Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA)
Intended status: Standards Track J. Salowey
Expires: April 9, 2009 Cisco Systems
W. Hardaker
Sparta, Inc.
October 6, 2008
Secure Shell Transport Model for SNMP
draft-ietf-isms-secshell-12
Status of This Memo
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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
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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 . . . . . . . . . . . . . . . . 10
3.1.3. Authentication Protocol Support . . . . . . . . . . . 11
3.1.4. Privacy Protocol Support . . . . . . . . . . . . . . . 11
3.1.5. Protection against Message Replay, Delay and
Redirection . . . . . . . . . . . . . . . . . . . . . 11
3.1.6. SSH Subsystem . . . . . . . . . . . . . . . . . . . . 12
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 12
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 13
4. Cached Information and References . . . . . . . . . . . . . . 13
4.1. securityStateReference . . . . . . . . . . . . . . . . . . 14
4.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. Transport information . . . . . . . . . . . . . . . . 15
4.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 15
4.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 16
4.2.4. Session Information . . . . . . . . . . . . . . . . . 16
4.3. Secure Shell Transport Model Cached Information . . . . . 17
4.3.1. tmSecurityName . . . . . . . . . . . . . . . . . . . . 17
4.3.2. tmSessionID . . . . . . . . . . . . . . . . . . . . . 17
4.3.3. session state . . . . . . . . . . . . . . . . . . . . 17
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 18
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 18
5.2. Procedures for an Outgoing Message . . . . . . . . . . . . 19
5.3. Establishing a Session . . . . . . . . . . . . . . . . . . 20
5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 22
6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 22
6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 22
6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 23
6.3. Relationship to Other MIB Modules . . . . . . . . . . . . 23
6.3.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 23
7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 23
8. Operational Considerations . . . . . . . . . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9.1. noAuthPriv . . . . . . . . . . . . . . . . . . . . . . . . 31
9.2. Use with SNMPv1/v2c Messages . . . . . . . . . . . . . . . 31
9.3. Skipping Public Key Verification . . . . . . . . . . . . . 31
9.4. The 'none' MAC Algorithm . . . . . . . . . . . . . . . . . 32
9.5. MIB Module Security . . . . . . . . . . . . . . . . . . . 32
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
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11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 38
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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-Hellman 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. Cached Information and References
When performing SNMP processing, there are two levels of state
information that may need to be retained: the immediate state linking
a request-response pair, and potentially longer-term state relating
to transport and security.
The RFC3411 architecture uses caches to maintain the short-term
message state, and uses references in the ASIs to pass this
information between subsystems.
This document defines the requirements for a cache to handle the
longer-term transport state information, using a tmStateReference
parameter to pass this information between subsystems.
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 being processed gets
discarded, the state related to that message SHOULD also be
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discarded. If state information is available when a relationship
between engines is severed, such as the closing of a transport
session, the state information for that relationship SHOULD also be
discarded.
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.
4.1. securityStateReference
The securityStateReference parameter is defined in RFC3411. Its
primary purpose is to provide a mapping between a request and the
corresponding response. This cache is not accessible to Transport
Models, and an entry is typically only retained for the lifetime of a
request-response pair of messages.
4.2. tmStateReference
For each transport session, information about the transport security
is stored in a cache. The tmStateReference parameter is used to pass
model-specific and mechanism-specific parameters between the
Transport subsystem and transport-aware Security Models.
The tmStateReference cache will typically remain valid for the
duration of the transport session, and hence may be used for several
messages.
Since this cache is only used within an implementation, and not on-
the-wire, the precise contents and format are implementation-
dependent. However, for interoperability between Transport Models
and transport-aware Security Models, entries in this cache must
include at least the following fields:
transportDomain
transportAddress
tmSecurityName
tmRequestedSecurityLevel
tmTransportSecurityLevel
tmSameSecurity
tmSessionID
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4.2.1. Transport information
Information about the source of an incoming SNMP message is passed up
from the Transport subsystem as far as the Message Processing
subsystem. However these parameters are not included in the
processIncomingMsg ASI defined in RFC3411, and hence this information
is not directly available to the Security Model.
A transport-aware Security Model might wish to take account of the
transport protocol and originating address when authenticating the
request, and setting up the authorization parameters. It is
therefore necessary for the Transport Model to include this
information in the tmStateReference cache, so that it is accessible
to the Security Model.
o transportDomain: the transport protocol (and hence the Transport
Model) used to receive the incoming message
o transportAddress: the source of the incoming message.
Note that the ASIs used for processing an outgoing message all
include explicit transportDomain and transportAddress parameters.
These fields within the tmStateReference cache will typically not be
used for outgoing messages.
4.2.2. securityName
There are actually three distinct "identities" that can be identified
during the processing of an SNMP request over a secure transport:
o transport principal: the transport-authenticated identity, on
whose behalf the secure transport connection was (or should be)
established. This value is transport-, mechanism- and
implementation- specific, and is only used within a given
Transport Model.
o tmSecurityName: a human-readable name (in snmpAdminString format)
representing this transport identity. This value is transport-
and implementation-specific, and is only used (directly) by the
Transport and Security Models.
o securityName: a human-readable name (in snmpAdminString format)
representing the SNMP principal in a model-independent manner.
o Note that the transport principal may or may not be the same as
the tmSecurityName. Similarly, the tmSecurityName may or may not
be the same as the securityName as seen by the Application and
Access Control subsystems. In particular, a non-transport-aware
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Security Model will ignore tmSecurityName completely when
determining the SNMP securityName.
o However it is important that the mapping between the transport
principal and the SNMP securityName (for transport-aware Security
Models) is consistent and predictable, to allow configuration of
suitable access control and the establishment of transport
connections.
4.2.3. securityLevel
There are two distinct issues relating to security level as applied
to secure transports. For clarity, these are handled by separate
fields in the tmStateReference cache:
o tmTransportSecurityLevel: an indication from the Transport Model
of the level of security offered by this session. The Security
Model can use this to ensure that incoming messages were suitably
protected before acting on them.
o tmRequestedSecurityLevel: an indication from the Security Model of
the level of security required to be provided by the transport
protocol. The Transport Model can use this to ensure that
outgoing messages will not be sent over an insufficiently secure
session.
4.2.4. Session Information
For security reasons, if a secure transport session is closed between
the time a request message is received and the corresponding response
message is sent, then the response message SHOULD be discarded, even
if a new session has been established. The SNMPv3 WG decided that
this should be a SHOULD architecturally, and it is a security-model-
specific decision whether to REQUIRE this.
When processing an outgoing message, if tmSameSecurity is true, then
the tmSessionID MUST match the current transport session, otherwise
the message MUST be discarded, and the dispatcher notified that
sending the message failed.
o tmSameSecurity: this flag is used by a transport-aware Security
Model to indicate whether the Transport Model MUST enforce this
restriction.
o tmSessionID: in order to verify whether the session has changed,
the Transport Model must be able to compare the session used to
receive the original request with the one to be used to send the
response. This typically requires some form of session
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identifier. This value is only ever used by the Transport Model,
so the format and interpretation of this field are model-specific
and implementation-dependent.
4.3. Secure Shell Transport Model Cached Information
The Secure Shell Transport Model has specific responsibilities
regarding the cached information. See the Elements of Procedure for
detailed processing instructions on the use of the tmStateReference
fields by the SSH Transport Model.
4.3.1. tmSecurityName
The tmSecurityName MUST be a human-readable name (in snmpAdminString
format) representing the identity that has been authenticated by the
SSH layer.
The identity SHOULD be 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 the SSH user name is extracted from the SSH
layer is implementation-dependent.
The SSH protocol is not always clear on whether the user name field
must be filled in, so for some implementations, such as those using
GSSAPI authentication, it may be necessary to use a mapping algorithm
to transform a user name to a SSH identity compatible with the
parameters required by this transport. How a compatible SSH identity
is determined should be administratively configurable if such a
mapping is needed.
The securityName derived from the tmSecurityName by a security model
is used to configure notifications and access controls. Non-default
transport model transforms SHOULD generate a predictable identity
representing the principal.
4.3.2. tmSessionID
The tmSessionID must be refreshed upon each received 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.
4.3.3. session state
The per-session state that is referenced by tmStateReference may be
saved across multiple messages in a Local Configuration Datastore.
Additional session/connection state information might also be stored
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in a Local Configuration Datastore.
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) Create a tmStateReference cache for subsequent reference to the
information.
tmTransportDomain = snmpSSHDomain
tmTransportAddress = the address the message originated from,
determined in an implementation-dependent way
tmSecurityLevel = "authPriv"
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tmSecurityName = the ssh principal name
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:
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. Other
implementation dependent steps may also be needed.
1) Extract the tmTransportAddress, tmSecurityName, tmSameSecurity,
and tmSessionID from the tmStateReference. (SSHTM ignores the
provided tmTransportDomain and tmRequestedSecurityLevel.)
2) Using tmTransportAddress and tmSecurityName or some other
implementation dependent way, determine if a corresponding entry
in the LCD exists.
3) If there is a corresponding entry, and tmSameSecurity is true,
and tmSessionID does not match the session id stored in the LCD,
then increment the sshtmSessionNoAvailableSessions counter,
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discard the message and return the error indication in the
statusInformation. Processing of this message stops.
4) If there is no corresponding LCD entry, 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) 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.
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 destTransportAddress field may contain a user-name followed by an
'@' character (ASCII 0x40) that will indicate a specific user-name
string that should presented to the ssh server as the "user name" for
authentication purposes. This may be different than the passed
tmSecurityName value that should be used in the remaining steps
below. If there is no specified user-name in the
destTransportAddress then the tmSecurtityName should be used as the
user-name. The other parameters of the transport connection and the
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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 LCD containing, at a minimum, a cache of
the following information:
tmTransportAddress
tmSecurityName
3)In an implementation-specific manner, pass the calculated user-name
from step 1) 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 the user 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
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. Store the
session identifier in the corresponding LCD 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
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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.
1) Look up the session information in the LCD using the
tmTransportAddress and tmSecurityName or other implementation-
dependent mechanism.
2) If there is no entry, then closeSession processing is
completed.
3) Extract the session identifier from the LCD entry. 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.
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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]
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
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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
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
USA
jsalowey@cisco.com
Wes Hardaker
Sparta, Inc.
P.O. Box 382
Davis, CA 95617
USA
+1 530 792 1913
ietf@hardakers.net
"
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
"
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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
and including 8192 octets in size. Implementation of
larger values is encouraged whenever possible.
The securityName prefix to be associated with the
snmpSSHDomain is 'ssh'. This prefix may be used by security
models or other components to identify what secure transport
infrastructure authenticated a securityName. For further
details on the usage of this prefix, see the
[I-D.ietf-isms-tmsm-transport-security-model]
document and the snmpTsmConfigurationUsePrefix in the
SNMP-TSM-MIB."
::= { snmpDomains yy }
-- RFC Ed.: Please replace the I-D reference with a proper one once it
-- has been published. Note: xml2rfc doesn't handle refs within artwork
-- RFC Ed.: replace yy with IANA-assigned number and
-- remove this note
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-- RFC Ed.: replace 'ssh' with the actual IANA assigned prefix string
-- if 'ssh' is not assigned to this document.
SnmpSSHAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"Represents either a hostname or IP address, along with a port
number and an optional username.
The beginning of the address specification may contain a
username followed by an '@' (ASCII character 0x40). This
portion of the address will indicate the user name that should
be used when authenticating to an SSH server. If missing, the
SNMP securityName should be used. After the optional user name
field and '@' character comes the hostname.
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 ('[' ASCII character 0x5B and ']' ASCII character
0x5D), 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
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TransportAddressType or TransportDomain as a pair.
When this textual convention is used as a syntax of an
index object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2, STD 58. 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 }
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sshtmSessionUserAuthFailures OBJECT-TYPE
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 }
-- ************************************************
-- 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 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
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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.
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.
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9.1. noAuthPriv
SSH provides the "none" userauth method, which is normally rejected
by servers and used only to find out what userauth methods are
supported. However, it is legal for a server to accept this method,
which has the effect of not authenticating the SSH client to the SSH
server. Doing this does not compromise authentication of the SSH
server to the SSH client, nor does it compromise data confidentiality
or data integrity.
SSH supports anonymous access. If 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
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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
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
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instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
10. IANA Considerations
IANA is requested to assign:
1. a TCP port number in the range 1..1023 in the
http://www.iana.org/assignments/port-numbers registry which will
be the default port for SNMP over 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. "ssh" as the corresponding prefix for the snmpSSHDomain in the
SNMP Transport Model registry;
5. "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, and Dave Shields for an outstanding job wordsmithing the
existing document to improve organization and clarity.
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.,
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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.
[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.,
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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-13
(work in progress),
August 2008.
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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 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
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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.
[I-D.ietf-isms-transport-security-model] Harrington, D., "Transport
Security Model for SNMP", d
raft-ietf-isms-transport-
security-model-08 (work in
progress), July 2008.
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.
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
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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 -11- to -12
updated "Cached Information and References" to match other ISMS
documents.
Added separate subsection on Secure Shell Transport Model Cached
Information.
Added IANA considerations to establish a registry for domains and
corresponding prefixes.
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.
Modified EOP to use sshtmLCDTable
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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.
Added support for user@ prefixing in the SSH Transport Address
definition and EOP.
Added support for the "ssh" prefix to the transport address
definition and IANA considerations section.
Removed the LCD tables and related configuration since the user@
transport address prefixing and the TSM user prefix changes change
makes it no longer needed.
From -07- to -08
Updated MIB
update MIB security considerations
develop sameSession and tmSessionID support
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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 table into the transport model MIB from the
transport subsystem MIB
added and then removed Appendix A - Notification Tables
Configuration (see Transport Security Model)
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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.
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
created 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 sections 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-"
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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
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 secshell RFCs
Modified text related to issues# 1, 2, 8, 11, 13, 14, 16, 18, 19,
20, 29, 30, and 32.
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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
Wes Hardaker
Sparta, Inc.
P.O. Box 382
Davis, CA 95617
US
Phone: +1 530 792 1913
EMail: ietf@hardakers.net
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