15/08/13MIB for Dummies - Net-SNMP Wiki MIB for Dummies From Net-SNMP Wiki MFD Tutorial Contents 1 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 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:mib2c MFD:ifXTable:Data Structures MFD:ifXTable:Data Access MFD:ifXTable:Get Data MFD:ifXTable:Testing Gets MFD:ifXTable:Set Data MFD:ifXTable:Testing Sets 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 for Dummies 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/netsnmp;a=blob_plain;f=local/mib2c.mfd.conf;hb=HEAD) " and is what you would pass to the -c argument of 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. www.net-snmp.org/wiki/index.php/MIB_for_Dummies 1/4 The table container will have a row request context for each row in the table. and allows the rest of the template code to use simple C data structures that you are probably more familiar with. MIB context The MIB context is a generated structure which is used to store the MIB indexes for a row. They are the user context . you can use this pointer instead of a global variable. (Later in the tutorial. we'll talk about using existing structures. other than to save it for the user.) Row Request Context The row request context ties together all the other contexts. The mib2c MFD configuration file will generate a data context structure with sufficient storage for all the objects defined in the MIB being implemented. Data Structures There are 4 important data structures used in an MFD module.php/MIB_for_Dummies 2/4 . User Context The user context is a pointer provided by the user (ie. Data Context The data context structure should contain all the data needed to get or set a value. with a clearly defined purpose. since the lexicographical ordering required by SNMP specification isn't always intuitive.Net-SNMP Wiki 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.15/08/13 MIB for Dummies . and the index specified in the MIB you have to implement might not match the order in which your data is currently stored. the data context and the the row request context .net-snmp.org/wiki/index. The data lookup for MFD modules uses the netsnmp_container interface. 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. If your module needs access to some external data. the programmer: You!) during module initialization. the mib context . It is not used by the MFD code. Data ordering can be tricky in SNMP. www.conf). Most of the template functions are short and simple. 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. But the MFD configuration file can help you out with this problem too. mib2c will generate template code that hides much of the SNMP (or Net-SNMP) specific details for implementing a module. When using the MFD configuration file (mib2c.mfd. is the container-cached method. When a request is received for a table. it is only recommended for small datastores. a get_next routine is called repeatedly to loop over every item in the datastore(s). www. Your data store may be a linked list. a function will be generated to extract the data value from the data structure located during the data lookup phase. a database or an API to some device. While the basic principle is simple. however. All of these functions will be discussed in more detail later in the tutorial. Due to the inefficency of this method. the netsnmp_container interface is used. There are currently two different interfaces between the MFD template code and the netsnmp_container used to locate data for a request.15/08/13 MIB for Dummies . and (optionally) kept around a configurable number of seconds for future requests. several functions are generated per table. for tasks which only need to be performed once. Iterator (unsorted-external) The unsorted-external method is a wrapper around the netsnmp_continer container_iterator. For each object defined in your mib module.net-snmp. This combines two features of net-snmp: the cache handler and Containers (netsnmp_container). so there are multiple functions generated for each object. the flexiblity makes this method the preferred method for most situations. or when memory contraints are very tight and response times are not important. even if multiple objects are set for a given index. the data manipulation routines will be called. Instead of trying to deal with all of the possiblities in the template code. the first time a request is received for a table. SET requests are processed in multiple steps. These include functions for syntax checks. undo setup. a text file. Data manipulation Once the appropriate data structure is retrieved from the data store. That function accesses any datastore(s) that contain data. This method lets you wrap a netsnmp_container around an existing datastore/access method.Net-SNMP Wiki Data Lookup There are lots of different ways to access data. which is generic enough to handle just about any situation. More details on this cache helper can be found on the cache handler page. Quite simply.php/MIB_for_Dummies 3/4 . with a very simple interface. a custom netsnmp_container can be used. Two examples are consistency checks and commit changes. The agent will then select the appropriate row for the request. a cache_load routine is called with a pointer to a netsnmp_container. Be warned. value set and undo value set. The container is then used to find the rows for the incoming request. For read-write objects.org/wiki/index. and adds all the rows to the container. value checks. This method is also very flexible. that the implementation for the container's find-next method must handle SNMP's lexicographical ordering rules. Direct For advanced users. Additionlly. things get more complicated. container-cached The default method. net-snmp. TBD . Retrieved from "http://www. Implementing SET request functions Basic Tables These examples are fairly basic. The process of implementing a module using the MFD configuration file can be broken down into several steps. row creation?) Advanced Tables These examples show how to deal with complicated issues that sometimes come up while implementing MIB modules. MFD:IF-MIB Tables ifTable: A simple table with a single integer index which gets it's data from a text file. (create multiple rowreq_ctx objects and insert them into the container) Implementing GET request functions In the MYTABLE_data_get.c file.h file. Generating the template code by running mib2c appropriately mib2c -c mib2c.net-snmp. and should suffice for most simple MIB tables and data stores. readonly implementation) ifXtable: A simple table with a single integer index which gets it's data from a text file. fill out each COLUMN_get() to copy the data from your row_req cache object to the COLUMN_val_ptr objects. (Partial.c file.org/wiki/index. adding column storage items to the MYTABLE_rowreq_ctx_s structure edit the MYTABLE_data_access.org/wiki/index. www. read-only implementation) Intermediate Tables These examples are sill fairly basic. You may wish to refer to this list in the future as it is a good highlevel todo list for every module you implement. TBD (set support.mfd.conf Implementing a netsnmp_container for data access edit the MYTABLE. but we are starting to add a few twists..php?title=MIB_for_Dummies&oldid=5279" This page was last modified on 28 July 2011.php/MIB_for_Dummies 4/4 ..15/08/13 MIB for Dummies .Net-SNMP Wiki Getting Started There are several example implementations. at 18:17. filling out the MYTABLE_container_load function to load all the data into a container cache.. and caches it. (Partial.