Difference between revisions of "TUT:mib2c scalar"

Latest revision as of 14:07, 18 April 2012

The mib2c scalar configuration file generates a bare-bones skeleton handler for each scalar object. You must then write code to provide data for the object, handling each mode. For a complete example, see the IP-MIB scalars implementation in the source tree. Here is an excerpt:

int ipAddressSpinLockValue;

int
handle_ipAddressSpinLock(netsnmp_mib_handler *handler,
                         netsnmp_handler_registration *reginfo,
                         netsnmp_agent_request_info *reqinfo,
                         netsnmp_request_info *requests);

/** Initializes the ip module */
void init_ip_scalars(void)
{
    static oid      ipAddressSpinLock_oid[] = { 1, 3, 6, 1, 2, 1, 4, 33 };

    DEBUGMSGTL(("ip_scalar", "Initializing\n"));

    netsnmp_register_scalar(netsnmp_create_handler_registration
                            ("ipAddressSpinLock", handle_ipAddressSpinLock,
                             ipAddressSpinLock_oid,
                             OID_LENGTH(ipAddressSpinLock_oid),
                             HANDLER_CAN_RWRITE));

    /* Initialize spin lock with random value */
    ipAddressSpinLockValue = (int) random();

}

int
handle_ipAddressSpinLock(netsnmp_mib_handler *handler,
                          netsnmp_handler_registration *reginfo,
                          netsnmp_agent_request_info   *reqinfo,
                          netsnmp_request_info         *requests)
{
    long   value;

    /* We are never called for a GETNEXT if it's registered as a
       "instance", as it's "magically" handled for us.  */

    /* a instance handler also only hands us one request at a time, so
       we don't need to loop over a list of requests; we'll only get one. */

    switch(reqinfo->mode) {

        case MODE_GET:
            snmp_set_var_typed_value(requests->requestvb, ASN_INTEGER,
                                     (u_char *)&ipAddressSpinLockValue,
                                     sizeof(ipAddressSpinLockValue));
            break;

#ifndef NETSNMP_NO_WRITE_SUPPORT
        /*
         * SET REQUEST
         *
         * multiple states in the transaction.  See:
         * http://www.net-snmp.org/tutorial-5/toolkit/mib_module/set-actions.jpg
         */
        case MODE_SET_RESERVE1:
        case MODE_SET_RESERVE2:
            /* just check the value */
            value =  *(requests->requestvb->val.integer);
            if (value != ipAddressSpinLockValue)
                netsnmp_set_request_error(reqinfo, requests, SNMP_ERR_INCONSISTENTVALUE);
            break;

        case MODE_SET_FREE:
            break;

        case MODE_SET_ACTION:
            /* perform the final spinlock check and increase its value */
            value =  *(requests->requestvb->val.integer);
            if (value != ipAddressSpinLockValue) {
                netsnmp_set_request_error(reqinfo, requests, SNMP_ERR_INCONSISTENTVALUE);
            } else {
                ipAddressSpinLockValue++;
                /* and check it for overflow */
                if (ipAddressSpinLockValue > 2147483647 || ipAddressSpinLockValue < 0)
                    ipAddressSpinLockValue = 0;
            }
            break;

        case MODE_SET_COMMIT:
            break;

        case MODE_SET_UNDO:
             break;
#endif /* !NETSNMP_NO_WRITE_SUPPORT */

        default:
            /* we should never get here, so this is a really bad error */
            snmp_log(LOG_ERR, "unknown mode (%d) in handle_ipAddressSpinLock\n", reqinfo->mode );
            return SNMP_ERR_GENERR;
    }

    return SNMP_ERR_NOERROR;
}

Tutorial Sections

About the SNMP Protocol

These tutorial links talk about SNMP generically and how the protocol itself works. They are good introductory reading 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 the example commands in these tutorials works if you try it yourself, as they're all examples that talk to our online Net-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.
• 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 snmpd
• Common 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 management requests and a notification receiver (snmptrapd) for receiving SNMP notifications.

• SNMP Agent (snmpd) Configuration
  • Configuration Basics
  • Access Control (VACM)
  • snmpconf
• SNMP Notification Receiver (snmptrapd)
  • Configuring snmptrapd
  • Configuring 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 own commands, add extensions to the agent to support your own MIBs and perform specialized processing of notifications.

• 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, and describes how the Agent Helpers work under the hood.
  • 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 you
    • General mib2c Overview
    • Using the mib2c-update script to recode your code
    • mib2c.mfd.conf tutorial
• Header files and autoconf

Debugging SNMP Applications and Agents

All our tools and applications have extensive debugging output. These tutorials talk about how the debugging system 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 code
• Using -Ddump to display packet breakdowns
• Debugging using GDB

Operating System Specific Tutorials

• Building With Visual Studio 2005 Express
  • Using the command line and nmake
• Building Net-SNMP 64-bit with Visual C++ 2010 Express
  • Using the command line and nmake (VC++ 2010 Express)
• Net-Snmp on Ubuntu
• Net-SNMP and lm-sensors on Ubuntu 10.04
• Net-SNMP for windows:
  • How to install: [1]
  • How to configure: [2]