both
roles depending on the SNMP application types supported in the
implementation. Where distinction is required, the application names
of Command Generator, Command Responder, Notification Originator,
Notification Receiver, 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.
The User-Based Security Model (USM) [RFC3414] is a mandatory-to-
implement Security Model in STD 62. While SSH and USM frequently
refer to a user, the terminology preferred 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, or a combination of these
within an administrative domain.
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.
Note to RFC Editor - if the previous paragraph and this note have not
been removed, please send the document back to the editor to remove
this.
1.3. Modularity
The reader is expected to have read and understood the description of
the SNMP architecture, as defined in [RFC3411], and the Transport
Subsystem architecture extension specified in "Transport Subsystem
for the Simple Network Management Protocol" [I-D.ietf-isms-tmsm].
This memo describes the Secure Shell Transport Model for SNMP, a
specific SNMP transport model to be used within the SNMP transport
subsystem to provide authentication, encryption, and integrity
checking of SNMP messages.
In keeping with the RFC 3411 design decision to use self-contained
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documents, this document defines the elements of procedure and
associated MIB module objects which are needed for processing the
Secure Shell Transport Model for SNMP.
This modularity of specification is not meant to be interpreted as
imposing any specific requirements on implementation.
1.4. Motivation
Version 3 of the Simple Network Management Protocol (SNMPv3) added
security to the protocol. The User-based 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 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.
This memo describes a transport model that will make use of the
existing and commonly deployed Secure Shell security infrastructure.
This transport model is designed to meet the security and operational
needs of network administrators, maximize usability in operational
environments to achieve high deployment success and at the same time
minimize implementation and deployment costs to minimize deployment
time.
This document addresses the requirement for the SSH client to
authenticate the SSH server, for the SSH server to authenticate the
SSH client, and describes how SNMP can make use of the authenticated
identities in authorization policies for data access, in a manner
that is independent of any specific access control model.
This document addresses the requirement to utilize client
authentication and key exchange methods which support different
security infrastructures and provide different security properties.
This document describes how to use client authentication as described
in "SSH Authentication Protocol" [RFC4252]. The SSH Transport Model
should work with any of the ssh-userauth methods including the
"publickey", "password", "hostbased", "none", "keyboard-interactive",
"gssapi-with-mic", ."gssapi-keyex", "gssapi", and "external-keyx"
(see http://www.iana.org/assignments/ssh-parameters). The use of the
"none" authentication method is NOT RECOMMENDED, as described in
Security Considerations. Local accounts may be supported through the
use of the publickey, hostbased or password methods. The password
method allows for integration with deployed password infrastructure
such as AAA servers using the RADIUS protocol [RFC2865]. The SSH
Transport Model SHOULD be able to take advantage of future defined
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ssh-userauth methods, such as those that might make use of X.509
certificate credentials.
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 [RFC4742].
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.
1.5. Constraints
The design of this SNMP Transport Model is influenced by the
following constraints:
1. In times of network stress, the transport 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).
2. When the network is not under stress, the transport model and its
underlying security mechanisms MAY depend upon the ready
availability of other network services.
3. It may not be possible for the transport model to determine when
the network is under stress.
4. A transport model should require no changes to the SNMP
architecture.
5. A transport model should require no changes to the underlying
protocol.
2. 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
protocol components, and add-on methods for user authentication:
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.
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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.
o Generic Message Exchange Authentication [RFC4256] is a general
purpose authentication method for the SSH protocol, suitable for
interactive authentications where the authentication data should
be entered via a keyboard
o Generic Security Service Application Program Interface (GSS-API)
Authentication and Key Exchange for the Secure Shell (SSH)
Protocol [RFC4462] describes methods for using the GSS-API for
authentication and key exchange in SSH. It defines an SSH user
authentication method that uses a specified GSS-API mechanism to
authenticate a user, and a family of SSH key exchange methods that
use GSS-API to authenticate a Diffie-Hellman key exchange.
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.
3. How SSHTM Fits into the Transport Subsystem
A transport model plugs into the Transport Subsystem. The SSH
Transport Model thus fits between the underlying SSH transport layer
and the message dispatcher [RFC3411].
The SSH Transport Model will establish a channel between itself and
the SSH Transport Model of another SNMP engine. The sending
transport model passes unencrypted messages from the dispatcher to
SSH to be encrypted, and the receiving transport model accepts
decrypted incoming messages from SSH and passes them to the
dispatcher.
After an SSH Transport model channel is established, then SNMP
messages can conceptually be sent through the channel from one SNMP
message dispatcher to another SNMP message dispatcher. Multiple SNMP
messages MAY be passed through the same channel.
The SSH Transport Model of an SNMP engine will perform the
translation between SSH-specific security parameters and SNMP-
specific, model-independent parameters.
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3.1. Security Capabilities of this Model
3.1.1. Threats
The Secure Shell Transport Model provides protection against the
threats identified by the RFC 3411 architecture [RFC3411]:
1. Message stream modification - SSH provides for verification that
each received message has not been modified during its
transmission through the network.
2. Information modification - SSH provides for verification that the
contents of each received message has not been modified during
its transmission through the network, data has not been altered
or destroyed in an unauthorized manner, nor have data sequences
been altered to an extent greater than can occur non-maliciously.
3. Masquerade - SSH 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 - the SSH Transport Model provides that the contents
of each received SNMP message are protected from disclosure to
unauthorized persons.
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.
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3.1.2. Message 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].
The SSH Transport Model determines from SSH the identity of the
authenticated principal, and the type and address associated with an
incoming message, and the SSH Transport Model 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 SSH Transport Model layer.
The SNMPv3 WG deliberately avoided this and settled for an assertion
by the security model that the requirements of securityLevel were met
The SSH Transport Model has no mechanisms by which it can test
whether an underlying SSH connection provides auth or priv, so the
SSH Transport Model trusts that the underlying SSH connection has
been properly configured to support authPriv security
characteristics.
The SSH Transport Model does not know about the algorithms or options
to open SSH sessions that match different securityLevels. For
interoperability of the trust assumptions between SNMP engines, an
SSH Transport Model-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 the SSH
Transport Model as authPriv securityLevel.
The security protocols used in the Secure Shell Authentication
Protocol [RFC4252] and the Secure Shell Transport Layer Protocol
[RFC4253] 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.
3.1.3. Authentication Protocol
The SSH Transport Model should support any server or 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 - and
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keyboard interactive and others.
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 [RFC4590] to integrate with RADIUS or Diameter. At
some point in the processing, these mechanisms require the password
be made available as clear text on the device that is authenticating
the password which might introduce 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.
3.1.4. Privacy Protocol
The SSH transport layer protocol provides strong encryption, server
authentication, and integrity protection.
3.1.5. Protection against Message Replay, Delay and Redirection
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 Diffie-Helman exchange, both sides will generate
new random numbers for each exchange, which means the encryption and
integrity keys will be distinct for every session.
3.1.6. SSH Subsystem
This document describes the use of an SSH subsystem for SNMP to make
SNMP usage distinct from other usages.
SSH subsystems of type "snmp" are opened by the SSH Transport Model
during the elements of procedure for an outgoing SNMP message. Since
the sender of a message initiates the creation of an SSH session if
needed, the SSH session will already exist for an incoming message or
the incoming message would never reach the SSH Transport Model.
Implementations MAY choose to instantiate SSH sessions in
anticipation of outgoing messages. This approach might be useful to
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ensure that an SSH session to a given target can be established
before it becomes important to send a message over the SSH session.
Of course, there is no guarantee that a pre-established session will
still be valid when needed.
SSH sessions are uniquely identified within the SSH Transport Model
by the combination of transportAddressType, transportAddress,
securityName, and securityLevel associated with each session.
3.2. Security Parameter Passing
For incoming messages, SSH-specific security parameters are
translated by the transport model into security parameters
independent of the transport and security models. The transport
model accepts messages from the SSH subsystem, and records the
transport-related and SSH-security-related information, including the
authenticated identity, in a cache referenced by tmStateReference,
and passes the WholeMsg and the tmStateReference to the dispatcher
using the receiveMessage() ASI (Application Service Interface).
For outgoing messages, the transport model takes input provided by
the dispatcher in the sendMessage() ASI. The SSH Transport Model
converts that information into suitable security parameters for SSH,
establishes sessions as needed, and passes messages to the SSH
subsystem for sending.
3.3. 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 are usually unmanned
automated processes. As a result, it may 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
containing a notification receiver.
The targets to whom notifications should be sent is typically
determined and configured by a network administrator. The SNMP-
TARGET-MIB module [RFC3413] contains objects for defining management
targets, including transport domains and addresses and security
parameters, for applications such as notifications and proxy.
For the SSH Transport Model, transport type and address are
configured in the snmpTargetAddrTable, and the 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
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receive notifications or perform proxy.
These MIB modules may be configured using SNMP or other
implementation-dependent mechanisms, such as CLI scripting or loading
a configuration file. It may be necessary to provide additional
implementation-specific configuration of SSH parameters.
4. Passing Security Parameters
For the SSH Transport Model, the session state needs to be maintained
using tmStateReference. RFC3411 discusses a securityStateReference,
which is not accessible to the Transport Subsystem.
4.1. tmStateReference
Upon opening each SSH connection, the SSH Transport Model stores
model- and mechanism-specific information about the connection in a
cache, referenced by tmStateReference.
For interoperability with Security Model designs, the state
referenced by tmStateReference MUST include the following fields
(with sample values). See the Elements of Procedure for detailed
processing instructions on the use of these fields by the SSH
Transport Model.
tmTransport = snmpSSHDomain
tmAddress = an snmpSSHAddress
tmRequestedSecurityLevel = ["noAuthNoPriv" | "authNoPriv" |
"authPriv" ]
tmTransportSecurityLevel = "authPriv"
tmSecurityName = the principal name [to be] authenticated by SSH.
See the section on tmSecurityName below.
tmSameSecurity = true or false, depending on whether the Security
Model requires that an outgoing response be sent using the same
security parameters as were used for the incoming request or for
any other security-model-dependent reason. See the section on
tmSameSecurity below.
The state referenced by tmStateReference for an SSH Transport Model
should also contain an implementation-dependent identifier (e.g.,
tmSessionID) that can be used to determine whether the SSH session
available for sending an outgoing message is the same SSH session as
was used when receiving the corresponding incoming message.
The tmStateReference is used to pass references containing the
appropriate SSH session information from the transport model for
subsequent processing.
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The state referenced by tmStateReference may be saved across multiple
messages in a Local Configuration Datastore (LCD).
A Transport Model will maintain any mapping between transport-
specific security parameters and tmTransportSecurityLevel and
tmSecurityName, and will verify for outgoing messages that the
transport-provided security is at least as strong as
tmRequestedSecurityLevel..
The SSH Transport Model implementation has the responsibility for
explicitly releasing the complete tmStateReference and associated
information when a session is closed.
Since the contents of a cache are meaningful only within an
implementation, and not on-the-wire, the format of the cache and the
LCD are implementation-specific.
4.2. tmSecurityName
How the SSH identity is extracted from the SSH layer is
implementation-dependent. How the SSH identity is mapped to a
tmSecurityName should be administratively configurable.
[TODO: standardize some dynamic mechanisms for SSHTM, per auth-
protocol, such as user-auth, host-auth, etc. Make the list of
authProtocols expandable, and provide default algorithms.] [DISCUSS:
this cannot be implementation-dependent. It used to identify the
layer 8 principal for use in such things as logging and for access
control policy assignment. it must generate a predictable
securityName representing the principal, regardless of the
authentication mechanism. USM provides this by pre-configuration of
the mapping of the auth protocol and auth-specific credentials to a
securityName, in the usmUserTable. We MUST have the similar mapping,
preferably done dynamically rather than statically. Therefore either
the dynamic mapping algorithm MUST be standardized. or we MUST have a
static mapping.]
tmSecurityName is a human-readable name in SnmpAdminString format
that is mapped from the identity that has been successfully
authenticated by SSH. By default, tmSecurityName is determined from
the value of the user name field of the SSH_MSG_USERAUTH_REQUEST
message for which a SSH_MSG_USERAUTH_SUCCESS has been received.
As described in RFC4252 section 5, all authentication requests,
regardless of authentication mechanism, MUST use the same message
format, which includes a byte to indicate SSH_MSG_USERAUTH_REQUEST,
and a user name field. How the authenticated user name is made
available to the SNMP implementation is SSH-implementation dependent.
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4.3. tmSameSecurity
If a Secure Shell transport session is closed between the time a
request message is received and the corresponding response message is
sent, then the response message MUST be discarded, even if a new SSH
session has been established. The SSH Transport Model does not know
whether a message contains a request or response (at least
architecturally, this is not available to the transport model;
implementations may choose to make this available for simplicity.)
Each Security Model that supports the tmStateReference cache will
pass a tmSameSecurity parameter in the tmStateReference cache for
outgoing messages to indicate whether the same security MUST be used
for the outgoing message as was used for the corresponding incoming
message (e.g., a request-response pair). The tmStateReference for
the Secure Shell Transport Model may also include an existing SSH-
specific transport session identifier in an implementation-dependent
format.
If the tmSameSecurity is indicated, but the session identified in the
tmStateReference does not match the current established SSH transport
session, i.e., it is not the same SSH security session, the message
MUST be discarded, and the dispatcher should be notified that the
sending of the message failed.
5. Elements of Procedure
Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an
SNMP entity. The Secure Shell Transport Model uses some of these
conceptual data flows when communicating between subsystems. These
RFC 3411-defined data flows are referred to here as public
interfaces.
To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule,
if state information is available when a message gets discarded, the
message-state information should also be released, and if state
information is available when a session is closed, the session state
information should also be released.
An error indication may return an OID and value for an incremented
counter and a value for securityLevel, and values for contextEngineID
and contextName for the counter, and the securityStateReference if
the information is available at the point where the error is
detected. ContextEngineID and contextName are not accessible to
Transport Models, so contextEngineID is set to the local value of
snmpEngineID, and contextName is set to the default context for error
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counters.
5.1. Procedures for an Incoming Message
For an incoming message, the SSH Transport Model will put information
from the SSH layer into a cache referenced by tmStateReference.
1) The SSH Transport Model queries the associated SSH engine, in
an implementation-dependent manner, to determine the transport and
security parameters for the received message.
tmTransportDomain = snmpSSHDomain
tmTransportAddress = a snmpSSHAddress
tmSecurityLevel = "authPriv"
tmsSecurityName = the principal name authenticated by SSH. 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. How this name is
extracted from the SSH environment and how it is translated
into a tmSecurityName is implementation-dependent.
2) If one does not exist, the SSH Transport Model creates an entry
in a Local Configuration Datastore, in an implementation-dependent
format, containing the information and any implementation-specific
parameters desired, and creates a tmStateReference for subsequent
reference to the information.
Then the Transport model passes the message to the Dispatcher using
the following ASI:
statusInformation =
receiveMessage(
IN transportDomain -- domain for the received message
IN transportAddress -- address for the received message
IN wholeMessage -- the whole SNMP message from SSH
IN wholeMessageLength -- the length of the SNMP message
IN tmStateReference -- (NEW) transport info
)
5.2. Procedures for an Outgoing Message
The Dispatcher passes the information to the Transport Model using
the ASI defined in the transport subsystem:
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statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- (NEW) transport info
)
The SSH Transport Model performs the following tasks:
1) Determine the target index by extracting the transportDomain,
transportAddress, securityName, and securityLevel from the
tmStateReference.
2) Lookup the session in the Local Configuration Datastore using
the target index
3) If tmSameSecurity specified in the tmStateReference is true,
and there is no session associated with the target that has the
same session identifier (e.g., tmSessionID) as that specified in
the tmStateReference, then increment the
sshtmSessionNoAvailableSessions counter, discard the message and
return the error indication in the statusInformation. Processing
of this message stops.
4) If there is no session open associated with the target index,
then call openSession().
4a) If an error is returned from OpenSession(), then discard the
message and return the error indication returned by OpenSession()
in the statusInformation.
4b) If openSession() is successful, then store any implementation-
specific information in the LCD for subsequent use.
5) Extract any implementation-specific parameters from the LCD
6) Pass the wholeMessage to SSH for encapsulation as data in an
SSH message.
5.3. Establishing a Session
The Secure Shell Transport Model provides the following application
service interface (ASI) to describe the data passed between the
Transport Model and the SSH service. It is an implementation
decision how such data is passed.
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statusInformation =
openSession(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN maxMessageSize -- of the sending SNMP entity
OUT tmStateReference -- (NEW) transport info
)
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 be done automatically for an SNMP application that
initiates a transaction, such as a Command Generator or a
Notification Originator or a Proxy Forwarder.
1) Using destTransportDomain and destTransportAddress, the client
will establish an SSH transport connection using the SSH transport
protocol, authenticate the server, and exchange keys for message
integrity and encryption. The parameters of the transport connection
and the credentials used to authenticate are provided in an
implementation-dependent manner.
If the attempt to establish a connection is unsuccessful, or server
authentication fails, then sshtmSessionOpenErrors is incremented, and
an openSession error indication is returned, and openSession
processing stops.
2) The provided transport domain, transport address, securityName and
securityLevel are used to lookup an associated entry in the Local
Configuration Datastore (LCD). Any 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.
In an implementation-specific manner, pass the username extracted
from the LCD to the SSH layer. [TODO: this may need to be
standardized.]
3)The client will then invoke an SSH authentications service to
authenticate the user, such as that described in the SSH
authentication protocol [RFC4252]. The credentials used to
authenticate are provided in an implementation-dependent manner.
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If the authentication is unsuccessful, then the transport connection
is closed, tmStateReference is released, the message is discarded,
the sshtmSessionUserAuthFailures counter is incremented, an error
indication is returned to the calling module, and processing stops
for this message.
4) Once the principal has been successfully authenticated, the client
will invoke the "ssh- connection" service, also known as the SSH
connection protocol [RFC4254].
5) After the ssh-connection service is established, the client will
request a channel of type "session" in an implementation-dependent
manner. If unsuccessful, the sshtmSessionChannelOpenFailures counter
is incremented, an error indication is returned to the calling
module, and processing stops for this message.
6) If successful, this will result in an SSH session. The
destTransportDomain and the destTransportAddress, plus any
implementation-dependent identifier for the channel should be
retained so they can be added to the LCD for subsequent use.
7) Once the SSH session has been established, the client 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 Transport Model MUST default to
providing access to the "SNMP" SSH subsystem if the SSH session is
established using the IANA-assigned TCP port. Servers SHOULD be
configurable to allow access to the SNMP SSH subsystem over other
ports.
8) Create an entry in a Local Configuration Datastore containing the
provided transportDomain, transportAddress, securityName,
securityLevel, and SSH-specific parameters and create a
tmStateReference to reference the entry.
5.4. Closing a Session
The Secure Shell Transport Model provides the following ASI to close
a session:
statusInformation =
closeSession(
IN tmStateReference -- transport info
)
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The following describes the procedure to follow to close a session
between a client and sever . This process is followed by any SNMP
engine closing the corresponding SNMP session.
1) Extract the transportDomain, transportAddress, securityName,
and securityLevel from the tmStateReference.
2) Lookup the session in the Local Configuration Datastore using
the target index
3) If there is no session open associated with the target index,
then closeSession processing is completed.
4) Extract any implementation-specific parameters from the LCD
5) Have SSH close the specified session.
6. MIB Module Overview
This MIB module provides management of the Secure Shell Transport
Model. It defines some needed textual conventions, and some
statistics.
6.1. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree
is organized as a set of related objects. The overall structure and
assignment of objects to their subtrees, and the intended purpose of
each subtree, is shown below.
6.2. Textual Conventions
Generic and Common Textual Conventions used in this document can be
found summarized at http://www.ops.ietf.org/mib-common-tcs.html
6.3. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing the SSH Transport Model. In particular, it
is assumed that an entity implementing the SSHTM-MIB will implement
the SNMPv2-MIB [RFC3418], the SNMP-FRAMEWORK-MIB [RFC3411] and the
SNMP-TRANSPORT-MIB [I-D.ietf-isms-tmsm].
This MIB module is for managing SSH Transport Model information.
This MIB module models a sample Local Configuration Datastore.
6.3.1. MIB Modules Required for IMPORTS
The following MIB module imports items from [RFC2578], [RFC2579],
[RFC2580].
This MIB module also references [RFC3490] and [RFC3986]
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7. MIB Module Definition
SSHTM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, mib-2, snmpDomains,
Counter32
FROM SNMPv2-SMI
TEXTUAL-CONVENTION
FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
;
sshtmMIB MODULE-IDENTITY
LAST-UPDATED "200710140000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
+49 6221 90511-15
quittek@netlab.nec.de
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@iu-bremen.de
Co-editors:
David Harrington
Huawei Technologies USA
1700 Alma Drive
Plano Texas 75075
USA
+1 603-436-8634
ietfdbh@comcast.net
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Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
jsalowey@cisco.com
"
DESCRIPTION "The Secure Shell Transport Model MIB
Copyright (C) The IETF Trust (2007). 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 "200710140000Z"
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 SNMP-SSH-TM-MIB
-- ---------------------------------------------------------- --
sshtmNotifications OBJECT IDENTIFIER ::= { sshtmMIB 0 }
sshtmObjects OBJECT IDENTIFIER ::= { sshtmMIB 1 }
sshtmConformance OBJECT IDENTIFIER ::= { sshtmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
snmpSSHDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over SSH transport domain. The corresponding transport
address is of type SnmpSSHAddress.
When an SNMP entity uses the snmpSSHDomain transport
model, it must be capable of accepting messages up to
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and including 8192 octets in size. Implementation of
larger values is encouraged whenever possible."
::= { snmpDomains yy }
-- RFC Ed.: replace yy with IANA-assigned number and
-- remove this note
SnmpSSHAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"Represents either a hostname with a port number or an IP
address with a port number.
The hostname must be encoded in ASCII, as specified in
RFC3490 (Internationalizing Domain Names in Applications)
followed by a colon ':' (ASCII character 0x3A) and a
decimal port number in ASCII. The name SHOULD be fully
qualified whenever possible.
An IPv4 address must be a dotted decimal format followed
by a colon ':' (ASCII character 0x3A) and a decimal port
number in ASCII.
An IPv6 address must be a colon separated format,
surrounded by brackets, followed by a colon ':' (ASCII
character 0x3A) and a decimal port number in ASCII.
Values of this textual convention may not be directly useable
as transport-layer addressing information, and may 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 snmpSSHAddress 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
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index object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2, STD 58. It is
RECOMMENDED that all MIB documents using this textual
convention make explicit any limitations on index
component lengths that management software must observe.
This may be done either by including SIZE constraints on
the index components or by specifying applicable
constraints in the conceptual row DESCRIPTION clause or
in the surrounding documentation.
"
REFERENCE
"RFC3896, Uniform Resource Identifier (URI): Generic Syntax"
SYNTAX OCTET STRING (SIZE (1..255))
-- The sshtmSession Group
sshtmSession OBJECT IDENTIFIER ::= { sshtmObjects 1 }
sshtmSessionOpens OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request has been
executed, whether it succeeded or failed.
"
::= { sshtmSession 1 }
sshtmSessionCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times a closeSession() request has been
executed, whether it succeeded or failed.
"
::= { sshtmSession 2 }
sshtmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request
failed to open a Session, for any reason.
"
::= { sshtmSession 3 }
sshtmSessionUserAuthFailures OBJECT-TYPE
SYNTAX Counter32
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MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request
failed due to user authentication failures.
"
::= { sshtmSession 4 }
sshtmSessionChannelOpenFailures OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an openSession() request
failed due to channel open failures.
"
::= { sshtmSession 5 }
sshtmSessionNoAvailableSessions OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of times an outgoing message
was dropped because the same
session was no longer available.
"
::= { sshtmSession 6 }
-- ************************************************
-- sshtmMIB - Conformance Information
-- ************************************************
sshtmCompliances OBJECT IDENTIFIER ::= { sshtmConformance 1 }
sshtmGroups OBJECT IDENTIFIER ::= { sshtmConformance 2 }
-- ************************************************
-- Compliance statements
-- ************************************************
sshtmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
SNMP-SSH-TM-MIB"
MODULE
MANDATORY-GROUPS { sshtmGroup }
::= { sshtmCompliances 1 }
-- ************************************************
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-- Units of conformance
-- ************************************************
sshtmGroup OBJECT-GROUP
OBJECTS {
sshtmSessionOpens,
sshtmSessionCloses,
sshtmSessionOpenErrors,
sshtmSessionUserAuthFailures,
sshtmSessionChannelOpenFailures,
sshtmSessionNoAvailableSessions
}
STATUS current
DESCRIPTION "A collection of objects for maintaining
information of an SNMP engine which implements the
SNMP Secure Shell Transport Model.
"
::= { sshtmGroups 2 }
END
8. Operational Considerations
The SSH Transport Model 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, a UDP transport
model should be considered instead of the SSH Transport Model.
The SSH Transport Model defines a single well-known default port for
all traffic types. Administrators might choose to define one port
for SNMP request-response traffic, but configure notifications to be
sent to a different port, by using the snmpTargetAddrTable, for
example.
If the SSH Transport Model is configured to utilize AAA services,
operators should consider configuring support for a local
authentication mechanisms, such as local passwords, so SNMP can
continue operating during times of network stress.
The SSH protocol has its own windowing mechanism. RFC 4254 says: The
window size specifies how many bytes the other party can send before
it must wait for the window to be adjusted. Both parties use the
following message to adjust the window. The SSH specifications leave
it open when such window adjustment messages are created. Some
implementations have been found to send window adjustment messages
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whenever received data has been passed to the application. Since
window adjustment messages are padded, encrypted, hmac'ed, and
wrapped, this results in noticeable bandwidth and processing
overhead, which can be avoided by sending window adjustment messages
less frequently.
The SSH protocol requires the execution of CPU intensive calculations
to establish a session key during session establishment. This means
that short lived sessions become computationally expensive compared
to USM, which does not have a notion of a session key. Other
transport security protocols such as TLS support a session resumption
feature that allows reusing a cached session key. Such a mechanism
does not exist for SSH and thus SNMP applications should keep SSH
sessions for longer time periods.
9. Security Considerations
This document describes a transport model that permits SNMP to
utilize SSH security services. The security threats and how the SSH
Transport Model mitigates those threats is covered in detail
throughout this memo.
The SSH Transport Model 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 the SSH Transport Model.
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 covered in [RFC4251].
The SSH Transport Model 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. The SSH
Transport Model simply trusts that these are properly configured 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
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server to the SSH client, nor does it compromise data confidentiality
or data integrity.
SSH supports anonymous access. If the SSH Transport Model can
extract from SSH an 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. The SSH
Transport Model SHOULD NOT be used with an SSH connection with the
"none" userauth method.
9.2. Use with SNMPv1/v2c Messages
The SNMPv1 and SNMPv2c message processing described in RFC3584 (BCP
74) [RFC3584] always selects the SNMPv1(1) Security Model for an
SNMPv1 message, or the SNMPv2c(2) Security Model for an SNMPv2c
message. When running SNMPv1/SNMPv2c over a secure transport like
the SSH Transport Model, the securityName and securityLevel used for
access control decisions are then derived from the community string,
not the authenticated identity and securityLevel provided by the SSH
Transport Model.
9.3. 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 SSH Transport Model.
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9.4. 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 SSH Transport Model.
9.5. MIB Module Security
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 The readable objects in this MIB module are not sensitive.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPSec or
SSH), even then, there is no control as to who on the secure network
is allowed to access and GET/SET (read/change/create/delete) the
objects in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410] section 8), including
full support for the USM and the SSH Transport Model cryptographic
mechanisms (for authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
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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 an SSH Transport Model as
defined in this document,
2. an SMI number under mib-2, for the MIB module in this document,
3. an SMI number under snmpDomains, for the snmpSSHDomain,
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.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple
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Network Management Protocol (SNMP)
Applications", STD 62, RFC 3413, December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based
Security Model (USM) for version 3 of the
Simple Network Management Protocol (SNMPv3)",
STD 62, RFC 3414, December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB)
for the Simple Network Management Protocol
(SNMP)", STD 62, RFC 3418, December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in
Applications (IDNA)", RFC 3490, March 2003.
[RFC3584] Frye, R., Levi, D., Routhier, S., and B.
Wijnen, "Coexistence between Version 1, Version
2, and Version 3 of the Internet-standard
Network Management Framework", BCP 74,
RFC 3584, August 2003.
[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
Subsystem for the Simple Network Management
Protocol (SNMP)", draft-ietf-isms-tmsm-11 (work
in progress), November 2007.
12.2. Informative References
[RFC1994] Simpson, W., "PPP Challenge Handshake
Authentication Protocol (CHAP)", RFC 1994,
August 1996.
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Internet-Draft Secure Shell Transport Model for SNMP February 2008
[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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic
Syntax", STD 66, RFC 3986, January 2005.
[RFC4256] Cusack, F. and M. Forssen, "Generic Message
Exchange Authentication for the Secure Shell
Protocol (SSH)", RFC 4256, January 2006.
[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.
[RFC4590] Sterman, B., Sadolevsky, D., Schwartz, D.,
Williams, D., and W. Beck, "RADIUS Extension
for Digest Authentication", RFC 4590,
July 2006.
[RFC4742] Wasserman, M. and T. Goddard, "Using the
NETCONF Configuration Protocol over Secure
SHell (SSH)", RFC 4742, December 2006.
Appendix A. Open Issues
We need to reach consensus on some issues.
Here is the current list of issues from the SSH Transport Model
document where we need to reach consensus.
o Issue #2: In USM, there is a mapping table that permits one user
to have multiple methods for authentication, that map to a common
securityName. Since SSH supports multiple authentication
mechanisms, do we need to specify how these mechanism-specific
identities map to a common securityName? This is important to
permit admins to configure the TARGET-MIB, for example, with one
common identity rather than mechanism-specific identities.
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o Issue #3: Mapping from the LCD identity to an SSH mechanisms-
specific identity. This may just be the opposite transform of
Issue #2.
o Issue #5: what are the elements of procedure if you run for
example SNMPv3/USM over SSHTM? The ASIs do not have parameters to
identify two methods of authentication, and it is unclear how an
outgoing message request would specify both SNMPv3/USM and SSHTM
should be used, and which securityName/Level should be used for
each.
o Issue #6: We have not resolved whether the principal associated
with a notification receiver must be a principal (aka user) or
whether a hostname is adequate. In SNMPv3, the access controls
are symmetrical - it is a user-level principal that access
controls apply to, whether for R/R or notify applications. Is it
acceptable to have user-level for R/R and host-level for notify
functionality? A user that is not allowed to GET an object might
be able to have the value of the object reported in a
notification, or vice-versa. This is not much different that a
principal having two different identities, one for R/R and another
for notifications, or an admin configuring systems to send
notifications to a different principal than those who do R/R
processing. The WG needs to discuss this and reach some consensus
on whether this is an issue or not, and how we want to proceed.
o Issue #8: Should we allow transport models to select the
corresponding security model by providing an additional parameter
- the securityModel parameter - to tmStateReference, which would
override the securityModel parameter extracted from a message
header? Doing this would resolve Issue #5, and would allow the
transport security model to be used with all SNMP message
versions.
TODO:
finalize error processing in EOP
Appendix B. Change Log
From -09- to -10
Issue #1: Made release of cached session info an implementation
requirement on session close.
Issue #4: UTF-8 syntax of userauth user name matches syntax of
SnmpAdminString.
Issue #7: Resolved to not describe how an SSH session is closed.
From -08- to -09
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Updated MIB assignment to by rfc4181 compatible
update MIB security considerations with coexistence issues
update sameSession and tmSessionID support
Fixed note about terminology, for consistency with SNMPv3.
From -07- to -08
Updated MIB
update MIB security considerations
develop sameSession and tmSessionID support
Added a note about terminology, for consistency with SNMPv3 rather
than with RFC2828.
Removed reference to mappings other than the identity function.
From -06- to -07
removed section on SSH to EngineID mappings, since engineIDs are
not exposed to the transport model
removed references to engineIDs and discovery
removed references to securityModel.
added security considerations warning about using with SNMPv1/v2c
messages.
added keyboard interactive discussion
noted some implementation-dependent points
removed references to transportModel; we use the transport domain
as a model identifier.
cleaned up ASIs
modified MIB to be under snmpModules
changed transportAddressSSH to snmpSSHDomain style addressing
From -05- to -06
replaced transportDomainSSH with RFC3417-style snmpSSHDomain
replaced transportAddressSSH with RFC3417-style snmpSSHAddress
Changed recvMessage to receiveMessage, and modified OUT to IN to
match TMSM.
From -04- to -05
added sshtmUserTable
moved session tabel into the transport model MIB from the
transport subsystem MIB
added and then removed Appendix A - Notification Tables
Configuration (see Transport Security Model)
made this document a specification of a transport model, rather
than a security model in two parts. Eliminated TMSP and MPSP and
replaced them with "transport model" and "security model".
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Removed security-model-specific processing from this document.
Removed discussion of snmpv3/v1/v2c message format co-existence
changed tmSessionReference back to tmStateReference
"From -03- to -04-"
changed tmStateReference to tmSessionReference
"From -02- to -03-"
rewrote almost all sections
merged ASI section and Elements of Procedure sections
removed references to the SSH user, in preference to SSH client
updated references
creayted a conventions section to identify common terminology.
rewrote sections on how SSH addresses threats
rewrote mapping SSH to engineID
eliminated discovery section
detailed the Elements of Procedure
eliminated secrtions on msgFlags, transport parameters
resolved issues of opening notifications
eliminated sessionID (TMSM needs to be updated to match)
eliminated use of tmsSessiontable except as an example
updated Security Considerations
"From -01- to -02-"
Added TransportDomainSSH and Address
Removed implementation considerations
Changed all "user auth" to "client auth"
Removed unnecessary MIB module objects
updated references
improved consistency of references to TMSM as architectural
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 the SSH Transport Model always equals
authpriv
removed discussion of using SSHSM with SNMPv1/v2c
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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
Huawei Technologies (USA)
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1 603 436 8634
EMail: dharrington@huawei.com
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
EMail: jsalowey@cisco.com
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