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SNMP Simple Network Management Protocol

Miguel Leitão Nov 2012

Introduction

NMS (Network Management System) Hardware and software used to monitor and administer

computer networks.

SNMP (Simple Network Management Protocol) Application-layer protocol for managing TCP/IP based networks.

Runs over UDP, which runs over IP.

SNMP Agent Device (e.g. Router) running software that understands SNMP

language.

MIB (Management Information Base) Database of info conforming to SMI.

SMI (Structure of Management Information)

Standard that defines how to create a MIB.

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SNMP Overview

Defines mechanism for remote management of network

devices (routers, bridges, etc.)

Fundamental principle: all device management done by

simple variable value manipulation.

Approach:

standard means for specifying quantities recognized by devices.

protocol for requesting, returning, notifying of changes of values.

SNMP Message Protocol

Messages use UDP

port 161 (requests/responses)

port 162 (notifications)

Message types:

GetRequest: request values of variables from device

GetNextRequest: request value of variable following

the one supplied

GetResponse: return values

SetRequest: instruct device to set values of variables

Trap: from device - notify monitor / manager of value

change

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SNMP encapsulation

Ethernet

Frame IP Packet

UDP Datagram SNMP Message CRC

SNMP Environment

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TCP/IP Protocol Suite

Agents: usually routers or servers to monitor and control.

Manager: usually a host, that controls and monitors a set of agents.

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Management overview

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1

2

3

Get Request

SNMP packet

4

ResponseSNMP packet

5

6

MIB

MIB: Management Information Base

Related to a managed device.

Description of a collection of

Management Objects

Can be specified using MIB Modules

Specified in Structure of Management

Information (SMI),

set of rules for specifying management information

adapted subset of the Abstract Syntax Notation One

(ASN.1)

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Managed Objects

Objects may be defined from instances of Object-Types.

Each objects is identified by:

Object IDentifier (OID): Machine recognizable

Name: Human readable

Example

(Name of the Object) OBJECT-TYPE

Property 1

Property 2

..............

::= OBJECT IDENTIFIER

OID

OID: Object IDentifier

Hierachical structure managed by ISO

mgmt object is identified as:

{ iso(1) org(3) dod(6) internet(1) mgmt(2) }

or simply:

{1.3.6.1.2}

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MIB – Management Information Base

MIB Format

OBJECT-TYPE

String that describes the MIB object.

Object IDentifier (OID).

SYNTAX Defines what kind of info is stored in

the MIB object.

ACCESS READ-ONLY, READ-WRITE.

STATUS State of object in regards the SNMP

community.

DESCRIPTION Reason why the MIB object exists.

Standard MIB Object:

sysUpTime OBJECT-TYPE

SYNTAX Time-Ticks

ACCESS read-only

STATUS mandatory

DESCRIPTION

“Time since the

network management

portion of the system

was last re-initialised.

::= {system 3}

SNMP Commands

SNMP has 5 different functions referred to as

Protocol Data Units (PDU’s), which are:

(1) GetRequest, aka Get

(2) GetNextRequest, aka GetNext

(3) GetResponse, aka Response

(4) SetRequest, aka Set

(5) Trap

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SNMP Commands [Get]

GetRequest [Get]

Most common PDU.

Used to ask SNMP agent for value of a particular

MIB agent.

NMS sends out 1 Get PDU for each instance,

which has a unique OID string.

SNMP Commands [GetNext]

GetNextRequest [GetNext]

NMS application uses GetNext to ‘walk’ down a table within a MIB.

Designed to ask for the OID and value of the MIB instance that comes after the one asked for.

Once the agent responds the NMS application can increment its count and generate a GetNext.

This can continue until the NMS application detects that the OID has changed, i.e. it has reached the end of the table.

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SNMP Commands [GetResponse]

GetResponse [Response]

Response to a Get, GetNext or Set.

SNMP agent responds to all requests or

commands via this PDU.

SNMP Commands [SetRequest]

SetRequest [Set]

Issued by an NMS application to change the

value of a MIB variable.

NMS must provide the correct permissions when

using the set PDU.

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SNMP Commands [Trap]

Trap

Asynchronous notification.

SNMP agents can be programmed to send a trap

when a certain set of circumstances arise.

Circumstances can be view as thresholds, i.e. a

trap may be sent when the temperature of the

core breaches a predefined level.

SNMP PDUs

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TC

P/IP

Pro

toco

l S

uite

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SNMP message

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TC

P/I

P P

roto

co

l S

uite

PDU format

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TC

P/IP

Pro

toco

l S

uite

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PDU types

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TCP/IP Protocol Suite

Errors

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TCP/IP Protocol Suite

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Value Encoding

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TC

P/I

P P

roto

co

l S

uite

Data Types

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TCP/IP Protocol Suite

Tag filed identifies the Data Type

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Value examples

34 TCP/IP Protocol Suite

Integer: 14

String: “HI”

Value examples

35 TCP/IP Protocol Suite

ObjectIndentifier: 1.3.6.1

IPv4 Addr: 131.21.14.8

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Message example

36 TC

P/I

P P

roto

co

l S

uite

OID

NULL

GetRequest

SNMPv2

SNMPv2 added two new PDUs

1. GETBULK REQUEST

2. INFORM

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SNMPv3

SNMPv3 added two new features:

• Security

• Message authentication,

• Confidentiality

• Integrity.

• Remote administration.

SNMP Tools

Command Line Interface

snmpwalk

…

Graphical User Interface

iReasoning’s MIB Browser

www.ireasoning.com

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iReasoning MIB Browser

MIB structure

Details of MIB structure.

iReasoning MIB Browser

Values