Network Working Group                                      D. Harrington
Internet-Draft                                 Huawei Technologies (USA)
Intended status: Standards Track                              J. Salowey
Expires: May 11, 2008                                      Cisco Systems
                                                        November 8, 2007


                 Secure Shell Transport Model for SNMP
                      draft-ietf-isms-secshell-09

Status of This Memo

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   This Internet-Draft will expire on May 11, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo describes a Transport Model for the Simple Network
   Management Protocol, using the Secure Shell protocol (SSH).

   This memo also defines a portion of the Management Information Base
   (MIB) for use with network management protocols in TCP/IP based
   internets.  In particular it defines objects for monitoring and
   managing the Secure Shell Transport Model for SNMP.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The Internet-Standard Management Framework . . . . . . . .  4
     1.2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Modularity . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Constraints  . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  The Secure Shell Protocol  . . . . . . . . . . . . . . . . . .  7
   3.  How SSHTM Fits into the Transport Subsystem  . . . . . . . . .  8
     3.1.  Security Capabilities of this Model  . . . . . . . . . . .  9
       3.1.1.  Threats  . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.2.  Message Authentication Issues  . . . . . . . . . . . . 10
       3.1.3.  Authentication Protocol  . . . . . . . . . . . . . . . 10
       3.1.4.  Privacy Protocol . . . . . . . . . . . . . . . . . . . 11
       3.1.5.  Protection against Message Replay, Delay and
               Redirection  . . . . . . . . . . . . . . . . . . . . . 11
       3.1.6.  SSH Subsystem  . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Security Parameter Passing . . . . . . . . . . . . . . . . 12
     3.3.  Notifications and Proxy  . . . . . . . . . . . . . . . . . 12
   4.  Passing Security Parameters  . . . . . . . . . . . . . . . . . 13
     4.1.  tmStateReference . . . . . . . . . . . . . . . . . . . . . 13
     4.2.  tmSecurityName . . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  tmSameSecurity . . . . . . . . . . . . . . . . . . . . . . 15
   5.  Elements of Procedure  . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Procedures for an Incoming Message . . . . . . . . . . . . 16
     5.2.  Procedures for an Outgoing Message . . . . . . . . . . . . 17
     5.3.  Establishing a Session . . . . . . . . . . . . . . . . . . 18
     5.4.  Closing a Session  . . . . . . . . . . . . . . . . . . . . 20
   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 . . . . . . . . . . . 21
   7.  MIB Module Definition  . . . . . . . . . . . . . . . . . . . . 21
   8.  Operational Considerations . . . . . . . . . . . . . . . . . . 26
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
     9.1.  noAuthPriv . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.2.  Use with SNMPv1/v2c Messages . . . . . . . . . . . . . . . 28
     9.3.  Skipping Public Key Verification . . . . . . . . . . . . . 28
     9.4.  The 'none' MAC Algorithm . . . . . . . . . . . . . . . . . 29
     9.5.  MIB Module Security  . . . . . . . . . . . . . . . . . . . 29
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     12.2. Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Open Issues . . . . . . . . . . . . . . . . . . . . . 33



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   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 33


















































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1.  Introduction

   This memo describes a Transport Model for the Simple Network
   Management Protocol, using the Secure Shell protocol (SSH) [RFC4251]
   within a transport subsystem [I-D.ietf-isms-tmsm].  The transport
   model specified in this memo is referred to as the Secure Shell
   Transport Model (SSHTM).

   This memo also defines a portion of the Management Information Base
   (MIB) for use with network management protocols in TCP/IP based
   internets.  In particular it defines objects for monitoring and
   managing the Secure Shell Transport Model for SNMP.

   It is important to understand the SNMP architecture [RFC3411] and the
   terminology of the architecture to understand where the Transport
   Model described in this memo fits into the architecture and interacts
   with other subsystems within the architecture.

1.1.  The Internet-Standard Management Framework

   For a detailed overview of the documents that describe the current
   Internet-Standard Management Framework, please refer to section 7 of
   RFC 3410 [RFC3410].

   Managed objects are accessed via a virtual information store, termed
   the Management Information Base or MIB.  MIB objects are generally
   accessed through the Simple Network Management Protocol (SNMP).
   Objects in the MIB are defined using the mechanisms defined in the
   Structure of Management Information (SMI).  This memo specifies a MIB
   module that is compliant to the SMIv2, which is described in STD 58,
   RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
   [RFC2580].

1.2.  Conventions

   For consistency with SNMP-related specifications, this document
   favors terminology as defined in STD62 rather than favoring
   terminology that is consistent with non-SNMP specifications.  This is
   consistent with the IESG decision to not require the SNMPv3
   terminology be modified to match the usage of other non-SNMP
   specifications when SNMPv3 was advanced to Full Standard.

   Authentication in this document typically refers to the English
   meaning of "serving to prove the authenticity of" the message, not
   data source authentication or peer identity authentication.

   The terms "manager" and "agent" are not used in this document,
   because in the RFC 3411 architecture [RFC3411], all SNMP entities



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   have the capability of acting in either manager or agent or in both
   roles depending on the SNMP application types supported in the
   implementation.  Where distinction is required, the application names
   of Command Generator, Command Responder, Notification Originator,
   Notification Receiver, and Proxy Forwarder are used.  See "SNMP
   Applications" [RFC3413] for further information.

   Throughout this document, the terms "client" and "server" are used to
   refer to the two ends of the SSH transport connection.  The client
   actively opens the SSH connection, and the server passively listens
   for the incoming SSH connection.  Either SNMP entity may act as
   client or as server, as discussed further below.

   The User-Based Security Model (USM) [RFC3414] is a mandatory-to-
   implement Security Model in STD 62.  While SSH and USM frequently
   refer to a user, the terminology preferred in RFC3411 [RFC3411] and
   in this memo is "principal".  A principal is the "who" on whose
   behalf services are provided or processing takes place.  A principal
   can be, among other things, an individual acting in a particular
   role; a set of individuals, with each acting in a particular role; an
   application or a set of applications, or a combination of these
   within an administrative domain.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Sections requiring further editing are identified by [todo] markers
   in the text.  Points requiring further WG research and discussion are
   identified by [discuss] markers in the text.

   Note to RFC Editor - if the previous paragraph and this note have not
   been removed, please send the document back to the editor to remove
   this.

1.3.  Modularity

   The reader is expected to have read and understood the description of
   the SNMP architecture, as defined in [RFC3411], and the Transport
   Subsystem architecture extension specified in "Transport Subsystem
   for the Simple Network Management Protocol" [I-D.ietf-isms-tmsm].

   This memo describes the Secure Shell Transport Model for SNMP, a
   specific SNMP transport model to be used within the SNMP transport
   subsystem to provide authentication, encryption, and integrity
   checking of SNMP messages.

   In keeping with the RFC 3411 design decision to use self-contained



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   documents, this document defines the elements of procedure and
   associated MIB module objects which are needed for processing the
   Secure Shell Transport Model for SNMP.

   This modularity of specification is not meant to be interpreted as
   imposing any specific requirements on implementation.

1.4.  Motivation

   Version 3 of the Simple Network Management Protocol (SNMPv3) added
   security to the protocol.  The User-based Security Model (USM)
   [RFC3414] was designed to be independent of other existing security
   infrastructures, to ensure it could function when third party
   authentication services were not available, such as in a broken
   network.  As a result, USM utilizes a separate user and key
   management infrastructure.  Operators have reported that deploying
   another user and key management infrastructure in order to use SNMPv3
   is a reason for not deploying SNMPv3.

   This memo describes a transport model that will make use of the
   existing and commonly deployed Secure Shell security infrastructure.
   This transport model is designed to meet the security and operational
   needs of network administrators, maximize usability in operational
   environments to achieve high deployment success and at the same time
   minimize implementation and deployment costs to minimize deployment
   time.

   This document addresses the requirement for the SSH client to
   authenticate the SSH server, for the SSH server to authenticate the
   SSH client, and describes how SNMP can make use of the authenticated
   identities in authorization policies for data access, in a manner
   that is independent of any specific access control model.

   This document addresses the requirement to utilize client
   authentication and key exchange methods which support different
   security infrastructures and provide different security properties.
   This document describes how to use client authentication as described
   in "SSH Authentication Protocol" [RFC4252].  The SSH Transport Model
   should work with any of the ssh-userauth methods including the
   "publickey", "password", "hostbased", "none", "keyboard-interactive",
   "gssapi-with-mic", ."gssapi-keyex", "gssapi", and "external-keyx"
   (see http://www.iana.org/assignments/ssh-parameters).  The use of the
   "none" authentication method is NOT RECOMMENDED, as described in
   Security Considerations.  Local accounts may be supported through the
   use of the publickey, hostbased or password methods.  The password
   method allows for integration with deployed password infrastructure
   such as AAA servers using the RADIUS protocol [RFC2865].  The SSH
   Transport Model SHOULD be able to take advantage of future defined



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   ssh-userauth methods, such as those that might make use of X.509
   certificate credentials.

   It is desirable to use mechanisms that could unify the approach for
   administrative security for SNMPv3 and Command Line interfaces (CLI)
   and other management interfaces.  The use of security services
   provided by Secure Shell is the approach commonly used for the CLI,
   and is the approach being adopted for use with NETCONF [RFC4742].
   This memo describes a method for invoking and running the SNMP
   protocol within a Secure Shell (SSH) session as an SSH subsystem.

   This memo describes how SNMP can be used within a Secure Shell (SSH)
   session, using the SSH connection protocol [RFC4254] over the SSH
   transport protocol, using SSH user-auth [RFC4252] for authentication.

   There are a number of challenges to be addressed to map Secure Shell
   authentication method parameters into the SNMP architecture so that
   SNMP continues to work without any surprises.  These are discussed in
   detail below.

1.5.  Constraints

   The design of this SNMP Transport Model is influenced by the
   following constraints:
   1.  In times of network stress, the transport protocol and its
       underlying security mechanisms SHOULD NOT depend upon the ready
       availability of other network services (e.g., Network Time
       Protocol (NTP) or AAA protocols).
   2.  When the network is not under stress, the transport model and its
       underlying security mechanisms MAY depend upon the ready
       availability of other network services.
   3.  It may not be possible for the transport model to determine when
       the network is under stress.
   4.  A transport model should require no changes to the SNMP
       architecture.
   5.  A transport model should require no changes to the underlying
       protocol.

2.  The Secure Shell Protocol

   SSH is a protocol for secure remote login and other secure network
   services over an insecure network.  It consists of three major
   protocol components, and add-on methods for user authentication:
   o  The Transport Layer Protocol [RFC4253] provides server
      authentication, and message confidentiality and integrity.  It may
      optionally also provide compression.  The transport layer will
      typically be run over a TCP/IP connection, but might also be used
      on top of any other reliable data stream.



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   o  The User Authentication Protocol [RFC4252] authenticates the
      client-side principal to the server.  It runs over the transport
      layer protocol.
   o  The Connection Protocol [RFC4254] multiplexes the encrypted tunnel
      into several logical channels.  It runs over the transport after
      successfully authenticating the principal.
   o  Generic Message Exchange Authentication [RFC4256] is a general
      purpose authentication method for the SSH protocol, suitable for
      interactive authentications where the authentication data should
      be entered via a keyboard
   o  Generic Security Service Application Program Interface (GSS-API)
      Authentication and Key Exchange for the Secure Shell (SSH)
      Protocol [RFC4462] describes methods for using the GSS-API for
      authentication and key exchange in SSH.  It defines an SSH user
      authentication method that uses a specified GSS-API mechanism to
      authenticate a user, and a family of SSH key exchange methods that
      use GSS-API to authenticate a Diffie-Hellman key exchange.
   The client sends a service request once a secure transport layer
   connection has been established.  A second service request is sent
   after client authentication is complete.  This allows new protocols
   to be defined and coexist with the protocols listed above.

   The connection protocol provides channels that can be used for a wide
   range of purposes.  Standard methods are provided for setting up
   secure interactive shell sessions and for forwarding ("tunneling")
   arbitrary TCP/IP ports and X11 connections.

3.  How SSHTM Fits into the Transport Subsystem

   A transport model plugs into the Transport Subsystem.  The SSH
   Transport Model thus fits between the underlying SSH transport layer
   and the message dispatcher [RFC3411].

   The SSH Transport Model will establish a channel between itself and
   the SSH Transport Model of another SNMP engine.  The sending
   transport model passes unencrypted messages from the dispatcher to
   SSH to be encrypyed, and the receiving transport model accepts
   decrypted incoming messages from SSH and passes them to the
   disptacher.

   After an SSH Transport model channel is established, then SNMP
   messages can conceptually be sent through the channel from one SNMP
   message dispatcher to another SNMP message dispatcher.  Multiple SNMP
   messages MAY be passed through the same channel.

   The SSH Transport Model of an SNMP engine will perform the
   translation between SSH-specific security parameters and SNMP-
   specific, model-independent parameters.



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3.1.  Security Capabilities of this Model

3.1.1.  Threats

   The Secure Shell Transport Model provides protection against the
   threats identified by the RFC 3411 architecture [RFC3411]:

   1.  Message stream modification - SSH provides for verification that
       each received message has not been modified during its
       transmission through the network.
   2.  Information modification - SSH provides for verification that the
       contents of each received message has not been modified during
       its transmission through the network, data has not been altered
       or destroyed in an unauthorized manner, nor have data sequences
       been altered to an extent greater than can occur non-maliciously.
   3.  Masquerade - SSH provides for both verification of the identity
       of the SSH server and verification of the identity of the SSH
       client - the principal on whose behalf a received SNMP message
       claims to have been generated.  It is not possible to assure the
       specific principal that originated a received SNMP message;
       rather, it is the principal on whose behalf the message was
       originated that is authenticated.  SSH provides verification of
       the identity of the SSH server through the SSH Transport Protocol
       server authentication [RFC4253].
   4.  Verification of principal identity is important for use with the
       SNMP access control subsystem, to ensure that only authorized
       principals have access to potentially sensitive data.  The SSH
       user identity will be used to map to an SNMP model-independent
       securityName for use with SNMP access control.
   5.  Authenticating both the SSH server and the SSH client ensures the
       authenticity of the SNMP engine that provides MIB data, whether
       that engine resides on the server or client side of the
       association.  Operators or management applications might act upon
       the data they receive (e.g., raise an alarm for an operator,
       modify the configuration of the device that sent the
       notification, modify the configuration of other devices in the
       network as the result of the notification, and so on), so it is
       important to know that the provider of MIB data is authentic.
   6.  Disclosure - the SSH Transport Model provides that the contents
       of each received SNMP message are protected from disclosure to
       unauthorized persons.
   7.  Replay - SSH ensures that cryptographic keys established at the
       beginning of the SSH session and stored in the SSH session state
       are fresh new session keys generated for each session.  These are
       used to authenticate and encrypt data, and to prevent replay
       across sessions.  SSH uses sequence information to prevent the
       replay and reordering of messages within a session.




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3.1.2.  Message Authentication Issues

   The RFC 3411 architecture recognizes three levels of security:
      - without authentication and without privacy (noAuthNoPriv)
      - with authentication but without privacy (authNoPriv)
      - with authentication and with privacy (authPriv)

   The Secure Shell protocol provides support for encryption and data
   integrity.  While it is technically possible to support no
   authentication and no encryption in SSH it is NOT RECOMMENDED by
   [RFC4253].

   The SSH Transport Model determines from SSH the identity of the
   authenticated principal, and the type and address associated with an
   incoming message, and the SSH Transport Model provides this
   information to SSH for an outgoing message.  The transport layer
   algorithms used to provide authentication, data integrity and
   encryption SHOULD NOT be exposed to the SSH Transport Model layer.
   The SNMPv3 WG deliberately avoided this and settled for an assertion
   by the security model that the requirements of securityLevel were met
   The SSH Transport Model has no mechanisms by which it can test
   whether an underlying SSH connection provides auth or priv, so the
   SSH Transport Model trusts that the underlying SSH connection has
   been properly configured to support authPriv security
   characteristics.

   The SSH Transport Model does not know about the algorithms or options
   to open SSH sessions that match different securityLevels.  For
   interoperability of the trust assumptions between SNMP engines, an
   SSH Transport Model-compliant implementation MUST use an SSH
   connection that provides authentication, data integrity and
   encryption that meets the highest level of SNMP security (authPriv).
   Outgoing messages requested by SNMP applications and specified with a
   lesser securityLevel (noAuthNoPriv or authNoPriv) are sent by the SSH
   Transport Model as authPriv securityLevel.

   The security protocols used in the Secure Shell Authentication
   Protocol [RFC4252] and the Secure Shell Transport Layer Protocol
   [RFC4253] are considered acceptably secure at the time of writing.
   However, the procedures allow for new authentication and privacy
   methods to be specified at a future time if the need arises.

3.1.3.  Authentication Protocol

   The SSH Transport Model should support any server or client
   authentication mechanism supported by SSH.  This includes the three
   authentication methods described in the SSH Authentication Protocol
   document [RFC4252] - publickey, password, and host-based - and



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   keyboard interactive and others.

   The password authentication mechanism allows for integration with
   deployed password based infrastructure.  It is possible to hand a
   password to a service such as RADIUS [RFC2865] or Diameter [RFC3588]
   for validation.  The validation could be done using the user-name and
   user-password attributes.  It is also possible to use a different
   password validation protocol such as CHAP [RFC1994] or digest
   authentication [RFC4590] to integrate with RADIUS or Diameter.  At
   some point in the processing, these mechanisms require the password
   be made available as clear text on the device that is authenticating
   the password which might introduce threats to the authentication
   infrastructure.

   GSSKeyex [RFC4462] provides a framework for the addition of client
   authentication mechanisms which support different security
   infrastructures and provide different security properties.
   Additional authentication mechanisms, such as one that supports X.509
   certificates, may be added to SSH in the future.

3.1.4.  Privacy Protocol

   The SSH transport layer protocol provides strong encryption, server
   authentication, and integrity protection.

3.1.5.  Protection against Message Replay, Delay and Redirection

   SSH uses sequence numbers and integrity checks to protect against
   replay and reordering of messages within a connection.

   SSH also provides protection against replay of entire sessions.  In a
   properly-implemented Diffie-Helman exchange, both sides will generate
   new random numbers for each exchange, which means the encryption and
   integrity keys will be distinct for every session.

3.1.6.  SSH Subsystem

   This document describes the use of an SSH subsystem for SNMP to make
   SNMP usage distinct from other usages.

   SSH subsystems of type "snmp" are opened by the SSH Transport Model
   during the elements of procedure for an outgoing SNMP message.  Since
   the sender of a message initiates the creation of an SSH session if
   needed, the SSH session will already exist for an incoming message or
   the incoming message would never reach the SSH Transport Model.

   Implementations MAY choose to instantiate SSH sessions in
   anticipation of outgoing messages.  This approach might be useful to



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   ensure that an SSH session to a given target can be established
   before it becomes important to send a message over the SSH session.
   Of course, there is no guarantee that a pre-established session will
   still be valid when needed.

   SSH sessions are uniquely identified within the SSH Transport Model
   by the combination of transportAddressType, transportAddress,
   securityName, and securityLevel associated with each session.

3.2.  Security Parameter Passing

   For incoming messages, SSH-specific security parameters are
   translated by the transport model into security parameters
   independent of the transport and security models.  The transport
   model accepts messages from the SSH subsystem, and records the
   transport-related and SSH-security-related information, including the
   authenticated identity, in a cache referenced by tmStateReference,
   and passes the WholeMsg and the tmStateReference to the dispatcher
   using the receiveMessage() ASI (Application Service Interface).

   For outgoing messages, the transport model takes input provided by
   the dispatcher in the sendMessage() ASI.  The SSH Transport Model
   converts that information into suitable security parameters for SSH,
   establishes sessions as needed, and passes messages to the SSH
   subsystem for sending.

3.3.  Notifications and Proxy

   SSH connections may be initiated by command generators or by
   notification originators.  Command generators are frequently operated
   by a human, but notification originators are usually unmanned
   automated processes.  As a result, it may be necessary to provision
   authentication credentials on the SNMP engine containing the
   notification originator, or use a third party key provider such as
   Kerberos, so the engine can successfully authenticate to an engine
   containing a notification receiver.

   The targets to whom notifications should be sent is typically
   determined and configured by a network administrator.  The SNMP-
   TARGET-MIB module [RFC3413] contains objects for defining management
   targets, including transport domains and addresses and security
   parameters, for applications such as notifications and proxy.

   For the SSH Transport Model, transport type and address are
   configured in the snmpTargetAddrTable, and the securityName, and
   securityLevel parameters are configured in the snmpTargetParamsTable.
   The default approach is for an administrator to statically
   preconfigure this information to identify the targets authorized to



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   receive notifications or perform proxy.

   These MIB modules may be configured using SNMP or other
   implementation-dependent mechanisms, such as CLI scripting or loading
   a configuration file.  It may be necessary to provide additional
   implementation-specific configuration of SSH parameters.

4.  Passing Security Parameters

   For the SSH Transport Model, the session state needs to be maintained
   using tmStateReference.  RFC3411 discusses a securityStateReference,
   which is not accessible to the Transport Subsystem.

4.1.  tmStateReference

   Upon opening each SSH connection, the SSH Transport Model stores
   model- and mechanism-specific information about the connection in a
   cache, referenced by tmStateReference.

   For interoperability with Security Model designs, the state
   referenced by tmStateReference MUST include the following fields
   (with sample values).  See the Elements of Procedure for detailed
   processing instructions on the use of these fields by the SSH
   Transport Model.
      tmTransport = snmpSSHDomain
      tmAddress = an snmpSSHAddress
      tmRequestedSecurityLevel = ["noAuthNoPriv" | "authNoPriv" |
      "authPriv" ]
      tmTransportSecurityLevel = "authPriv"
      tmSecurityName = the principal name [to be] authenticated by SSH.
      See the section on tmSecurityName below.
      tmSameSecurity = true or false, depending on whether the Security
      Model requires that an outgoing response be sent using the same
      security parameters as were used fo rthe incoming request or for
      any other security-model-dependent reason.  See the section on
      tmSameSecurity below.


   The state referenced by tmStateReference for an SSH Transport Model
   should also contain an implementation-dependent identifier (e.g.,
   tmSessionID) that can be used to determine whether the SSH session
   available for sending an outgoing message is the same SSH session as
   was used when receiving the corresponding incoming message.

   The tmStateReference is used to pass references containing the
   appropriate SSH session information from the transport model for
   subsequent processing.




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   The state referenced by tmStateReference may be saved across multiple
   messages in a Local Configuration Datastore (LCD).

   A Transport Model will maintain any mapping between transport-
   specific security parameters and tmTransportSecurityLevel and
   tmSecurityName, and will verify for outgoing messages that the
   transport-provided security is at least as strong as
   tmRequestedSecurityLevel..

   The SSH Transport Model has the responsibility for explicitly
   releasing the complete tmStateReference and deleting the associated
   information when a session is closed.  [DISCUSS: does thi smean an
   admin cannot preconfigure informationi for connections?  How does
   this work with the TARGET-MIB?]

   Since the contents of a cache are meaningful only within an
   implementation, and not on-the-wire, the format of the cache and the
   LCD are implementation-specific.

4.2.  tmSecurityName

   How the SSH identity is extracted from the SSH layer is
   implementation-dependent.  How the SSH identity is mapped to a
   tmSecurityName should be administratively configurable.

   [TODO: standardize some dynamic mechanisms for SSHTM, per auth-
   protocol, such as user-auth, host-auth, etc.  Make the list of
   authProtocols expandable, and provide default algorithms.]  [DISCUSS:
   this cannot be implementation-dependent.  It used to identify the
   layer 8 principal for use in such things as logging and for access
   control policy assignment. it must generate a predictable
   securityName representing the principal, regardless of the
   authentication mechanism.  USM provides this by pre-configuration of
   the mapping of the auth protocol and auth-specific credentials to a
   securityName, in the usmUserTable.  We MUST have the similar mapping,
   preferably done dynamically rather than statically.  Therefore either
   the dynamic mapping algoruithm MUST be standardzied. or we MUST have
   a static mapping.]

   tmSecurityName is a human-readable name in SnmpAdminString format
   that is mapped from the identity that has been successfully
   authenticated by SSH.  By defulat, 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,



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   and a user name field.  How the authenticated user name is made
   available to the SNMP implementation is SSH-implementation dependent.

   The SSH userauth user name field is in ISO-10646 UTF-8 encoding
   [RFC3629]. tmSecurityName is an SnmpAdminString, represented using
   the ISO/IEC IS 10646-1 character set, encoded as an octet string
   using the UTF-8 transformation format described in [RFC2279].

   [DISCUSS: get expert review to understand differences between 2279
   UTF-8 encoding and 3629 UTF-8 encoding.  Are SnmpAdminsStrings and
   SSH user names fully compatible?  If not, where are the
   incompatibilities?  Does the SnmpAdminString format need to be
   deprecated and replaced with a format compatible with RFC3629?  If
   so, should tmSecurityName be in 3629 format? wil that still be
   compatible with VACM and USM and other existing SNMPv3 usages?]

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
   acrhitecturally, this is not available to the transport model;
   implementations may choose to make this available for simplicity.)

   Each Security Model that supports the tmStateReference cache will
   pass a tmSameSecurity parameter in the tmStateReference cache for
   outgoing messages to indicate whether the same security MUST be used
   for the outgoing message as was used for the corresponding incoming
   message (e.g., a request-response pair).  The tmStateReference for
   the Secure Shell Transport Model may also include an existing SSH-
   specific transport session identifier in an implementation-dependent
   format.

   If the tmSameSecurity is indicated, but the session identified in the
   tmStateReference does not match the current established SSH transport
   session, i.e., it is not the same SSH security session, the message
   MUST be discarded, and the dispatcher should be notified that the
   sending of the message failed.

5.  Elements of Procedure

   Abstract service interfaces have been defined by RFC 3411 to describe
   the conceptual data flows between the various subsystems within an
   SNMP entity.  The Secure Shell Transport Model uses some of these
   conceptual data flows when communicating between subsystems.  These
   RFC 3411-defined data flows are referred to here as public



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   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

   For an incoming message, the SSH Transport Model will put information
   from the SSH layer into a cache referenced by tmStateReference.

      1) The SSH Transport Model queries the associated SSH engine, in
      an implementation-dependent manner, to determine the transport and
      security parameters for the received message.

         tmTransportDomain = snmpSSHDomain
         tmTransportAddress = a snmpSSHAddress
         tmSecurityLevel = "authPriv"
         tmsSecurityName = the principal name authenticated by SSH.  By
         default, the tmSecurityName is the name that has been
         successfully authenticated by SSH, from the user name field of
         the SSH_MSG_USERAUTH_REQUEST message.  How this name is
         extracted from the SSH environment and how it is translated
         into a tmSecurityName is implementation-dependent.
      2) If one does not exist, the SSH Transport Model creates an entry
      in a Local Configuration Datastore, in an implementation-dependent
      format, containing the information and any implementation-specific
      parameters desired, and creates a tmStateReference for subsequent
      reference to the information.

   Then the Transport model passes the message to the Dispatcher using
   the following ASI:







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   statusInformation =
   receiveMessage(
   IN   transportDomain       -- domain for the received message
   IN   transportAddress      -- address for the received message
   IN   wholeMessage          -- the whole SNMP message from SSH
   IN   wholeMessageLength    -- the length of the SNMP message
   IN   tmStateReference      -- (NEW) transport info
    )

5.2.  Procedures for an Outgoing Message

   The Dispatcher passes the information to the Transport Model using
   the ASI defined in the transport subsystem:


   statusInformation =
   sendMessage(
   IN   destTransportDomain           -- transport domain to be used
   IN   destTransportAddress          -- transport address to be used
   IN   outgoingMessage               -- the message to send
   IN   outgoingMessageLength         -- its length
   IN   tmStateReference              -- (NEW) transport info
   )

   The SSH Transport Model performs the following tasks:

      1) Determine the target index by extracting the transportDomain,
      transportAddress, securityName, and securityLevel from the
      tmStateReference.
      2) Lookup the session in the Local Configuration Datastore using
      the target index
      3) If tmSameSecurity specified in the tmStateReference is true,
      and there is no session associated with the target that has the
      same session identifier (e.g., tmSessionID) as that specified in
      the tmStateReference, then increment the
      sshtmSessionNoAvailableSessions counter, discard the message and
      return the error indication in the statusInformation.  Processing
      of this message stops.
      4) If there is no session open associated with the target index,
      then call openSession().
      4a) If an error is returned from OpenSession(), then discard the
      message and return the error indication returned by OpenSession()
      in the statusInformation.
      4b) If openSession() is successful, then store any implementation-
      specific information in the LCD for subsequent use.
      5) Extract any implementation-specific parameters from the LCD





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      6) Pass the wholeMessage to SSH for encapsulation as data in an
      SSH message.

5.3.  Establishing a Session

   The Secure Shell Transport Model provides the following application
   service interface (ASI) to describe the data passed between the
   Transport Model and the SSH service.  It is an implementation
   decision how such data is passed.

   statusInformation =
   openSession(
   IN   destTransportDomain      -- transport domain to be used
   IN   destTransportAddress     -- transport address to be used
   IN   securityName             -- on behalf of this principal
   IN   securityLevel            -- Level of Security requested
   IN   maxMessageSize           -- of the sending SNMP entity
   OUT  tmStateReference         -- (NEW) transport info
    )


   The following describes the procedure to follow to establish a
   session between a client and server to run SNMP over SSH.  This
   process is followed by any SNMP engine establishing a session for
   subsequent use.

   This will be done automatically for an SNMP application that
   initiates a transaction, such as a Command Generator or a
   Notification Originator or a Proxy Forwarder.

   1) Using destTransportDomain and destTransportAddress, the client
   will establish an SSH transport connection using the SSH transport
   protocol, authenticate the server, and exchange keys for message
   integrity and encryption.  The parameters of the transport connection
   and the credentials used to authenticate are provided in an
   implementation-dependent manner.

   If the attempt to establish a connection is unsuccessful, or server
   authentication fails, then sshtmSessionOpenErrors is incremented, and
   an openSession error indication is returned, and openSession
   processing stops.

   2) The provided transport domain, transport address, securityName and
   securityLevel are used to lookup an associated entry in the Local
   Configuration Datastore (LCD).  Any model-specific information
   concerning the principal at the destination is extracted.  This step
   allows preconfiguration of model-specific principals mapped to the
   transport/name/level, for example, for sending notifications.



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   In an implementation-specific manner, pass the username extracted
   from the LCD to the SSH layer.  [TODO: this may need to be
   standardized.]

   3)The client will then invoke an SSH authentications service to
   authenticate the user, such as that described in the SSH
   authentication protocol [RFC4252].  The credentials used to
   authenticate are provided in an implementation-dependent manner.

   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 unsucccessful, the sshtmSessionChannelOpenFailures
   counter is incremented, an error indication is returned to the
   calling module, and processing stops for this message.

   6) If successful, this will result in an SSH session.  The
   destTransportDomain and the destTransportAddress, plus any
   implementation-dependent identifer for the channel should be retained
   so they can be added to the LCD for subsequent use.

   7) Once the SSH session has been established, the client will invoke
   SNMP as an SSH subsystem, as indicated in the "subsystem" parameter.

   In order to allow SNMP traffic to be easily identified and filtered
   by firewalls and other network devices, servers associated with SNMP
   entities using the Secure Shell Transport Model MUST default to
   providing access to the "SNMP" SSH subsystem if the SSH session is
   established using the IANA-assigned TCP port.  Servers SHOULD be
   configurable to allow access to the SNMP SSH subsystem over other
   ports.

   8) Create an entry in a Local Configuration Datastore containing the
   provided transportDomain, transportAddress, securityName,
   securityLevel, and SSH-speciifc parameters and create a
   tmStateReference to reference the entry.






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5.4.  Closing a Session

   The Secure Shell Transport Model provides the following ASI to close
   a session:

   statusInformation =
   closeSession(
   IN  tmStateReference        -- transport info
    )



   The following describes the procedure to follow to close a session
   between a client and sever .  This process is followed by any SNMP
   engine closing the corresponding SNMP session.

      1) Extract the transportDomain, transportAddress, securityName,
      and securityLevel from the tmStateReference.
      2) Lookup the session in the Local Configuration Datastore using
      the target index
      3) If there is no session open associated with the target index,
      then closeSession processing is completed.
      4) Extract any implementation-specific parameters from the LCD
      5) Have SSH close the specified session.

6.  MIB Module Overview

   This MIB module provides management of the Secure Shell Transport
   Model.  It defines some needed textual conventions, and some
   statistics.

6.1.  Structure of the MIB Module

   Objects in this MIB module are arranged into subtrees.  Each subtree
   is organized as a set of related objects.  The overall structure and
   assignment of objects to their subtrees, and the intended purpose of
   each subtree, is shown below.

6.2.  Textual Conventions

   Generic and Common Textual Conventions used in this document can be
   found summarized at http://www.ops.ietf.org/mib-common-tcs.html

6.3.  Relationship to Other MIB Modules

   Some management objects defined in other MIB modules are applicable
   to an entity implementing the SSH Transport Model.  In particular, it
   is assumed that an entity implementing the SSHTM-MIB will implement



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   the SNMPv2-MIB [RFC3418], the SNMP-FRAMEWORK-MIB [RFC3411] and the
   SNMP-TRANSPORT-MIB [I-D.ietf-isms-tmsm].

   This MIB module is for managing SSH Transport Model information.
   This MIB module models a sample Local Configuration Datastore.

6.3.1.  MIB Modules Required for IMPORTS

   The following MIB module imports items from [RFC2578], [RFC2579],
   [RFC2580].

   This MIB module also references [RFC3490] and [RFC3986]

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



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                  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
                    "
       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 }



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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."
    ::= { 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



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         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
         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.



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                "
    ::= { sshtmSession 2 }

sshtmSessionOpenErrors  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request
               failed to open a Session, for any reason.
                "
    ::= { sshtmSession 3 }

sshtmSessionUserAuthFailures  OBJECT-TYPE
    SYNTAX       Counter32
    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 }




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-- ************************************************
-- 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
    }
    STATUS      current
    DESCRIPTION "A collection of objects for maintaining
                 information of an SNMP engine which implements the
                 SNMP Secure Shell Transport Model.
                "


    ::= { sshtmGroups 2 }


END


8.  Operational Considerations

   The SSH Transport Model will likely not work in conditions where
   access to the CLI has stopped working.  In situations where SNMP
   access has to work when the CLI has stopped working, a UDP transport
   model should be considered instead of the SSH Transport Model.

   The SSH Transport Model defines a single well-known default port for
   all traffic types.  Administrators might choose to define one port
   for SNMP request-response traffic, but configure notifications to be



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   sent to a different port, by using the snmpTargetAddrTable, for
   example.

   If the SSH Transport Model is configured to utilize AAA services,
   operators should consider configuring support for a local
   authentication mechanisms, such as local passwords, so SNMP can
   continue operating during times of network stress.

   The SSH protocol has its own windowing mechanism.  RFC 4254 says: The
   window size specifies how many bytes the other party can send before
   it must wait for the window to be adjusted.  Both parties use the
   following message to adjust the window.  The SSH specifications leave
   it open when such window adjustment messages are created.  Some
   implementations have been found to send window adjustment messages
   whenever received data has been passed to the application.  Since
   window adjustment messages are padded, encrypted, hmac'ed, and
   wrapped, this results in noticable bandwidth and processing overhead,
   which can be avoided by sending window adjustment messages less
   frequently.

   The SSH protocol requires the execution of CPU intensive calculations
   to establish a session key during session establishment.  This means
   that short lived sessions become computationally expensive compared
   to USM, which does not have a notion of a session key.  Other
   transport security protocols such as TLS support a session resumption
   feature that allows reusing a cached session key.  Such a mechanism
   does not exist for SSH and thus SNMP applications should keep SSH
   sessions for longer time periods.

9.  Security Considerations

   This document describes a transport model that permits SNMP to
   utilize SSH security services.  The security threats and how the SSH
   Transport Model mitigates those threats is covered in detail
   throughout this memo.

   The SSH Transport Model relies on SSH mutual authentication, binding
   of keys, confidentiality and integrity.  Any authentication method
   that meets the requirements of the SSH architecture will provide the
   properties of mutual authentication and binding of keys.  While SSH
   does support turning off confidentiality and integrity, they SHOULD
   NOT be turned off when used with the SSH Transport Model.

   SSHv2 provides Perfect Forward Security (PFS) for encryption keys.
   PFS is a major design goal of SSH, and any well-designed keyex
   algorithm will provide it.

   The security implications of using SSH are covered in [RFC4251].



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   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.

   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



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   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
   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).



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   Further, deployment of SNMP versions prior to SNMPv3 is NOT
   RECOMMENDED.  Instead, it is RECOMMENDED to deploy SNMPv3 and to
   enable cryptographic security.  It is then a customer/operator
   responsibility to ensure that the SNMP entity giving access to an
   instance of this MIB module is properly configured to give access to
   the objects only to those principals (users) that have legitimate
   rights to indeed GET or SET (change/create/delete) them.

10.  IANA Considerations

   IANA is requested to assign:
   1.  a TCP port number in the range 1..1023 in the
       http://www.iana.org/assignments/port-numbers registry which will
       be the default port for SNMP over 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.

   [RFC2279]             Yergeau, F., "UTF-8, a transformation format of
                         ISO 10646", RFC 2279, January 1998.

   [RFC2578]             McCloghrie, K., Ed., Perkins, D., Ed., and J.
                         Schoenwaelder, Ed., "Structure of Management
                         Information Version 2 (SMIv2)", STD 58,
                         RFC 2578, April 1999.

   [RFC2579]             McCloghrie, K., Ed., Perkins, D., Ed., and J.
                         Schoenwaelder, Ed., "Textual Conventions for
                         SMIv2", STD 58, RFC 2579, April 1999.

   [RFC2580]             McCloghrie, K., Perkins, D., and J.
                         Schoenwaelder, "Conformance Statements for
                         SMIv2", STD 58, RFC 2580, April 1999.



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   [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., 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.




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   [I-D.ietf-isms-tmsm]  Harrington, D. and J. Schoenwaelder, "Transport
                         Subsystem for the Simple Network Management
                         Protocol (SNMP)", draft-ietf-isms-tmsm-10 (work
                         in progress), September 2007.

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.

   [RFC3629]             Yergeau, F., "UTF-8, a transformation format of
                         ISO 10646", STD 63, RFC 3629, November 2003.

   [RFC3986]             Berners-Lee, T., Fielding, R., and L. Masinter,
                         "Uniform Resource Identifier (URI): Generic
                         Syntax", STD 66, RFC 3986, January 2005.

   [RFC4256]             Cusack, F. and M. Forssen, "Generic Message
                         Exchange Authentication for the Secure Shell
                         Protocol (SSH)", RFC 4256, January 2006.

   [RFC4462]             Hutzelman, J., Salowey, J., Galbraith, J., and
                         V. Welch, "Generic Security Service Application
                         Program Interface (GSS-API) Authentication and
                         Key Exchange for the Secure Shell (SSH)
                         Protocol", RFC 4462, May 2006.

   [RFC4590]             Sterman, B., Sadolevsky, D., Schwartz, D.,
                         Williams, D., and W. Beck, "RADIUS Extension
                         for Digest Authentication", RFC 4590,
                         July 2006.

   [RFC4742]             Wasserman, M. and T. Goddard, "Using the
                         NETCONF Configuration Protocol over Secure
                         SHell (SSH)", RFC 4742, December 2006.






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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  Consistency with TSM needs to be done
   o  SSH transport domain and transport address definitions -
      consistency across WGs
   o  configuring notification originators

   TODO:

      finalize error processing in EOP

Appendix B.  Change Log

   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

      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.




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      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.
      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-"



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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 SecSH RFCs
      Modified text related to issues# 1, 2, 8, 11, 13, 14, 16, 18, 19,
      20, 29, 30, and 32.
      updated security considerations
      removed Juergen Schoenwaelder from authors, at his request
      ran the mib module through smilint

Authors' Addresses

   David Harrington
   Huawei Technologies (USA)
   1700 Alma Dr. Suite 100
   Plano, TX 75075
   USA

   Phone: +1 603 436 8634
   EMail: dharrington@huawei.com









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   Joseph Salowey
   Cisco Systems
   2901 3rd Ave
   Seattle, WA 98121
   USA

   EMail: jsalowey@cisco.com












































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Full Copyright Statement

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