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IETF RFC 5592
Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)
Last modified on Wednesday, July 1st, 2009
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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.
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Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
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than English.
Harrington, et al. Standards Track PAGE 1 
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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
Harrington, et al. Standards Track PAGE 2
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4251]
within a Transport Subsystem [RFC 5590]. 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 [RFC 3411] 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 [RFC 3410].
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 [RFC 2578], STD 58, RFC 2579 [RFC 2579] and STD 58, RFC 2580
[RFC 2580].
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 [RFC 2119].
Harrington, et al. Standards Track PAGE 3
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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" [RFC 3413] for further
information.
The User-based Security Model (USM) [RFC 3414] 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 [RFC 3411] 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.
Harrington, et al. Standards Track PAGE 4
RFC 5592 Secure Shell Transport Model for SNMP June 2009
1.3. Modularity
The reader is expected to have read and understood the description of
the SNMP architecture, as defined in [RFC 3411], and the Transport
Subsystem architecture extension specified in "Transport Subsystem
for the Simple Network Management Protocol (SNMP)" [RFC 5590].
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)
[RFC 3414] 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.
Harrington, et al. Standards Track PAGE 5
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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" [RFC 4252]. 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 [RFC 2865]. 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 [RFC 4742].
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 [RFC 4254] over the SSH
transport protocol, and using ssh-userauth [RFC 4252] 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).
Harrington, et al. Standards Track PAGE 6
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4253] 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 [RFC 4252] authenticates the
client-side principal to the server. It runs over the Transport
Layer Protocol.
o The Connection Protocol [RFC 4254] multiplexes the encrypted tunnel
into several logical channels. It runs over the transport after
successfully authenticating the principal.
o Generic Message Exchange Authentication [RFC 4256] 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" [RFC 4462] 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.
Harrington, et al. Standards Track PAGE 7
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 5590]
within the SNMP architecture. The SSH Transport Model thus fits
between the underlying SSH transport layer and the Message Dispatcher
[RFC 3411].
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 [RFC 3411]:
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.
Harrington, et al. Standards Track PAGE 8
RFC 5592 Secure Shell Transport Model for SNMP June 2009
SSH provides for verification of the identity of the SSH server
through the SSH transport protocol server authentication
[RFC 4253]. 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 [RFC 4252]. 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
[RFC 4253].
Harrington, et al. Standards Track PAGE 9
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4252] and the Secure Shell Transport Layer Protocol
[RFC 4253] 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 [RFC 4252] (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 [RFC 2865] or Diameter [RFC 3588]
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) [RFC 1994] or digest authentication
[RFC 5090] 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.
Harrington, et al. Standards Track PAGE 10
RFC 5592 Secure Shell Transport Model for SNMP June 2009
GSS-API key exchange [RFC 4462] 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 [RFC 4252]).
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.
Harrington, et al. Standards Track PAGE 11
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3413]
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-
Harrington, et al. Standards Track PAGE 12
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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" [RFC 5590] 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.
Harrington, et al. Standards Track PAGE 13
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3411] and
further augmented by [RFC 5590] 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.
Harrington, et al. Standards Track PAGE 14
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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).
Harrington, et al. Standards Track PAGE 15
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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:
Harrington, et al. Standards Track PAGE 16
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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.
Harrington, et al. Standards Track PAGE 17
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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.
Harrington, et al. Standards Track PAGE 18
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4252].
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.
Harrington, et al. Standards Track PAGE 19
RFC 5592 Secure Shell Transport Model for SNMP June 2009
4. The client should invoke the "ssh-connection" service (also known
as the SSH connection protocol [RFC 4254]), 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.
Harrington, et al. Standards Track PAGE 20
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3418] and the SNMP-FRAMEWORK-MIB
[RFC 3411]. It is expected that an entity implementing this MIB will
also support the Transport Security Model [RFC 5591] 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 [RFC 2578], [RFC 2579], and
[RFC 2580].
This MIB module also references [RFC 1033], [RFC 4252], [RFC 3490], and
[RFC 3986].
Harrington, et al. Standards Track PAGE 21
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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
Harrington, et al. Standards Track PAGE 22
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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,
Harrington, et al. Standards Track PAGE 23
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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
Harrington, et al. Standards Track PAGE 24
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4252]). 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 RFC 1033);
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.
Harrington, et al. Standards Track PAGE 25
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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
Harrington, et al. Standards Track PAGE 26
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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
Harrington, et al. Standards Track PAGE 27
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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.
"
Harrington, et al. Standards Track PAGE 28
RFC 5592 Secure Shell Transport Model for SNMP June 2009
::= { 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 [RFC 5591].
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
Harrington, et al. Standards Track PAGE 29
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 4251].
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.
Harrington, et al. Standards Track PAGE 30
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3414] 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.
Harrington, et al. Standards Track PAGE 31
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3584] (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.
Harrington, et al. Standards Track PAGE 32
RFC 5592 Secure Shell Transport Model for SNMP June 2009
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 [RFC 3410], Section 8),
including full support for cryptographic mechanisms for
authentication and privacy, such as those found in the User-based
Security Model [RFC 3414], the Transport Security Model [RFC 5591], 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 [RFC 5590].
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.
Harrington, et al. Standards Track PAGE 33
RFC 5592 Secure Shell Transport Model for SNMP June 2009
Additionally, helpful document reviews were received from Juergen
Schoenwaelder.
12. References
12.1. Normative References
[RFC 1033] Lottor, M., "Domain administrators operations guide",
RFC 1033, November 1987.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC 2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC 2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC 3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC 3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC 3414] 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.
[RFC 3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC 3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
Harrington, et al. Standards Track PAGE 34
RFC 5592 Secure Shell Transport Model for SNMP June 2009
[RFC 3584] 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.
[RFC 4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC 4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC 4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC 4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
[RFC 5590] Harrington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)",
RFC 5590, June 2009.
12.2. Informative References
[RFC 1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC 2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC 3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[RFC 3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC 4256] Cusack, F. and M. Forssen, "Generic Message Exchange
Authentication for the Secure Shell Protocol (SSH)",
RFC 4256, January 2006.
Harrington, et al. Standards Track PAGE 35
RFC 5592 Secure Shell Transport Model for SNMP June 2009
[RFC 4462] 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.
[RFC 4742] Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure SHell (SSH)", RFC 4742,
December 2006.
[RFC 5090] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
and W. Beck, "RADIUS Extension for Digest Authentication",
RFC 5090, February 2008.
[RFC 5591] 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
Harrington, et al. Standards Track PAGE 36
Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)
RFC TOTAL SIZE: 82822 bytes
PUBLICATION DATE: Wednesday, July 1st, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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