Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Intended status: Standards Track J. Salowey
Expires: January 12, 2009 Cisco Systems
July 11, 2008
Secure Shell Transport Model for SNMP
draft-ietf-isms-secshell-11
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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1.1. The Internet-Standard Management Framework . . . . . . . . 3
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Modularity . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Constraints . . . . . . . . . . . . . . . . . . . . . . . 6
2. The Secure Shell Protocol . . . . . . . . . . . . . . . . . . 6
3. How SSHTM Fits into the Transport Subsystem . . . . . . . . . 7
3.1. Security Capabilities of this Model . . . . . . . . . . . 8
3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.2. Message Authentication . . . . . . . . . . . . . . . . 9
3.1.3. Authentication Protocol Support . . . . . . . . . . . 10
3.1.4. Privacy Protocol Support . . . . . . . . . . . . . . . 10
3.1.5. Protection against Message Replay, Delay and
Redirection . . . . . . . . . . . . . . . . . . . . . 10
3.1.6. SSH Subsystem . . . . . . . . . . . . . . . . . . . . 11
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 11
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 12
4. Passing Security Parameters . . . . . . . . . . . . . . . . . 12
4.1. tmStateReference . . . . . . . . . . . . . . . . . . . . . 12
4.2. tmSecurityName . . . . . . . . . . . . . . . . . . . . . . 13
4.3. tmSameSecurity . . . . . . . . . . . . . . . . . . . . . . 14
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 15
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 15
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 . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
9.1. noAuthPriv . . . . . . . . . . . . . . . . . . . . . . . . 31
9.2. Use with SNMPv1/v2c Messages . . . . . . . . . . . . . . . 32
9.3. Skipping Public Key Verification . . . . . . . . . . . . . 32
9.4. The 'none' MAC Algorithm . . . . . . . . . . . . . . . . . 32
9.5. MIB Module Security . . . . . . . . . . . . . . . . . . . 33
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 37
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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:
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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.
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
[I-D.ietf-isms-tmsm]. 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
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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.
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. 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 is provided to the transport model, so it can be
used to map to an SNMP model-independent securityName for use
with SNMP access control and notification configuration. (The
identity may undergo various transforms before it maps to the
securityName.)
5. Authenticating both the SSH server and the SSH client ensures the
authenticity of the SNMP engine that provides MIB data.
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
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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.
3.1.2. Message Authentication
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
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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 Support
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
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 [RFC5090] 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. [DISCUSS: do we really need this paragraph?
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 Support
The SSH transport model supports any privacy protocol used with SSH.
[DISCUSS: The authentication support section goes into significant
detail; should the same be done here?]
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.
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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.
[DISCUSS: If a notification originator opens a subsystem called
"snmp" and a command generator opens a subsystem called "snmp", will
that be confusing to SSH? ]
Implementations MAY choose to instantiate SSH sessions in
anticipation of outgoing messages. This approach might be useful to
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.
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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
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, some 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. This connection information
might be consistent across multiple messages, and might also be
stored in the sshtmLCDTable.
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.
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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).
The per-session state that is referenced by tmStateReference may be
saved across multiple messages in a Local Configuration Datastore
(sshtmLCDTable).
The tmStateReference cache should be generated anew for each message,
so that it 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 (e.g., a response
to a request), when tmSameSecurity is set.
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
"garbage collection" - releasing any associated tmStateReference when
a session is closed.
4.2. tmSecurityName
How the SSH identity is extracted from the SSH layer is
implementation-dependent.
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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.
The SSH protocol is not always clear on whether the user name field
must be filled in, so for some implementations, it may be necessary
to use a non-default mapping algorithm. How the SSH identity is
mapped to a tmSecurityName should be administratively configurable.
The sshtmLCDTransformPolicy object is used during the elements of
procedure to determine whether the default transform or an
alternative tranbsform is used.
The tmSecurityName may ultimately be used to identify the layer 8
principal for use in such things as logging and for access control
policy assignment. For example, USM provides this by pre-
configuration of the mapping of the auth protocol and auth-specific
credentials to a securityName, in the usmUserTable. Transport model
transforms SHOULD generate a predictable tmSecurityName representing
the principal.
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 SHOULD also include a tmSessionID to
indicate which SSH session should be used for the corresponding
response.
If the tmSameSecurity is indicated, but the session identified in the
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tmStateReference does not match the current established SSH session
for the tmTransportDomain, tmTransportAddress, tmSecurityName, and
tmSecurityLevel, then 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
counters.
5.1. Procedures for an Incoming Message
1) The SSH Transport Model queries the SSH engine, in an
implementation-dependent manner, to determine the transportAddress,
the principal name authenticated by SSH, and a session identifier.
By default, the principal name is the value of the user name field of
the SSH_MSG_USERAUTH_REQUEST message for which a
SSH_MSG_USERAUTH_SUCCESS has been received. How this name is
extracted from the SSH environment is implementation-dependent.
2) Using the sshtmLCDTransformPolicy, calculate a corresponding
tmSecurityName value. Determine if there is a row in the
sshtmLCDTable with the transport address and an
sshtmLCDTmSecurityName that matches the calculated tmSecurityName
value.
3) If one does not exist, create an entry in the sshtmLCDTable:
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sshtmTransportAddress = transport address
sshtmLCDTmSecurityName = calculated tmSecurityName
sshtmLCDPrincipal=the SSH principal
sshtmLCDSessionID=the SSH session identifier
sshtmStorageType=volatile
sshtmRowStatus=createAndGo
4) Create a tmStateReference cache for subsequent reference to the
information.
tmTransportDomain = snmpSSHDomain
tmTransportAddress = sshtmLCDTransportAddress
tmSecurityLevel = "authPriv"
tmSecurityName = sshtmLCDTmSecurityName
tmSessionID = an implementation-dependent value that can be used
to detect when a session has closed and been replaced by another
session. The value in tmStateReference should identify the
session over which the message was received.
Then the Transport model passes the message to the Dispatcher using
the following ASI:
statusInformation =
receiveMessage(
IN transportDomain -- snmpSSHDomain
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) Extract the tmTransportAddress, tmSecurityName, tmSameSecurity,
and tmSessionID from the tmStateReference. (SSHTM ignores the
provided tmTransportDomain and tmRequestedSecurityLevel.)
2) Using tmTransportAddress and tmSecurityName, determine if a
corresponding row exists in the sshtmLCDTable.
3) If there is a corresponding row, and tmSameSecurity is true,
and tmSessionID does not match sshtmLCDSessionID, 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 corresponding row, then call openSession() with
the tmTransportAddress and tmSecurityName as parameters.
4b) If an error is returned from OpenSession(), then discard the
message, and return the error indication returned by OpenSession()
in the statusInformation.
5) Determine any SSH-implementation-specific parameters from the
sshtmLCDTable
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 SSH
Transport Model and the SSH service. It is an implementation
decision how such data is passed.
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statusInformation =
openSession(
IN destTransportAddress -- transport address to be used
IN tmSecurityName -- on behalf of this principal
IN maxMessageSize -- of the sending SNMP entity
)
The following describes the procedure to follow to establish a
session between a client and server to run SNMP over SSH. This
process is used 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 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) Create an entry in the sshtmLCDTable:
sshtmLCDTransportAddress = tmTransportAddress
sshtmLCDTmSecurityName = tmSecurityName
Using the sshtmLCDTransformPolicy, calculate the corresponding
sshtmLCDPrincipal value.
3)In an implementation-specific manner, pass the sshtmLCDPrincipal to
the SSH layer. The client will then invoke an SSH authentications
service to authenticate the principal, such as that described in the
SSH authentication protocol [RFC4252]. The credentials used to
authenticate sshtmLCDPrincipal are determined in an implementation-
dependent manner.
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
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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 transport connection is closed,
tmStateReference is released, the message is discarded, 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. Set the
sshtmLCDSessionID in the corresponding entry. Increment the
sshtmSessionOpens counter.
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.
5.4. Closing a Session
The Secure Shell Transport Model provides the following ASI to close
a session:
statusInformation =
closeSession(
IN tmTransportAddress -- transport address to be used
IN tmSecurityName -- on behalf of this principal
)
The following describes the procedure to follow to close a session
between a client and sever . This process is followed by any SNMP
engine to close an SSH session. It is implementation-dependent when
a session should be closed.
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1) Lookup the session in the sshtmLCDTable using the
tmTransportAddress and tmSecurityName.
3) If there is no entry, then closeSession processing is
completed.
4) Extract sshtmLCDPrincipal and sshtmLCDSessionID. Have SSH
close the session. Increment the sshtmSessionCloses counter.
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 for the
Transport Model (not for SSH or an associated security model).
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 }
sshtmLCD OBJECT IDENTIFIER ::= { sshtmObjects 2 }
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
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SYNTAX Counter32
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 }
-- The table of users for the Transport Security Model
-- This table can support users of multiple transport models
sshtmLCDSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
Command Generator Applications to coordinate their
use of facilities to alter the sshtmLCDTable.
"
::= { sshtmLCD 1 }
-- The transform policy for the SSH Transport Model
sshtmLCDTransformPolicy OBJECT-TYPE
SYNTAX INTEGER { default(1),
private(2)
}
MAX-ACCESS read-write
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STATUS current
DESCRIPTION
"The policy that should be used to perform transforms
between the SSH identity and tmSecurityName, and vice-versa.
default (1) - for incoming messages, the value passed in
the user name field of SSH user_auth is assigned
to both sshtmLCDTmSecurityName and sshtmLCDPrincipal.
For outgoing messages, the value passed in tmSecurityName
is assigned to both sshtmLCDTmSecurityName and
sshtmLCDPrincipal.
private (2) - use an implementation-specific mapping
algorithm for the transform. If the algorithm does not yield
a mapping, no entry should be created for the identity in
the sshtmLCDTable. It is implementation-dependent
whether a private algorithm is supported.
"
DEFVAL { default }
::= { sshtmLCD 2 }
sshtmLCDTable OBJECT-TYPE
SYNTAX SEQUENCE OF sshtmLCDEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of users configured in the SSH
Transport Model Local Configuration Datastore (LCD).
Rows in this table can be instantiated when an
authenticated identity is passed to the transport model
from SSH, and they can be instantiated by a command
generator.
To instantiate a new row in this table, the
sshtmLCDSpinLock should be used to prevent conflicts.
1) GET(sshtmLCDSpinLock.0) and save in sValue.
2) SET(sshtmLCDSpinLock.0=sValue,
sshtmLCDTransportAddress=(desired value),
sshtmLCDTmSecurityName=(desired value),
sshtmLCDPrincipal=(desired value),
sshtmLCDSessionID=sValue,
sshtmLCDStorageType=(desired value),
sshtmLCDStatus=createAndGo)
"
::= { sshtmLCD 3 }
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sshtmLCDEntry OBJECT-TYPE
SYNTAX sshtmLCDEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A user configured in the Local
Configuration Datastore (LCD) for the SSH
Transport Model.
To maintain modularity of design, and to avoid
side-effects, only the SSH Transport Model
(or a SET operation) should modify this table.
For all entries in this table, the transportDomain
is snmpSSHTransportDomain, and the security level
is authpriv.
"
INDEX { sshtmLCDTransportAddress,
sshtmLCDTmSecurityName
}
::= { sshtmLCDTable 1 }
sshtmLCDEntry ::= SEQUENCE
{
sshtmLCDTransportAddress TransportAddress,
sshtmLCDTmSecurityName SnmpAdminString,
sshtmLCDPrincipal SnmpAdminString,
sshtmLCDSessionID Integer32
sshtmLCDStorageType StorageType,
sshtmLCDRowStatus RowStatus
}
sshtmLCDTransportAddress OBJECT-TYPE
SYNTAX TransportAddress
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This object contains a transport address.
"
::= { sshtmLCDEntry 2 }
sshtmLCDTmSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A human readable string representing the user.
This value is passed in tmSecurityName.
The default transformation of the SSH user name to
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tmSecurityName and vice versa is the identity function
so the sshtmLCDTmSecurityName is usually the same as
the sshtmLCDPrincipal.
"
::= { sshtmLCDEntry 3 }
sshtmLCDPrincipal OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A human readable string used to determine, in an
implementation-dependent manner, which SSH
credentials are used during authentication of the
prinicipal.
By default, this is the SSH user name.
"
::= { sshtmLCDEntry 5 }
sshtmLCDStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The storage type for this conceptual row.
Conceptual rows having the value readOnly, permanent,
or nonVolatile must persist across reinitializations of
the management subsystem.
Conceptual rows having the value 'volatile' must not
persist across reinitializations of the management
subsystem.
It is an implementation issue to decide if a SET for
a readOnly or permanent row is accepted at all. In
some contexts this may make sense, in others it may
not. If a SET for a readOnly or permanent row is not
accepted at all, then a 'wrongValue' error must be
returned.
"
DEFVAL { volatile }
::= { sshtmLCDEntry 6 }
sshtmLCDRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The status of this conceptual row.
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Until instances of all corresponding columns are
appropriately configured, the value of the
corresponding instance of sshtmLCDStatus
is 'notReady'.
The sshtmLCDPrincipal value should only be
changed when the value of this object
is 'active'.
"
::= { sshtmLCDEntry 7 }
-- ************************************************
-- 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 }
-- ************************************************
-- Units of conformance
-- ************************************************
sshtmGroup OBJECT-GROUP
OBJECTS {
sshtmSessionOpens,
sshtmSessionCloses,
sshtmSessionOpenErrors,
sshtmSessionUserAuthFailures,
sshtmSessionChannelOpenFailures,
sshtmSessionNoAvailableSessions,
sshtmLCDSpinLock,
sshtmLCDTransportAddress,
sshtmLCDTmSecurityName,
sshtmLCDPrincipal,
sshtmLCDStorageType,
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sshtmLCDRowStatus
}
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 two well-known default ports, one for
request/response traffic, and one port that listens for
notifications.
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
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
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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.
To initiate SSH connections, an entity must be configured with SSH
client credentials and information to authenticate the server. While
hosts are often configured to be SSH clients, most internetworking
devices are not. To send notifications over SSHTM, the
internetworking device will need to be configured to be SSH clients.
How this credential configuration is done is implementation and
deployment specific. A scalable IETF standard protocol for
configuration or key management is RECOMMENDED.
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
server to the SSH client, nor does it compromise data confidentiality
or data integrity.
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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.
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
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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.
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
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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
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.
[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-12 (work
in progress), February 2008.
12.2. Informative References
[RFC1994] Simpson, W., "PPP Challenge Handshake
Authentication Protocol (CHAP)", RFC 1994,
August 1996.
[RFC3410] Case, J., Mundy, R., Partain, D., and B.
Stewart, "Introduction and Applicability
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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.
[RFC5090] Sterman, B., Sadolevsky, D., Schwartz, D.,
Williams, D., and W. Beck, "RADIUS Extension
for Digest Authentication", RFC 5090,
February 2008.
[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.
o Issue #3: Mapping from the sshtmLCDTable identity to an SSH
mechanisms-specific identity. This may just be the opposite
transform of Issue #2.
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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.
TODO:
finalize error processing in EOP
Appendix B. Change Log
From -10- to -11
Changed LCD to sshtmLCDTable so it would not be confused with the
snmpTsmLCD.
Removed the text that said the format and content of the LCD is
implementation-specific, since we now have a MIB module to
standardize the format and content.
Designed sshtmLCDTable to reflect there is only one
transportDomain and one securityLevel supported by this transport
model.
Used sshtmLCDTmSecurityName to reflect that the values in this
table and the values in the tmStateReference are usually the same
for some fields.
Added operational considerations about SSH client credential
distribution.
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Modified EOP to use sshtmLCDTable
Resolved 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. - The consensus is that we will not allow the transport
model to specify the security model.
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
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
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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".
Removed security-model-specific processing from this document.
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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"
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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
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
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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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