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What is SNMP?
SNMP (Simple Network
Management Protocol) is a widely available protocol that facilitates management
of networked devices from a central location. An SNMP Architecture has 3 major
elements:
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Managers (software) are responsible for communicating with (and
managing) network devices that implement SNMP Agents (also software).
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Agents reside in devices such as workstations, switches, routers,
microwave radios, printers, and provide information to Managers.
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MIBs (Management Information Base) describe data objects to be
managed by an Agent within a device. MIBs are actually just text files, and
values of MIB data objects are the topic of conversation between Managers and
Agents.
We need to mention Object Identifiers
(OIDs) here. Each MIB object definition has a unique OID, which is a dotted
list of integers. For example, the OID for data object "hrDiskStorageCapacity"
is ".1.3.6.1.2.1.25.3.6.1.4".
When Managers and Agents communicate they refer to MIB data objects using
OIDs.
An OID sent with a corresponding value {OID,Value} is referred to as
"binding".
Manager & Agent
Communications
Managers can be viewed as
Clients and Agents as Servers. The operations between Managers and Agents are
quite simple:
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get commands are sent by a Manager to an Agent to request data
values defined by a MIB.
The agent will send a response with the
requested values.
Closely related requests are getnext and
getbulk.
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A Manager can also send
set commands to an Agent. If the MIB
defines a data object as read-write, then the Agent will accept the data value
sent with the set command and process it
(store it or execute appropriate actions).
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Agents will send
unsolicited traps (alarms) to Managers to
alert them to important events.
Typical Uses
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Monitoring device
performance
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Detecting device faults (or
recovery from faults)
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Collecting long term
performance data (trend reports)
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Remote device configuration
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Remote device control
The operations of SNMP are
"simple", but the totality of SNMP is not -- typical ramp-up time for an
individual (on their own) to learn how to effectively use SNMP can easily be six
months or more.
SNMP Standards and
Versions
SNMP Standards are described
in RFC (Request for Comments) documents published by the IETF (Internet
Engineering Task Force). Standards Topics can generally be categorized into:
Messaging Protocols
Three such protocols are
defined:
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SNMPv1 was the first protocol introduced, and is still widely
used. It implements get,
getnext,
getresponse, and trap
operations.
Security for SNMPv1 is based on a "community string" that is sent with each
message, and functions as a password. If the Manager includes the correct
password in a request to an Agent, the agent will send a response. The
community string is sent in the clear (not encrypted) and thus the security
it provides is quite weak.
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SNMPv2 usually refers to SNMPv2c
(other SNMPv2's were proposed, but only SNMPv2c currently has significant
support).
SNMPv2c introduced the ability to transmit SMIv2 MIB-objects of type
"Counter64".
SNMPv2c also provides for expanded messaging operations:
getbulk, inform,
report, and a new
v2trap operation (same functionality as the
SNMPv1 trap). Enhanced agent error
responses were also added.
SNMPv2c uses the same community string security as SNMPv1.
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SNMPv3 is the most recent introduction, and is a major step
forward in improving security. Security enhancements include:
User Authentication --
verification of the identity of the SNMP Entity (Manager or Agent) sending
the message. Authentication is based on definitions of users, and on shared
secret keys used to produce a message hashed authentication code sent with
each message.
Encryption -- the message payload
can be optionally encrypted based on a second shared key.
VACM (View Access Control Model)
-- Agents can be configured to control who can access which MIB objects
under agent management. For example, User "Barney Bigwig" can access
critical read-write control data, while User "Robbie Repair" can access only
read-only data.
Message Timeless Checks ensure
that messages are not delayed or replayed.
MIB Syntax Standards
MIBs are written in either
of two syntax standards:
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SMIv1 is the earliest version, and was introduced along with
SNMPv1. SMIv1 MIBs are very functional and quite common.
Data objects defined by SMIv1 MIBs can be transmitted between Managers and
Agents using any of the 3 messaging protocols (SNMPv1, SNMPv2c, SNMPv3).
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SMIv2 was introduced along with SNMPv2c. Major enhancements
include:
Introduction of new data base types, including the "Counter64".
Although derived data types could be defined using SMIv1, SMIv2 formalizes
this syntax, and also defines some "Standard Textual Conventions" such as
"Display String".
Improvements that enhance the documentation of MIB files and objects, such
as:
-- Module Identity Statement
-- Conformance Statements
-- Improved NOTIFICATION-TYPE syntax replaces TRAP-TYPE syntax
An SMIv2 MIB can usually be translated into an SMIv1 MIB. The primary
exceptions are: a) A Counter64 object has no SMIv1 counterpart, b) An
improperly coded NOTIFICATION-TYPE object cannot be translated to an SMIv1
TRAP-TYPE object with the same OID.
Data defined by SMIv2 MIB Objects can be transmitted between Managers and
Agents by any of the 3 messaging protocols. The only exception to this is
that Counter64 data cannot be transported in SNMPv1 messages.
"Standard MIB" Definitions
Depending on the origin
(author) of a MIB, we can categorize MIBs into either:
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Standard MIBs are authored by persons associated with the IETF.
For example, a Standard Printer MIB exists, and printer manufacturers
commonly implement Agent support for this MIB (in addition to their own
Enterprise MIBs).
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Enterprise MIBs are authored by non-standards-committee
organizations (e.g., Cisco, HP, Chateau Systems). All such organizations
must apply for a unique "Enterprise ID" issued by IANA (Internet Assigned
Number Authority). Enterprise MIB objects are then organized under these
unique assigned OIDs.
Standard MIB Examples:
For more information on how
to further your knowledge of the SNMP Protocol, click on the SNMP Training Link
below.
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