





Network Working Group                                      D. Harrington
Request for Comments: 5592                     Huawei Technologies (USA)
Category: Standards Track                                     J. Salowey
                                                           Cisco Systems
                                                             W. Hardaker
                                               Cobham Analytic Solutions
                                                               June 2009


                  Secure Shell Transport Model for the
               Simple Network Management Protocol (SNMP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.









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Abstract

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

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

Table of Contents

   1. Introduction ....................................................3
      1.1. The Internet-Standard Management Framework .................3
      1.2. Conventions ................................................3
      1.3. Modularity .................................................5
      1.4. Motivation .................................................5
      1.5. Constraints ................................................6
   2. The Secure Shell Protocol .......................................7
   3. How SSHTM Fits into the Transport Subsystem .....................8
      3.1. Security Capabilities of this Model ........................8
           3.1.1. Threats .............................................8
           3.1.2. Message Authentication ..............................9
           3.1.3. Authentication Protocol Support ....................10
           3.1.4. SSH Subsystem ......................................11
      3.2. Security Parameter Passing ................................12
      3.3. Notifications and Proxy ...................................12
   4. Cached Information and References ..............................13
      4.1. Secure Shell Transport Model Cached Information ...........13
           4.1.1. tmSecurityName .....................................13
           4.1.2. tmSessionID ........................................14
           4.1.3. Session State ......................................14
   5. Elements of Procedure ..........................................14
      5.1. Procedures for an Incoming Message ........................15
      5.2. Procedures for Sending an Outgoing Message ................17
      5.3. Establishing a Session ....................................18
      5.4. Closing a Session .........................................20
   6. MIB Module Overview ............................................21
      6.1. Structure of the MIB Module ...............................21
      6.2. Textual Conventions .......................................21
      6.3. Relationship to Other MIB Modules .........................21
           6.3.1. MIB Modules Required for IMPORTS ...................21
   7. MIB Module Definition ..........................................22
   8. Operational Considerations .....................................29
   9. Security Considerations ........................................30
      9.1. Skipping Public Key Verification ..........................31
      9.2. Notification Authorization Considerations .................31
      9.3. SSH User and Key Selection ................................31



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      9.4. Conceptual Differences between USM and SSHTM ..............31
      9.5. The 'none' MAC Algorithm ..................................32
      9.6. Use with SNMPv1/v2c Messages ..............................32
      9.7. MIB Module Security .......................................32
   10. IANA Considerations ...........................................33
   11. Acknowledgments ...............................................33
   12. References ....................................................34
      12.1. Normative References .....................................34
      12.2. Informative References ...................................35

1.  Introduction

   This memo describes a Transport Model for the Simple Network
   Management Protocol, using the Secure Shell (SSH) protocol [RFC4251]
   within a Transport Subsystem [RFC5590].  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

   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].



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   Lowercase versions of the keywords should be read as in normal
   English.  They will usually, but not always, be used in a context
   that relates to compatibility with the RFC 3411 architecture or the
   subsystem defined here but that might have no impact on on-the-wire
   compatibility.  These terms are used as guidance for designers of
   proposed IETF models to make the designs compatible with RFC 3411
   subsystems and Abstract Service Interfaces (ASIs).  Implementers are
   free to implement differently.  Some usages of these lowercase terms
   are simply normal English usage.

   For consistency with SNMP-related specifications, this document
   favors terminology as defined in STD 62, 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, all SNMP entities have the
   capability of acting as manager, agent, or both 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.

   The User-based Security Model (USM) [RFC3414] is a mandatory-to-
   implement Security Model in STD 62.  While the SSH and USM
   specifications frequently refer to a user, the terminology preferred
   in [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 each acting in a
   particular role, an application or a set of applications, or a
   combination of these within an administrative domain.

   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.






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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 (SNMP)" [RFC5590].

   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
   documents, this document defines the elements of procedure and
   associated MIB module objects that 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 having to
   deploy 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 and 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.






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   This document addresses the requirement to utilize client-
   authentication and key-exchange methods that support different
   security infrastructures and provide different security properties.
   This document describes how to use client authentication as described
   in "The Secure Shell (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 the SSH Protocol Parameters registry
   maintained by IANA).  The use of the "none" authentication method is
   NOT RECOMMENDED, as described in this document's Security
   Considerations.  Local accounts may be supported through the use of
   the publickey, hostbased, or password methods.  The password method
   allows for integration with a deployed password infrastructure, such
   as Authentication, Authorization, and Accounting (AAA) servers using
   the RADIUS protocol [RFC2865].  The SSH Transport Model SHOULD be
   able to take advantage of future-defined 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, and using ssh-userauth [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).





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   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 NOT require changes to the SNMP
       architecture.

   5.  A Transport Model SHOULD NOT require changes to the underlying
       security 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.

   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; it also defines a family of SSH key-exchange
      methods that use GSS-API to authenticate a Diffie-Hellman key
      exchange.





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   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 is a component of the Transport Subsystem [RFC5590]
   within the SNMP architecture.  The SSH Transport Model thus fits
   between the underlying SSH transport layer and the Message Dispatcher
   [RFC3411].

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

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

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

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.  Modification of Information - SSH provides for verification that
       the contents of each message have not been modified during its
       transmission through the network by digitally signing each SSH
       packet.

   2.  Masquerade - SSH provides for verification of the identity of the
       SSH server and the identity of the SSH client.




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       SSH provides for verification of the identity of the SSH server
       through the SSH transport protocol server authentication
       [RFC4253].  This allows an operator or management station to
       ensure the authenticity of the SNMP engine that provides MIB
       data.

       SSH provides a number of mechanisms for verification of the
       identity of the SSH client-side principal using the Secure Shell
       Authentication Protocol [RFC4252].  These include public key,
       password, and host-based mechanisms.  This allows the SNMP Access
       Control Subsystem to ensure that only authorized principals have
       access to potentially sensitive data.

       Verification of the client's 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.)

   3.  Message Stream Modification - SSH protects against malicious re-
       ordering or replaying of messages within a single SSH session by
       using sequence numbers and integrity checks.  SSH protects
       against replay of messages across SSH sessions by ensuring that
       the cryptographic keys used for encryption and integrity checks
       are generated afresh for each session.

   4.  Disclosure - SSH provides protection against the disclosure of
       information to unauthorized recipients or eavesdroppers by
       allowing for encryption of all traffic between SNMP engines.

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].



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   The SSH Transport Model determines from SSH the identity of the
   authenticated principal and the type and address associated with an
   incoming message, and provides this information to SSH for an
   outgoing message.  The SSH 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.

   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 specified with a securityLevel of noAuthNoPriv or
   authNoPriv are actually sent by the SSH Transport Model with
   authPriv-level protection.

   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), 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 the Challenge Handshake
   Authentication Protocol (CHAP) [RFC1994] or digest authentication
   [RFC5090] to integrate with RADIUS or Diameter.  At some point in the
   processing, these mechanisms require the password to be made
   available as cleartext on the device that is authenticating the
   password, which might introduce threats to the authentication
   infrastructure.





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   GSS-API key exchange [RFC4462] provides a framework for the addition
   of client-authentication mechanisms that 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.  SSH Subsystem

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

   An SSH Subsystem of type "snmp" is 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;
   otherwise, 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
   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 tmTransportAddress and tmSecurityName
   associated with each session.

   Because naming policies might differ between administrative domains,
   many SSH client software packages support a user@hostname:port
   addressing syntax that operators can use to align non-equivalent
   account names.  The SnmpSSHAddress Textual Convention echos this
   common SSH notation.

   When this notation is used in an SnmpSSHAddress, the SSH connection
   should be established with an SSH user name matching the "user"
   portion of the notation when establishing a session with the remote
   SSH server.  The user name must be encoded in UTF-8 (per [RFC4252]).
   The "user" portion may or may not match the tmSecurityName parameter
   passed from the Security Model.  If no "user@" portion is specified
   in the SnmpSSHAddress, then the SSH connection should be established
   using the tmSecurityName as the SSH user name when establishing a
   session with the remote SSH server.







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   The SnmpSSHAddress and tmSecurityName associated with an SSH session
   MUST remain constant during the life of the session.  Different
   SnmpSSHAddress values (with different hostnames, "user@" prefix
   names, and/or port numbers) will each result in individual SSH
   sessions.

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, 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 (Abstract 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 to 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 or proxy requests should be sent is
   typically determined and configured by a network administrator.  The
   SNMP-NOTIFICATION-MIB contains a list of targets to which
   notifications should be sent.  The SNMP-TARGET-MIB module [RFC3413]
   contains objects for defining these management targets, including
   transport domains and addresses and security parameters, for
   applications such as notification generators and proxy forwarders.

   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 received proxied messages.  Local access-



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   control processing needs to be performed by a notification originator
   before notifications are actually sent, and this processing is done
   using the configured securityName.  An important characteristic of
   this is that authorization is done prior to determining if the
   connection can succeed.  Thus, the locally configured securityName is
   entirely trusted within the notification originator.

   The SNMP-TARGET-MIB and NOTIFICATION-MIB 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 a potentially longer-term state relating
   to transport and security.  "Transport Subsystem for the Simple
   Network Management Protocol" [RFC5590] defines general requirements
   for caches and references.

   This document defines additional cache requirements related to the
   Secure Shell Transport Model.

4.1.  Secure Shell Transport Model Cached Information

   The Secure Shell Transport Model has specific responsibilities
   regarding the cached information.  See the Elements of Procedure in
   Section 5 for detailed processing instructions on the use of the
   tmStateReference fields by the SSH Transport Model.

4.1.1.  tmSecurityName

   The tmSecurityName MUST be a human-readable name (in snmpAdminString
   format) representing the identity that has been set according to the
   procedures in Section 5.  The tmSecurityName MUST be constant for all
   traffic passing through an SSHTM session.  Messages MUST NOT be sent
   through an existing SSH session that was established using a
   different tmSecurityName.

   On the SSH server side of a connection:

      The tmSecurityName should be the SSH user name.  How the SSH user
      name is extracted from the SSH layer is implementation-dependent.






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      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 an SSH identity to a tmSecurityName
      or to transform a tmSecurityName to an SSH identity.

      In other cases, the user name may not be verified by the server,
      so for these implementations, it may be necessary to obtain the
      user name from other credentials exchanged during the SSH
      exchange.

   On the SSH client side of a connection:

      The tmSecurityName is presented to the SSH Transport Model by the
      application (possibly because of configuration specified in the
      SNMP-TARGET-MIB).

   The securityName MAY be derived from the tmSecurityName by a Security
   Model and MAY be used to configure notifications and access controls
   in MIB modules.  Transport Models SHOULD generate a predictable
   tmSecurityName so operators will know what to use when configuring
   MIB modules that use securityNames derived from tmSecurityNames.

4.1.2.  tmSessionID

   The tmSessionID MUST be recorded per message at the time of receipt.
   When tmSameSecurity is set, the recorded tmSessionID can be used to
   determine whether the SSH session available for sending a
   corresponding outgoing message is the same SSH session as was used
   when receiving the incoming message (e.g., a response to a request).

4.1.3.  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
   in a Local Configuration Datastore.

5.  Elements of Procedure

   Abstract Service Interfaces have been defined by [RFC3411] and
   further augmented by [RFC5590] 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.






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   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 in statusInformation will typically include the
   Object Identifier (OID) and value for an incremented error counter.
   This may be accompanied by the requested securityLevel and the
   tmStateReference.  Per-message context information is not accessible
   to Transport Models, so for the returned counter OID and value,
   contextEngine would be set to the local value of snmpEngineID and
   contextName 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 address the
       message originated from, the user name authenticated by SSH, and
       a session identifier.

   2.  Determine the tmTransportAddress to be associated with the
       incoming message:

       A.  If this is a client-side SSH session, then the
           tmTransportAddress is set to the tmTransportAddress used to
           establish the session.  It MUST exactly include any "user@"
           prefix associated with the address provided to the
           openSession() ASI.

       B.  If this is a server-side SSH session and this is the first
           message received over the session, then the
           tmTransportAddress is set to the address the message
           originated from, determined in an implementation-dependent
           way.  This value MUST be constant for the entire SSH session,
           and future messages received MUST result in the
           tmTransportAddress being set to the same value.

       C.  If this is a server-side SSH session and this is not the
           first message received over the session, then the
           tmTransportAddress is set to the previously established
           tmTransportAddress for the session (the value from step B,
           determined from a previous incoming message).







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   3.  Determine the tmSecurityName to be associated with the incoming
       message:

       A.  If this is a client-side SSH session, then the tmSecurityName
           MUST be set to the tmSecurityName used to establish the
           session.

       B.  If this is a server-side SSH session and this is the first
           message received over the session, then the tmSecurityName is
           set to the SSH user name.  How the SSH user name is extracted
           from the SSH layer is implementation-dependent.  This value
           MUST be constant for the entire SSH session, and future
           messages received MUST result in the tmSecurityName being set
           to the same value.

       C.  If this is a server-side SSH session and this is not the
           first message received over the session, then the
           tmSecurityName is set to the previously established
           tmSecurityName for the session (the value from step B,
           determined from a previous incoming message).

   4.  Create a tmStateReference cache for subsequent reference to the
       information.

          tmTransportDomain = snmpSSHDomain

          tmTransportAddress = the derived tmTransportAddress from step
          2.

          tmSecurityName = the derived tmSecurityName from step 3.

          tmTransportSecurityLevel = "authPriv" (authentication and
          confidentiality MUST be used to comply with this Transport
          Model.)

          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 MUST uniquely
          identify the session over which the message was received.
          This session identifier MUST NOT be reused until there are no
          references to it remaining.

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







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   statusInformation =
   receiveMessage(
   IN   transportDomain       -- snmpSSHDomain
   IN   transportAddress      -- the tmTransportAddress for the 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 Sending 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.  If tmStateReference does not refer to a cache containing values
       for tmTransportDomain, tmTransportAddress, tmSecurityName,
       tmRequestedSecurityLevel, and tmSameSecurity, then increment the
       snmpSshtmSessionInvalidCaches counter, discard the message, and
       return the error indication in the statusInformation.  Processing
       of this message stops.

   2.  Extract the tmTransportDomain, tmTransportAddress,
       tmSecurityName, tmRequestedSecurityLevel, tmSameSecurity, and
       tmSessionID from the tmStateReference.

   3.  Identify an SSH session over which to send the messages:

       A.  If tmSameSecurity is true and there is no existing session
           with a matching tmSessionID, tmSecurityName, and
           tmTransportAddress, then increment the
           snmpSshtmSessionNoSessions counter, discard the message, and
           return the error indication in the statusInformation.
           Processing of this message stops.

       B.  If there is a session with a matching tmSessionID,
           tmTransportAddress, and tmSecurityName, then select that
           session.



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       C.  If there is a session that matches the tmTransportAddress and
           tmSecurityName, then select that session.

       D.  If the above steps failed to select a session to use, then
           call openSession() with the tmStateReference as a parameter.

           +  If openSession fails, then discard the message, release
              tmStateReference, and pass the error indication returned
              by openSession back to the calling module.  Processing of
              this message stops.

           +  If openSession succeeds, then record the
              destTransportDomain, destTransportAddress, tmSecurityname,
              and tmSessionID in an implementation-dependent manner.
              This will be needed when processing an incoming message.

   4.  Pass the wholeMessage to SSH for encapsulation as data in an SSH
       message over the identified SSH session.  Any necessary
       additional SSH-specific parameters should be provided in an
       implementation-dependent manner.

5.3.  Establishing a Session

   The Secure Shell Transport Model provides the following Abstract
   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   tmStateReference       -- transport information to be used
   OUT  tmStateReference       -- transport information to be used
   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, a notification
   originator, or a proxy forwarder.








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   1.  Increment the snmpSshtmSessionOpens counter.

   2.  Using tmTransportAddress, 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 transportAddress associated with a session
       MUST remain constant during the lifetime of the SSH session.
       Implementations may need to cache the transportAddress passed to
       the openSession API for later use when performing incoming
       message processing (see Section 5.1).

       1.  To authenticate the server, the client usually stores pairs
           (tmTransportAddress, server host public key) in an
           implementation-dependent manner.

       2.  The other parameters of the transport connection are provided
           in an implementation-dependent manner.

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

   3.  The client will then invoke an SSH authentication service to
       authenticate the principal, such as that described in the SSH
       authentication protocol [RFC4252].

       1.  If the tmTransportAddress field contains a user name followed
           by an '@' character (US-ASCII 0x40), that user name string
           should be presented to the SSH server as the "user name" for
           user-authentication purposes.  If there is no user name in
           the tmTransportAddress, then the tmSecurityName should be
           used as the user name.

       2.  The credentials used to authenticate the SSH principal are
           determined in an implementation-dependent manner.

       3.  In an implementation-specific manner, invoke the SSH user-
           authentication service using the calculated user name.

       4.  If the user authentication is unsuccessful, then the
           transport connection is closed, the
           snmpSshtmSessionUserAuthFailures counter is incremented, an
           error indication is returned to the calling module, and
           processing stops for this message.





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   4.  The client should invoke the "ssh-connection" service (also known
       as the SSH connection protocol [RFC4254]), and request a channel
       of type "session".  If unsuccessful, the transport connection is
       closed, the snmpSshtmSessionNoChannels counter is incremented, an
       error indication is returned to the calling module, and
       processing stops for this message.

   5.  The client invokes "snmp" as an SSH Subsystem, as indicated in
       the "subsystem" parameter.  If unsuccessful, the transport
       connection is closed, the snmpSshtmSessionNoSubsystems counter is
       incremented, an error indication is returned to the calling
       module, and processing stops for this message.

       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 ports (5161 and 5162).  Servers SHOULD be
       configurable to allow access to the SNMP SSH Subsystem over other
       ports.

   6.  Set tmSessionID in the tmStateReference cache to an
       implementation-dependent value to identify the session.

   7.  The tmSecurityName used to establish the SSH session must be the
       only tmSecurityName used with the session.  Incoming messages for
       the session MUST be associated with this tmSecurityName value.
       How this is accomplished is implementation-dependent.

5.4.  Closing a Session

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

   statusInformation =
   closeSession(
   IN   tmSessionID     -- session ID of session to be closed
   )

   The following describes the procedure to follow to close a session
   between a client and server.  This process is followed by any SNMP
   engine to close an SSH session.  It is implementation-dependent when
   a session should be closed.  The calling code should release the
   associated tmStateReference.






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   1.  Increment the snmpSshtmSessionCloses counter.

   2.  If there is no session corresponding to tmSessionID, then
       closeSession processing is complete.

   3.  Have SSH close the session associated with tmSessionID.

6.  MIB Module Overview

   This MIB module provides management of the Secure Shell Transport
   Model.  It defines an OID to identify the SNMP-over-SSH transport
   domain, a Textual Convention for SSH Addresses, and several
   statistics counters.

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 SNMP-SSH-TM-MIB will
   implement the SNMPv2-MIB [RFC3418] and the SNMP-FRAMEWORK-MIB
   [RFC3411].  It is expected that an entity implementing this MIB will
   also support the Transport Security Model [RFC5591] and, therefore,
   implement the SNMP-TSM-MIB.

   This MIB module is for monitoring SSH Transport Model information.

6.3.1.  MIB Modules Required for IMPORTS

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

   This MIB module also references [RFC1033], [RFC4252], [RFC3490], and
   [RFC3986].






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   This document uses TDomain Textual Conventions for the SNMP-internal
   MIB modules defined here for compatibility with the RFC 3413 MIB
   modules and the RFC 3411 Abstract Service Interfaces.

7.  MIB Module Definition

SNMP-SSH-TM-MIB DEFINITIONS ::= BEGIN

IMPORTS
    MODULE-IDENTITY, OBJECT-TYPE,
    OBJECT-IDENTITY, mib-2, snmpDomains,
    Counter32
      FROM SNMPv2-SMI -- RFC 2578
    TEXTUAL-CONVENTION
      FROM SNMPv2-TC -- RFC 2579
    MODULE-COMPLIANCE, OBJECT-GROUP
      FROM SNMPv2-CONF -- RFC 2580
    ;

snmpSshtmMIB MODULE-IDENTITY
    LAST-UPDATED "200906090000Z"
    ORGANIZATION "ISMS Working Group"
    CONTACT-INFO "WG-EMail:   isms@lists.ietf.org
                  Subscribe:  isms-request@lists.ietf.org

                  Chairs:
                    Juergen Quittek
                    NEC Europe Ltd.
                    Network Laboratories
                    Kurfuersten-Anlage 36
                    69115 Heidelberg
                    Germany
                    +49 6221 90511-15
                    quittek@netlab.nec.de

                    Juergen Schoenwaelder
                    Jacobs University Bremen
                    Campus Ring 1
                    28725 Bremen
                    Germany
                    +49 421 200-3587
                    j.schoenwaelder@jacobs-university.de

                  Co-editors:
                    David Harrington
                    Huawei Technologies USA
                    1700 Alma Drive
                    Plano Texas 75075



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                    USA
                    +1 603-436-8634
                    ietfdbh@comcast.net

                    Joseph Salowey
                    Cisco Systems
                    2901 3rd Ave
                    Seattle, WA 98121
                    USA
                    jsalowey@cisco.com

                    Wes Hardaker
                    Cobham Analytic Solutions
                    P.O. Box 382
                    Davis, CA  95617
                    USA
                    +1 530 792 1913
                    ietf@hardakers.net
                 "
    DESCRIPTION
       "The Secure Shell Transport Model MIB.

        Copyright (c) 2009 IETF Trust and the persons
        identified as authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, are permitted provided that the
        following conditions are met:

        - Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        - Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials
          provided with the distribution.

        - Neither the name of Internet Society, IETF or IETF Trust,
          nor the names of specific contributors, may be used to endorse
          or promote products derived from this software without
          specific prior written permission.

        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
        CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES,
        INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
        MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
        DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
        CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,



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        SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
        NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
        LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
        HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
        CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
        OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
        EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

        This version of this MIB module is part of RFC 5592;
        see the RFC itself for full legal notices."

    REVISION     "200906090000Z"
    DESCRIPTION  "The initial version, published in RFC 5592."

    ::= { mib-2 189 }

-- ---------------------------------------------------------- --
-- subtrees in the SNMP-SSH-TM-MIB
-- ---------------------------------------------------------- --

snmpSshtmNotifications    OBJECT IDENTIFIER ::= { snmpSshtmMIB 0 }
snmpSshtmObjects          OBJECT IDENTIFIER ::= { snmpSshtmMIB 1 }
snmpSshtmConformance      OBJECT IDENTIFIER ::= { snmpSshtmMIB 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 which secure transport
         infrastructure authenticated a securityName."
    ::= { snmpDomains 7 }

SnmpSSHAddress ::= TEXTUAL-CONVENTION
    DISPLAY-HINT "1a"
    STATUS      current



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    DESCRIPTION
        "Represents either a hostname or IP address, along with a port
         number and an optional user name.

         The beginning of the address specification may contain a
         user name followed by an '@' (US-ASCII character 0x40).  This
         portion of the address will indicate the user name that should
         be used when authenticating to an SSH server.  The user name
         must be encoded in UTF-8 (per [RFC4252]).  If missing, the
         SNMP securityName should be used.  After the optional user
         name field and '@' character comes the hostname or IP
         address.

         The hostname is always in US-ASCII (as per RFC1033);
         internationalized hostnames are encoded in US-ASCII as
         specified in RFC 3490.  The hostname is followed by a colon
         ':' (US-ASCII character 0x3A) and a decimal port number in
         US-ASCII.  The name SHOULD be fully qualified whenever
         possible.

         An IPv4 address must be in dotted decimal format followed
         by a colon ':' (US-ASCII character 0x3A) and a decimal port
         number in US-ASCII.

         An IPv6 address must be in colon-separated format, surrounded
         by square brackets ('[', US-ASCII character 0x5B, and ']',
         US-ASCII character 0x5D), followed by a colon ':' (US-ASCII
         character 0x3A) and a decimal port number in US-ASCII.

         Values of this Textual Convention might not be directly usable
         as transport-layer addressing information and may require
         runtime resolution.  As such, applications that write them
         must be prepared for handling errors if such values are
         not supported or cannot be resolved (if resolution occurs
         at the time of the management operation).

         The DESCRIPTION clause of TransportAddress objects that may
         have snmpSSHAddress values must fully describe how (and
         when) such names are to be resolved to IP addresses and vice
         versa.

         This Textual Convention SHOULD NOT be used directly in
         object definitions since it restricts addresses to a
         specific format.  However, if it is used, it MAY be used
         either on its own or in conjunction with
         TransportAddressType or TransportDomain as a pair.





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         When this Textual Convention is used as a syntax of an
         index object, there may be issues with the limit of 128
         sub-identifiers, which is 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
      "RFC 1033: DOMAIN ADMINISTRATORS OPERATIONS GUIDE
       RFC 3490: Internationalizing Domain Names in Applications
       RFC 3986: Uniform Resource Identifier (URI): Generic Syntax
       RFC 4252: The Secure Shell (SSH) Authentication Protocol"
    SYNTAX      OCTET STRING (SIZE (1..255))

-- The snmpSshtmSession Group

snmpSshtmSession       OBJECT IDENTIFIER ::= { snmpSshtmObjects 1 }

snmpSshtmSessionOpens  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request has been
                 executed as an SSH client, whether it succeeded or
                 failed.
                "
    ::= { snmpSshtmSession 1 }

snmpSshtmSessionCloses  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times a closeSession() request has been
                 executed as an SSH client, whether it succeeded or
                 failed.
                "
    ::= { snmpSshtmSession 2 }

snmpSshtmSessionOpenErrors  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current





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    DESCRIPTION "The number of times an openSession() request
                 failed to open a transport connection or failed to
                 authenticate the server.
                "
    ::= { snmpSshtmSession 3 }

snmpSshtmSessionUserAuthFailures  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request
                 failed to open a session as an SSH client due to
                 user-authentication failures.
                "
    ::= { snmpSshtmSession 4 }

snmpSshtmSessionNoChannels  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request
                 failed to open a session as an SSH client due to
                 channel-open failures.
                "
    ::= { snmpSshtmSession 5 }

snmpSshtmSessionNoSubsystems OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request
                 failed to open a session as an SSH client due to
                 inability to connect to the requested subsystem.
                "
    ::= { snmpSshtmSession 6 }

snmpSshtmSessionNoSessions  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.
                "
    ::= { snmpSshtmSession 7 }

snmpSshtmSessionInvalidCaches OBJECT-TYPE
    SYNTAX       Counter32



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    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of outgoing messages dropped because the
                 tmStateReference referred to an invalid cache.
                "
    ::= { snmpSshtmSession 8 }

-- ************************************************
-- snmpSshtmMIB - Conformance Information
-- ************************************************

snmpSshtmCompliances OBJECT IDENTIFIER ::= { snmpSshtmConformance 1 }

snmpSshtmGroups      OBJECT IDENTIFIER ::= { snmpSshtmConformance 2 }

-- ************************************************
-- Compliance statements
-- ************************************************

snmpSshtmCompliance MODULE-COMPLIANCE
    STATUS      current

    DESCRIPTION "The compliance statement for SNMP engines that
                 support the SNMP-SSH-TM-MIB."
    MODULE
        MANDATORY-GROUPS { snmpSshtmGroup }
    ::= { snmpSshtmCompliances 1 }

-- ************************************************
-- Units of conformance
-- ************************************************

snmpSshtmGroup OBJECT-GROUP
    OBJECTS {
      snmpSshtmSessionOpens,
      snmpSshtmSessionCloses,
      snmpSshtmSessionOpenErrors,
      snmpSshtmSessionUserAuthFailures,
      snmpSshtmSessionNoChannels,
      snmpSshtmSessionNoSubsystems,
      snmpSshtmSessionNoSessions,
      snmpSshtmSessionInvalidCaches
    }
    STATUS      current
    DESCRIPTION "A collection of objects for maintaining information
                 of an SNMP engine that implements the SNMP Secure
                 Shell Transport Model.
                "



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    ::= { snmpSshtmGroups 2 }

END

8.  Operational Considerations

   The SSH Transport Model will likely not work in conditions where
   remote access to the CLI has stopped working.  The SSH Transport
   Model assumes that TCP and IP continue to operate correctly between
   the communicating nodes.  Failures in either node, death of the
   deamon serving the communication, routing problems in the network
   between, firewalls that block the traffic, and other problems can
   prevent the SSH Transport Model from working.  In situations where
   management access has to be very reliable, operators should consider
   mitigating measures.  These measures may include dedicated
   management-only networks, point-to-point links, and the ability to
   use alternate protocols and transports.

   To have SNMP properly utilize the security services provided by SSH,
   the SSH Transport Model MUST be used with a Security Model that knows
   how to process a tmStateReference, such as the Transport Security
   Model for SNMP [RFC5591].

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

   The SSH protocol has its own window mechanism, defined in RFC 4254.
   The SSH specifications leave it open when window adjustment messages
   should be created, and some implementations send these whenever
   received data has been passed to the application.  There are
   noticeable bandwidth and processing overheads to handling such window
   adjustment messages, which can be avoided by sending them 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 plus information to authenticate the server.
   While hosts are often configured to be SSH clients, most



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   internetworking devices are not.  To send notifications over SSHTM,
   the internetworking device will need to be configured as an SSH
   client.  How this credential configuration is done is implementation-
   and deployment-specific.

9.  Security Considerations

   This memo 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.

   SSHv2 provides perfect forward secrecy (PFS) for encryption keys.
   PFS is a major design goal of SSH, and any well-designed key-exchange
   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 to verify that the correct key is being used.  The SSH
   Transport Model simply trusts that these are properly configured by
   the implementer and deployer.

   SSH provides the "none" userauth method.  The SSH Transport Model
   MUST NOT be used with an SSH connection with the "none" userauth
   method.  While SSH does support turning off confidentiality and
   integrity, they MUST NOT be turned off when used with the SSH
   Transport Model.

   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 an SSH identity to a tmSecurityName or to transform a
   tmSecurityName to an SSH identity.

   In other cases, the user name may not be verified by the server, so
   for these implementations, it may be necessary to obtain the user
   name from other credentials exchanged during the SSH exchange.







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9.1.  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
   Diffie-Hellman (DH) signed by public keys, this requires the client
   to know the host's public key a priori and to verify that the correct
   key is being used.  If this step is skipped, then authentication of
   the SSH server to the SSH client is not done.  Data confidentiality
   and data integrity protection to the server still exist, but these
   are of dubious value when an attacker can insert himself between the
   client and the real SSH server.  Note that some userauth methods may
   defend against this situation, but many of the common ones (including
   password and keyboard-interactive) do not and, in fact, depend on the
   fact that the server's identity has been verified (so passwords are
   not disclosed to an attacker).

   SSH MUST NOT be configured to skip public-key verification for use
   with the SSH Transport Model.

9.2.  Notification Authorization Considerations

   SNMP Notifications are authorized to be sent to a receiver based on
   the securityName used by the notification originator's SNMP engine.
   This authorization is performed before the message is actually sent
   and before the credentials of the remote receiver have been verified.
   Thus, the credentials presented by a notification receiver MUST match
   the expected value(s) for a given transport address, and ownership of
   the credentials MUST be properly cryptographically verified.

9.3.  SSH User and Key Selection

   If a "user@" prefix is used within an SnmpSSHAddress value to specify
   an SSH user name to use for authentication, then the key presented to
   the remote entity MUST be the key expected by the server for the
   "user".  This may be different than a locally cached key identified
   by the securityName value.

9.4.  Conceptual Differences between USM and SSHTM

   The User-based Security Model [RFC3414] employed symmetric
   cryptography and user-naming conventions.  SSH employs an asymmetric
   cryptography and naming model.  Unlike USM, cryptographic keys will
   be different on both sides of the SSH connection.  Both sides are
   responsible for verifying that the remote entity presents the right
   key.  The optional "user@" prefix component of the SnmpSSHAddress
   Textual Convention allows the client SNMP stack to associate the
   connection with a securityName that may be different than the SSH
   user name presented to the SSH server.



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9.5.  The 'none' MAC Algorithm

   SSH provides the "none" Message Authentication Code (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.

9.6.  Use with SNMPv1/v2c Messages

   The SNMPv1 and SNMPv2c message processing described in [RFC3584] (BCP
   74) always selects the SNMPv1 or SNMPv2c Security Models,
   respectively.  Both of these and the User-based Security Model
   typically used with SNMPv3 derive the securityName and securityLevel
   from the SNMP message received, even when the message was received
   over a secure transport.  Access control decisions are therefore made
   based on the contents of the SNMP message, rather than using the
   authenticated identity and securityLevel provided by the SSH
   Transport Model.

9.7.  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 information in the snmpSshtmSession group is generated locally
      when a client session is being opened or closed.  This information
      can reflect the configured capabilities of a remote SSH server,
      which could be helpful to an attacker for focusing an attack.








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   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 cryptographic mechanisms for
   authentication and privacy, such as those found in the User-based
   Security Model [RFC3414], the Transport Security Model [RFC5591], and
   the SSH Transport Model described in this document.

   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 has assigned:

   1.  Two TCP port numbers in the Port Numbers registry that will be
       the default ports for the SNMP-over-SSH Transport Model as
       defined in this document, and the SNMP-over-SSH Transport Model
       for notifications as defined in this document.  The assigned
       keywords and port numbers are "snmpssh" (5161) and "snmpssh-trap"
       (5162).

   2.  An SMI number (189) under mib-2, for the MIB module in this
       document.

   3.  An SMI number (7) under snmpDomains, for the snmpSSHDomain.

   4.  "ssh" as the corresponding prefix for the snmpSSHDomain in the
       SNMP Transport Domains registry; defined in [RFC5590].

   5.  "snmp" as a Connection Protocol Subsystem Name in the SSH
       Protocol Parameters registry.

11.  Acknowledgments

   The editors would like to thank Jeffrey Hutzelman for sharing his SSH
   insights, and Dave Shield for an outstanding job wordsmithing the
   existing document to improve organization and clarity.



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   Additionally, helpful document reviews were received from Juergen
   Schoenwaelder.

12.  References

12.1.  Normative References

   [RFC1033]  Lottor, M., "Domain administrators operations guide",
              RFC 1033, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

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

   [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.






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   [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.

   [RFC5590]  Harrington, D. and J. Schoenwaelder, "Transport Subsystem
              for the Simple Network Management Protocol (SNMP)",
              RFC 5590, June 2009.

12.2.  Informative References

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
              Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [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.







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   [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.

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

   [RFC5090]  Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
              and W. Beck, "RADIUS Extension for Digest Authentication",
              RFC 5090, February 2008.

   [RFC5591]  Harrington, D. and W. Hardaker, "Transport Security Model
              for the Simple Network Management Protocol (SNMP)",
              RFC 5591, June 2009.

Authors' Addresses

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

   Phone: +1 603 436 8634
   EMail: ietfdbh@comcast.net


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

   EMail: jsalowey@cisco.com


   Wes Hardaker
   Cobham Analytic Solutions
   P.O. Box 382
   Davis, CA  95617
   US

   Phone: +1 530 792 1913
   EMail: ietf@hardakers.net





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