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MFD Tutorial

MIB for Dummies

MFD:IF-MIB

MFD:ifTable

MFD:ifTable:mib2c

MFD:ifTable:Data Structures

MFD:ifTable:Data Access

MFD:ifTable:Get Data

MFD:ifTable:Testing

MFD:ifXTable

MFD:ifXTable:mib2cMFD:ifXTable:Data Structures

MFD:ifXTable:Data Access

MFD:ifXTable:Get Data

MFD:ifXTable:Testing Gets

MFD:ifXTable:Set Data

MFD:ifXTable:Testing Sets

MIB for Dummies

From Net-SNMP Wiki

Contents1 MIBs For Dummies

1.1 Introduction

1.2 Data Structures

1.2.1 User Context

1.2.2 MIB context

1.2.3 Data Context

1.2.4 Row Request Context

1.3 Data Lookup

1.3.1 container-cached

1.3.2 Iterator (unsorted-external)

1.3.3 Direct

1.4 Data manipulation

1.5 Getting Started

1.5.1 Basic Tables

1.5.2 Intermediate Tables

1.5.3 Advanced Tables

MIBs For Dummies

NOTE: this tutorial is based on the code for release 5.2. If you generate code from a release prior to 5.2

or with 5.3.x, your results may vary.

Power users or repeat readers, you may wish to jump to the list in the Getting Started node.

Introduction

One of the questions that comes up on our mailing lists a lot is:I just ran mib2c on my MIB. What do I do

now?. This tutorial, and the MIB forDummies code approach, is designed to make your life as easy as

possible. In this tutorial we'll walk you through the entire process of creating C code starting from only a MIB

definition. This should answer this question for you, assuming you're willing to use the MIB for Dummies

approach to coding.

One of the output code styles that mib2c supports is a style called "MIB for Dummies". This configuration file

is called "local/mib2c.mfd.conf (http://net-snmp.git.sourceforge.net/git/gitweb.cgi?p=net-snmp/net-

nmp;a=blob_plain;f=local/mib2c.mfd.conf;hb=HEAD) "and is what you would pass to the -c argument o

mib2c to get it to produce the style of code this tutorial is going to discuss. It will generate mib-module code

which is designed to plug into an agent when developing Net-SNMP agent extensions.
http://net-snmp.git.sourceforge.net/git/gitweb.cgi?p=net-snmp/net-snmp;a=blob_plain;f=local/mib2c.mfd.conf;hb=HEADhttp://www.net-snmp.org/wiki/index.php?title=MFD:ifXTable:Set_Data&action=edit&redlink=1http://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Advanced_Tableshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Basic_Tableshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Getting_Startedhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Data_manipulationhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Row_Request_Contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#MIB_contexthttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:Data_Accesshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#User_Contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Introductionhttp://www.net-snmp.org/wiki/index.php/MFD:ifTablehttp://net-snmp.git.sourceforge.net/git/gitweb.cgi?p=net-snmp/net-snmp;a=blob_plain;f=local/mib2c.mfd.conf;hb=HEADhttp://www.net-snmp.org/wiki/index.php/Mib2chttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Getting_Startedhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Advanced_Tableshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Intermediate_Tableshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Basic_Tableshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Getting_Startedhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Data_manipulationhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Directhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Iterator_.28unsorted-external.29http://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#container-cachedhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Data_Lookuphttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Row_Request_Contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Data_Contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#MIB_contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#User_Contexthttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Data_Structureshttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#Introductionhttp://www.net-snmp.org/wiki/index.php/MIB_for_Dummies#MIBs_For_Dummieshttp://www.net-snmp.org/wiki/index.php?title=MFD:ifXTable:Testing_Sets&action=edit&redlink=1http://www.net-snmp.org/wiki/index.php?title=MFD:ifXTable:Set_Data&action=edit&redlink=1http://www.net-snmp.org/wiki/index.php?title=MFD:ifXTable:Testing_Gets&action=edit&redlink=1http://www.net-snmp.org/wiki/index.php/MFD:ifXTable:Get_Datahttp://www.net-snmp.org/wiki/index.php/MFD:ifXTable:Data_Accesshttp://www.net-snmp.org/wiki/index.php/MFD:ifXTable:Data_Structureshttp://www.net-snmp.org/wiki/index.php/MFD:ifXTable:mib2chttp://www.net-snmp.org/wiki/index.php/MFD:ifXTablehttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:Testinghttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:Get_Datahttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:Data_Accesshttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:Data_Structureshttp://www.net-snmp.org/wiki/index.php/MFD:ifTable:mib2chttp://www.net-snmp.org/wiki/index.php/MFD:ifTablehttp://www.net-snmp.org/wiki/index.php/MFD:IF-MIB


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One of the primary motivations for writing the MIBs for Dummies configuration file was to reduce the amount of

SNMP knowledge needed to implement a MIB. When using the MFD configuration file (mib2c.mfd.conf),

mib2c will generate template code that hides much of the SNMP (or Net-SNMP) specific details for

implementing a module, and allows the rest of the template code to use simple C data structures that you are

probably more familiar with. Most of the template functions are short and simple, with a clearly defined purpose.

Another motivation was to separate the method used to locate the data for an incoming request to the SNMP

agent from the methods that manipulate the data. The data lookup for MFD modules uses the

netsnmp_container interface. Data ordering can be tricky in SNMP, since the lexicographical ordering required

by SNMP specification isn't always intuitive, and the index specified in the MIB you have to implement might

not match the order in which your data is currently stored. But the MFD configuration file can help you out with

this problem too.

The templates generated by the MFD configuration file fall into several categories:

data structures

contains the data used to answer a request

data lookup

finding the right data for a request

data manipulation

either returning existing values or setting new values.

Data Structures

There are 4 important data structures used in an MFD module. They are the user context, the mib context, the

data contextand the the row request context.

User Context

The user context is a pointer provided by the user (ie, the programmer: You!) during module initialization. It is

not used by the MFD code, other than to save it for the user. If your module needs access to some external

data, you can use this pointer instead of a global variable.

MIB context

The MIB context is a generated structure which is used to store the MIB indexes for a row.

Data Context

The data context structure should contain all the data needed to get or set a value. The mib2c MFD

configuration file will generate a data context structure with sufficient storage for all the objects defined in the

MIB being implemented. (Later in the tutorial, we'll talk about using existing structures.)

Row Request Context

The row request context ties together all the other contexts. The table container will have a row request context

for each row in the table.
http://www.net-snmp.org/wiki/index.php/Containers


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Data Lookup

There are lots of different ways to access data. Your data store may be a linked list, a text file, a database or an

API to some device. Instead of trying to deal with all of the possiblities in the template code, the

netsnmp_containerinterface is used.

There are currently two different interfaces between the MFD template code and the netsnmp_containerusedto locate data for a request.

container-cached

The default method, which is generic enough to handle just about any situation, is the container-cachedmethod.

This combines two features of net-snmp: the cache handler and Containers (netsnmp_container). Quite simply,

the first time a request is received for a table, a cache_loadroutine is called with a pointer to a

netsnmp_container. That function accesses any datastore(s) that contain data, and adds all the rows to the

container. The container is then used to find the rows for the incoming request, and (optionally) kept around a

configurable number of seconds for future requests. While the basic principle is simple, the flexiblity makes thismethod the preferred method for most situations. More details on this cache helpercan be found on the cache

handlerpage.

Iterator (unsorted-external)

The unsorted-externalmethod is a wrapper around the netsnmp_continercontainer_iterator. This method is

also very flexible, with a very simple interface. When a request is received for a table, a get_nextroutine is

called repeatedly to loop over every item in the datastore(s). The agent will then select the appropriate row for

the request. Due to the inefficency of this method, it is only recommended for small datastores, or when memorycontraints are very tight and response times are not important.

Direct

For advanced users, a custom netsnmp_containercan be used. This method lets you wrap a

netsnmp_containeraround an existing datastore/access method. Be warned, however, that the implementation

for the container'sfind-nextmethod must handle SNMP's lexicographical ordering rules.

Data manipulation

Once the appropriate data structure is retrieved from the data store, the data manipulation routines will be

called. For each object defined in your mib module, a function will be generated to extract the data value from

the data structure located during the data lookup phase.

For read-write objects, things get more complicated. SET requests are processed in multiple steps, so there are

multiple functions generated for each object. These include functions for syntax checks, value checks, undo

setup, value set and undo value set. Additionlly, several functions are generated per table, for tasks which only

need to be performed once, even if multiple objects are set for a given index. Two examples are consistency

checks and commit changes. All of these functions will be discussed in more detail later in the tutorial.
http://www.net-snmp.org/wiki/index.php/Cache_handlerhttp://www.net-snmp.org/wiki/index.php/Containershttp://www.net-snmp.org/wiki/index.php/Cache_handler


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Getting Started

There are several example implementations. The process of implementing a module using the MFD configuration

file can be broken down into several steps. You may wish to refer to this list in the future as it is a good high-

level todo list for every module you implement.

Generating the template code by running mib2c appropriately

mib2c -c mib2c.mfd.confImplementing a netsnmp_containerfor data access

edit theMYTABLE.h file, adding column storage items to theMYTABLE_rowreq_ctx_s structure

edit theMYTABLE_data_access.c file, filling out theMYTABLE_container_loadfunction to load

all the data into a container cache.

(create multiple rowreq_ctx objects and insert them into the container)

Implementing GET request functions

In theMYTABLE_data_get.c file, fill out each COLUMN_get() to copy the data from your

row_req cache object to the COLUMN_val_ptr objects.

Implementing SET request functions

Basic Tables

These examples are fairly basic, and should suffice for most simple MIB tables and data stores.

MFD:IF-MIB Tables

ifTable: A simple table with a single integer index which gets it's data from a text file. (Partial, read-

only implementation)

ifXtable: A simple table with a single integer index which gets it's data from a text file, and caches

it. (Partial, read-only implementation)

Intermediate Tables

These examples are sill fairly basic, but we are starting to add a few twists.

TBD (set support; row creation?)

Advanced Tables

These examples show how to deal with complicated issues that sometimes come up while implementing MIB

modules..

TBD ...

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