Secure Shell Transport Model for SNMP

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Abstract

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

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

1. Introduction
    1.1. The Internet-Standard Management Framework
    1.2. Conventions
    1.3. Modularity
    1.4. Motivation
    1.5. Constraints
2. The Secure Shell Protocol
3. How SSHTM Fits into the Transport Subsystem
    3.1. Security Capabilities of this Model
        3.1.1. Threats
        3.1.2. Message Authentication
        3.1.3. Authentication Protocol Support
        3.1.4. SSH Subsystem
    3.2. Security Parameter Passing
    3.3. Notifications and Proxy
4. Cached Information and References
    4.1. Secure Shell Transport Model Cached Information
        4.1.1. tmSecurityName
        4.1.2. tmSessionID
        4.1.3. Session State
5. Elements of Procedure
    5.1. Procedures for an Incoming Message
    5.2. Procedures for sending an Outgoing Message
    5.3. Establishing a Session
    5.4. Closing a Session
6. MIB Module Overview
    6.1. Structure of the MIB Module
    6.2. Textual Conventions
    6.3. Relationship to Other MIB Modules
        6.3.1. MIB Modules Required for IMPORTS
7. MIB Module Definition
8. Operational Considerations
9. Security Considerations
    9.1. Skipping Public Key Verification
    9.2. Notification Authorizaton Considerations
    9.3. SSH User and Key Selection
    9.4. Conceptual Differences Between USM and SSHTM
    9.5. The 'none' MAC Algorithm
    9.6. Use with SNMPv1/v2c Messages
    9.7. MIB Module Security
10. IANA Considerations
11. Acknowledgements
12. References
    12.1. Normative References
    12.2. Informative References
Appendix A. Change Log

1. Introduction

1.1. The Internet-Standard Management Framework

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.

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.

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

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 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] (Blumenthal, U. and B. Wijnen, “User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3),” December 2002.) 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] (Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Authentication Protocol,” January 2006.). 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] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.). 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] (Wasserman, M. and T. Goddard, “Using the NETCONF Configuration Protocol over Secure SHell (SSH),” December 2006.). This memo describes a method for invoking and running the SNMP protocol within a Secure Shell (SSH) session as an SSH subsystem.

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:

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:

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

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

3.1.2. Message Authentication

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 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.

3.1.3. Authentication Protocol Support

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 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 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 a SSH user name matching the "user" portion of the notation when establishing a session with the remote SSH server. 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.

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, 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 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] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.) 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 control processing needs to be performed 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

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 (Elements of Procedure) 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 (Elements of Procedure). 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.

The securityName MAY be derived from the tmSecurityName by a security model, and MAY be used to configure notifications and access controls. Transport models SHOULD generate a predictable tmSecurityName.

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

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 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

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

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:

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.

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

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

5.4. Closing a Session

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.

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

6.3.1. MIB Modules Required for IMPORTS

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 "200903090000Z"
    ORGANIZATION "ISMS Working Group"
    CONTACT-INFO "WG-EMail:   isms@lists.ietf.org
                  Subscribe:  isms-request@lists.ietf.org

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

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

                  Co-editors:
                     David Harrington
                     Huawei Technologies USA
                     1700 Alma Drive
                     Plano Texas 75075
                     USA
                     +1 603-436-8634
                     ietfdbh@comcast.net

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

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

                  Copyright (C) The IETF Trust (2009). 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     "200903090000Z"
    DESCRIPTION  "The initial version, published in RFC XXXX.
-- NOTE to RFC editor: replace XXXX with actual RFC number
--                     for this document and remove this note
                 "

    ::= { mib-2 xxxx }
-- RFC Ed.: replace xxxx with IANA-assigned number and
--          remove this note

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

sshtmNotifications    OBJECT IDENTIFIER ::= { sshtmMIB 0 }
sshtmObjects          OBJECT IDENTIFIER ::= { sshtmMIB 1 }
sshtmConformance      OBJECT IDENTIFIER ::= { sshtmMIB 2 }

-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------

snmpSSHDomain OBJECT-IDENTITY
    STATUS      current
    DESCRIPTION
        "The SNMP over SSH transport domain. The corresponding transport
         address is of type SnmpSSHAddress.

         When an SNMP entity uses the snmpSSHDomain transport
         model, it must be capable of accepting messages up to
         and including 8192 octets in size. Implementation of
         larger values is encouraged whenever possible.

         The securityName prefix to be associated with the
         snmpSSHDomain is 'ssh'. This prefix may be used by security
         models or other components to identify what secure transport
         infrastructure authenticated a securityName."
    ::= { snmpDomains yy }

-- RFC Ed.: Please replace the I-D reference with a proper one once it
-- has been published.

-- RFC Ed.: replace yy with IANA-assigned number and
--          remove this note

-- RFC Ed.: replace 'ssh' with the actual IANA assigned prefix string
--          if 'ssh' is not assigned to this document.

SnmpSSHAddress ::= TEXTUAL-CONVENTION
    DISPLAY-HINT "1a"
    STATUS      current
    DESCRIPTION
        "Represents either a hostname or IP address, along with a port
         number and an optional username.

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

         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 in dotted decimal format followed
         by a colon ':' (ASCII character 0x3A) and a decimal port
         number in ASCII.

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

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

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

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

         When this textual convention is used as a syntax of an
         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
      "RFC3986, 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 as an SSH client, 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 as an SSH client, whether it succeeded or
                 failed.
                "
    ::= { sshtmSession 2 }

sshtmSessionOpenErrors  OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of times an openSession() request
                 failed to open a transport connection, or failed to
                 authenticate the server.
                "
    ::= { sshtmSession 3 }

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

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

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 7 }

sshtmSessionInvalidCaches OBJECT-TYPE
    SYNTAX       Counter32
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The number of outgoing messages dropped because the
                 tmStateReference referred to an invalid cache.
                "
    ::= { sshtmSession 8 }

-- ************************************************
-- sshtmMIB - Conformance Information
-- ************************************************

sshtmCompliances OBJECT IDENTIFIER ::= { sshtmConformance 1 }

sshtmGroups      OBJECT IDENTIFIER ::= { sshtmConformance 2 }

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

sshtmCompliance MODULE-COMPLIANCE
    STATUS      current

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

-- ************************************************
-- Units of conformance
-- ************************************************
sshtmGroup OBJECT-GROUP
    OBJECTS {
      sshtmSessionOpens,
      sshtmSessionCloses,
      sshtmSessionOpenErrors,
      sshtmSessionUserAuthFailures,
      sshtmSessionChannelOpenFailures,
      sshtmSessionUnavailableSubsystems,
      sshtmSessionNoAvailableSessions,
      sshtmSessionInvalidCaches
    }
    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.

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 adjustments 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 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 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.

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 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.

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.

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 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 Authorizaton 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. It is thus critical that the credentials presented by a notification receiver MUST match the expected value(s) for a given transport address and that ownership of the credentials MUST be properly cryptographically verified.

9.3. SSH User and Key Selection

If a "user@" prefix is used within a SnmpSSHAddress value to specify a 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] (Blumenthal, U. and B. Wijnen, “User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3),” December 2002.) employed symmetric cryptography and naming user conventions. SSH employs an asymmetric cryptographic 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.

9.5. 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.

9.6. Use with SNMPv1/v2c Messages

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:

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.

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

-- note to RFC editor -- Please replace YYY and ZZZ in this document with the assigned port numbers and remove this note.

11. Acknowledgements

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.

Additionally, helpful document reviews were received from: Juergen Schoenwaelder.

[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC2578]	McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” STD 58, RFC 2578, April 1999 (TXT).
[RFC2579]	McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Textual Conventions for SMIv2,” STD 58, RFC 2579, April 1999 (TXT).
[RFC2580]	McCloghrie, K., Perkins, D., and J. Schoenwaelder, “Conformance Statements for SMIv2,” STD 58, RFC 2580, April 1999 (TXT).
[RFC2865]	Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” RFC 2865, June 2000 (TXT).
[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 (TXT).
[RFC3413]	Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” STD 62, RFC 3413, December 2002 (TXT).
[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 (TXT).
[RFC3418]	Presuhn, R., “Management Information Base (MIB) for the Simple Network Management Protocol (SNMP),” STD 62, RFC 3418, December 2002 (TXT).
[RFC3490]	Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” RFC 3490, March 2003 (TXT).
[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 (TXT).
[RFC4251]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Protocol Architecture,” RFC 4251, January 2006 (TXT).
[RFC4252]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Authentication Protocol,” RFC 4252, January 2006 (TXT).
[RFC4253]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Transport Layer Protocol,” RFC 4253, January 2006 (TXT).
[RFC4254]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Connection Protocol,” RFC 4254, January 2006 (TXT).
[I-D.ietf-isms-tmsm]	Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” draft-ietf-isms-tmsm-18 (work in progress), May 2009 (TXT).

[RFC1994]	Simpson, W., “PPP Challenge Handshake Authentication Protocol (CHAP),” RFC 1994, August 1996 (TXT).
[RFC3410]	Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” RFC 3410, December 2002 (TXT).
[RFC3588]	Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” RFC 3588, September 2003 (TXT).
[RFC3986]	Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[RFC4256]	Cusack, F. and M. Forssen, “Generic Message Exchange Authentication for the Secure Shell Protocol (SSH),” RFC 4256, January 2006 (TXT).
[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 (TXT).
[RFC5090]	Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D., and W. Beck, “RADIUS Extension for Digest Authentication,” RFC 5090, February 2008 (TXT).
[RFC4742]	Wasserman, M. and T. Goddard, “Using the NETCONF Configuration Protocol over Secure SHell (SSH),” RFC 4742, December 2006 (TXT).
[I-D.ietf-isms-transport-security-model]	Harrington, D. and W. Hardaker, “Transport Security Model for SNMP,” draft-ietf-isms-transport-security-model-14 (work in progress), May 2009 (TXT).

	David Harrington
	Huawei Technologies (USA)
	1700 Alma Dr. Suite 100
	Plano, TX 75075
	USA
Phone:	+1 603 436 8634
EMail:	dharrington@huawei.com

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

	Wes Hardaker
	Sparta, Inc.
	P.O. Box 382
	Davis, CA 95617
	US
Phone:	+1 530 792 1913
EMail:	ietf@hardakers.net

Secure Shell Transport Model for SNMPdraft-ietf-isms-secshell-15

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