TUT:mib2c scalar
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;
}
Contents
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:
- Writing mib2c config files
Application Configuration
All of our applications support configuration to allow you to customize how they behave.
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
- SNMP Notification Receiver (snmptrapd)
- Agent Monitoring
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
- 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
- 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
- Using -Ddump to display packet breakdowns
- Debugging using GDB
Operating System Specific Tutorials
- Building With Visual Studio 2005 Express
- Building Net-SNMP 64-bit with Visual C++ 2010 Express
- Net-Snmp on Ubuntu
- Net-SNMP and lm-sensors on Ubuntu 10.04
- Net-SNMP for windows:
