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TUT:Writing a MIB Module

From Net-SNMP Wiki

This page is the first in a series to guide you through the creation of custom extensions to the core Net-SNMPdaemon, usually called an SNMP "agent". In this article we will describe how to build your extension into thedaemon. The two following articles will describe how to put your changes into a shared object (see Writing aDynamically Loadable Object) and a separate daemon (see Writing a Subagent).

Extensions (or "MIB modules") usually make use of the API provided by Net-SNMP and are therefore writtenin C. For instructions on writing extensions in Perl, please read Extending SNMPd using Perl.

Contents

1 The "Big Picture"

2 Recommended Reading List3 Writing Your First Module

3.1 MIBs For Dummies and mib2c

3.2 A simple scalar attached to a variable3.3 A simple scalar with the value returned from code

3.4 A table of data, stored within the agent

3.5 Sending SNMP notifications (traps and informs) from inside the agent.

3.6 The older (and thus most backward compatible) ucd-snmp 4.X API

4 The MIB Module API

5 Compiling in your new MIB module6 Set Processing

7 Tutorial Sections7.1 About the SNMP Protocol

7.2 Net-SNMP Command Line Applications7.3 Application Configuration

7.4 Net-SNMP Daemons

7.5 Coding Tutorials

7.6 Debugging SNMP Applications and Agents

7.7 Operating System Specific Tutorials

The "Big Picture"

The main task of custom extensions is to register a callback function which handles requests for a specific OID.Version 5.0 of the Net-SNMP agent tries to make this task as easy as possible by providing so called helpermodules. Each request is handled in a chain in Net-SNMP, as described in the Agent Architecture. You caninject helper modules into this chain, so that helper modules are called before control reaches the actualcallback function. This can, for example, be used to install a cache in front of the actual data acquisition, whichmay be costly.

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Recommended Reading List

It is most helpful if you have the time to read through some documentation that describes how the Net-SNMPagent architecture works. Thus, if you're starting from scratch, the suggested reading order for information aboutthe agent is:

The Agent Architecture describes the internals of the agent, and how the handler system works.

The Agent Helpers list describes the available MIB helpers available, which will be used in detail below.

The rest of the tutorial below where you actually get to play with some code.

Can you skip some things on this list and jump straight to the real-code below? Absolutely! But you'll probablyregret it later.

Writing Your First Module

We will now walk you through the process of developing increasingly more complex MIB objects, beginningwith a scalar, then a table, persistent data, notifications, and so on.

MIBs For Dummies and mib2c

Writing modules for MIBs is a long and sometimes troublesome process, much like writing a parser. And justlike writing a parser, you don't have to do everything by hand: MIBs can already be processed by computerspretty well, so there is no need to start from square one every time. The tool to convert an existing MIB to someC code is called mib2c and is part of the Net-SNMP distribution. There is a MIB for Dummies tutorial on usingmib2c and local/mib2c.mfd.conf (http://net-snmp.git.sourceforge.net/git/gitweb.cgi?p=net-snmp/net-snmp;a=blob_plain;f=local/mib2c.mfd.conf;hb=HEAD) to generate a code template for a table.

A simple scalar attached to a variable

The first example, scalar_int.c (http://www.net-snmp.org/dev/agent/scalar__int_8c-example.html) , only consistsof an initialization function, init_scalar_int, which registers the module-global integer example1 with the

SNMP agent (aka. "daemon"). The prototype and name of the initialization function must follow this schema:

void init_module_name (void);

In the example code, the module's name is "scalar_int", hence the name of the initialization function.

Within the initialization function, the API function netsnmp_register_int_instance (http://www.net-snmp.org/dev/agent/group__instance.html#ga20) is called to actually register the integer at a specified OID. Theprototype of that API function is:

int netsnmp_register_int_instance (const char *name,

oid *reg_oid,

size_t reg_oid_len,

int *it,

Netsnmp_Node_Handler *subhandler);

The first argument, name, is a the short description of the object being registered. The second argument is theOID to assign to this object — the length of the OID is stored in the third argument. It is recommended to usethe OID_LENGTH macro which will only work with statically allocated OIDs (arrays work, pointers don't).

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For the sake of simplicity the default subhandler is used in the example code, therefore NULL is passed as fifth

argument. The default handler allows the integer to be read with a GET request as well as set with a SET request.

Simple integer registration functions, including read-only registration and the default handlers, are declared inagent/instance.h.

A simple scalar with the value returned from code

Here is some example code about writing a MIB Module (http://www.net-snmp.org/dev/agent/delayed__instance_8c-example.html) that describes how to implement a generic instance(not tied to a variable, like the above example) as well as how to delay answering queries for a bit (in theexample, an alarm is set to add a delay to the answer).

A table of data, stored within the agent

Here is a more complex example (http://www.net-snmp.org/dev/agent/data__set_8c-example.html) thatimplements a table, where the table data is completely contained within the agent.

Sending SNMP notifications (traps and informs) from inside the agent.

Here is an example of how to use the agent's internal notification sending API (http://www.net-snmp.org/dev/agent/notification_8c-example.html) to send a notification to all of the agent's trap receivers.

The older (and thus most backward compatible) ucd-snmp 4.X API

Of course, you can continue to write code using the older API set. A tutorial on it can be found here(http://www.net-snmp.org/tutorial/tutorial-4/toolkit/mib_module/) .

The MIB Module API

The new API is documented more completely here (http://www.net-snmp.org/dev/agent/group__handler.html) .Additionally, see the complete list of (mostly) documented available helpers and other information here(http://www.net-snmp.org/tutorial/tutorial-4/toolkit/mib_module/) .

Compiling in your new MIB module

Note: you will need to have previously installed the Net-SNMP Source Package on your systembefore proceeding.

There are a few ways to get your new MIB module loaded and accessible via SNMP requests. We'll discuss allthree ways separately. To make this easy to test the procedures outlined below, we've provided three simplemib modules which implement the three simple scalars in the NET-SNMP-TUTORIAL-MIB (http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mib_module/NET-SNMP-TUTORIAL-MIB.txt) MIB. To see how MIBscan be properly used by the tools, please see the mib-options tutorial.

1. Compile it into the master agent.

Lets assume you're going to compile in a new mib module. For our example, lets use the example mib module(http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mib_module/nstAgentModuleObject.c) and it's header file(http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mib_module/nstAgentModuleObject.h) . To do this, you

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would put the nstAgentModuleObject.h and nstAgentModuleObject.c files into the net-snmp source codedirectory. You do this by copying them into a agent/mibgroup/nstAgentModuleObject.h andagent/mibgroup/nstAgentModuleObject.c file.

Next, you have to configure the package to find them and compile them into the agent. To do this, you run theconfigure script giving it the extra module names you want it to load:

% ./configure --with-mib-modules="nstAgentModuleObject"

If you had multiple modules to include (like a second "XXX" module, for example), you can separate them withspaces inside the quotes (e.g., --with-mib-modules="nstAgentModuleObject XXX").

Note that nstAgentModuleObject is the prefix and the configure script will actually look for anstAgentModuleObject.h and a nstAgentModuleObject.c file. You must have a .h file and you can not get it towork with just a .c file.

Build your new agent with your new code in it by running make:

% make

Finally, install the whole lot by running make install:

% make install

Now that the agent is installed, you need to add some basic configuration (see other tutorials for moreconfiguration options). Here we just add a simple read-only and read-write community strings (tutget andtutset, respectively) for the tutorial MIB branch:

% echo "rocommunity tutget .1.3.6.1.4.1.8072.2.4" > /usr/local/etc/snmpd.conf

% echo "rwcommunity tutset .1.3.6.1.4.1.8072.2.4" > /usr/local/etc/snmpd.conf

You can test out the functionality by starting the snmpd agent:

% /usr/local/sbin/snmpd -f -L -d -p 9999

This runs the agent on a temporary port ( -p 9999) so that it doesn't need special priviledges, or interfere withyour 'normal' agent. Incoming and outgoing packets are printed (-d), to show what's happening, and the agent isrun as a 'non-daemon' command (-f -L) so that you can see these messages. Note that this ties up the currentshell, so you'll need to run the following checks in a different terminal window.

And then running snmpget and snmpset on the scalar object:

% snmpget -v2c -c tutget localhost:9999 NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0

NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 = INTEGER: 1

% snmpset -v2c -c tutset localhost:9999 NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 = 5

NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 = INTEGER: 5

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% snmpget -v2c -c tutget localhost:9999 NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 = INTEGER: 5

You can also compile your code into a "subagent" which then attaches itself to the master agent using theAgentX subagent protocol (http://www.scguild.com/agentx/) . Our libraries provide support to make this easy todo and this is discussed in greater detail in a later section.

Finally, you can also compile your code into pluggable shared object and tell the snmpd agent to load it. This isalso discussed in greater detail in a later section .

Set Processing

To process an SNMP-SET, the agent must use a series of calls to the mib module code to ensure thatprocessing of all sets in the incoming packet can be successful. This gives you or other mib modules the chanceto bail out early on in the transaction sequence and thus stop all of the transactions in the set from happening.This is important for continuity. However, it makes set code processing a bit more complex. Let's examine asimple state diagram that the master agent uses at each step of the way:

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destinationIn a perfect operation with no failures, we take the vertical path on the left. If any of the mib modules beingacted upon returns an error of any kind, we will branch to the right to one of the failure states where you mustclean up and possibly undo what you did in previous steps.

If you need even finer-grained SET processing with even more states, check out the baby_steps helper module.

Tutorial Sections

About the SNMP Protocol

These tutorial links talk about SNMP generically and how the protocol itself works. They are good introductoryreading material and the concepts are important to understand before diving into the later tutorials about Net-SNMP itself.

How SNMP Works: About the protocol itself (GETs, GETNEXTs, etc)What data is in SNMP: All about SNMP Management Information Bases (MIBs)

Securing SNMP: How to use the SNMP protocol securely

Net-SNMP Command Line Applications

These tutorial pages discuss the command line tools provided in the Net-SNMP suite of tools. Nearly all theexample commands in these tutorials works if you try it yourself, as they're all examples that talk to our onlineNet-SNMP test agent. Given them a shot!

snmptranslate: learning about the MIB tree.

snmpget: retrieving data from a host.

snmpgetnext: retrieving unknown indexed data.

snmpwalk: retrieving lots of data at once!snmptable: displaying a table.

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snmpset: peforming write operations.

snmpbulkget: communicates with a network entity using SNMP GETBULK request

snmpbulkwalk: retrieve a sub-tree of management values using SNMP GETBULK requests.snmptrap: Sending and receiving traps, and acting upon them.

Traps/informs with SNMPv3/USM: Sending and receiving SNMPv3/USM TRAPs and

INFORMs

Sending Traps/Informs via AgentX: Sending notifications from the command line through snmpdCommon command line options:

Using and loading MIBS

SNMPv3/USM Options

Using SNMPv3 over TLS and DTLS

Customized Output Formats

Writing mib2c config files

Application Configuration

All of our applications support configuration to allow you to customize how they behave.

Configuration files for Net-SNMP applications

Net-SNMP Daemons

Net-SNMP comes with two long-running daemons: a SNMP agent (snmpd) for responding to managementrequests and a notification receiver (snmptrapd) for receiving SNMP notifications.

SNMP Agent (snmpd) Configuration

Configuration Basics

Access Control (VACM)

snmpconf

SNMP Notification Receiver (snmptrapd)

Configuring snmptrapdConfiguring SNMPv3 notifications

Configuring snmptrapd to understand vendor-specific MIBS (Cisco)

Agent Monitoring

DisMan Monitoring

Monitoring with MRTG

Coding Tutorials

Net-SNMP comes with a highly flexible and extensible API. The API allows you to create your owncommands, add extensions to the agent to support your own MIBs and perform specialized processing ofnotifications.

Client / Manager Coding Tutorials

Writing a simple application

Writing a simple asynchronous application

Agent Coding Tutorials

The Agent Architecture page might be worth reading before or after the agent coding tutorials, anddescribes how the Agent Helpers work under the hood.

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Writing a mib module to serve information described by an SNMP MIB, and how to compile it

into the net-snmp snmpd agent.

Writing a Dynamically Loadable Object that can be loaded into the SNMP agent.

Writing a Subagent that can be run to attach to the snmpd master agent.

Writing a perl plugin to extend the agent using the NetSNMP::agent module.

Writing shell scripts to extend the agent

Using mib2c to help write an agent code template for youGeneral mib2c Overview

Using the mib2c-update script to recode your code

mib2c.mfd.conf tutorial (http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mfd/)

Header files and autoconf

Debugging SNMP Applications and Agents

All our tools and applications have extensive debugging output. These tutorials talk about how the debuggingsystem works and how you can add your own debugging statements to you code:

Debugging output printed using the -D command line option

Putting DEBUGMSG tokens in your codeUsing -Ddump to display packet breakdowns

Debugging using GDB

Operating System Specific Tutorials

Building With Visual Studio 2005 Express

Using the command line and nmake

Net-Snmp on Ubuntu

Net-SNMP and lm-sensors on Ubuntu 10.04Net-SNMP for windows:

How to install: [1] (http://www.netadmintools.com/art487.html)

How to configure: [2] (http://www.netadmintools.com/art488.html)

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