TUT:Writing a Dynamically Loadable Object

2007-04-05T13:22:08Z

Aw:

First off, you need to write a mib module first before you can do this part of the tutorial. We assume you have read and completed the [[TUT:Writing_a_MIB_Module | mib module portion]] of the toolkit tutorial. This part of the tutorial shows how to install a dynamic module into the agent, assuming you already have written a mib-module which we'll then compile into a dynamic module. For example purposes, we'll refer to some example MIB objects and code: the [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mib_module/NET-SNMP-TUTORIAL-MIB.txt NET-SNMP-TUTORIAL-MIB] MIB, and the [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/demon/nstAgentSubagentObject.c example mib module] and it's [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/demon/nstAgentSubagentObject.h header file].

Note: For this to work, you must have compiled the net-snmp package with dynamicly loadable module support turned on, as well as built it with --enable-shared turned on. (It's on by default, if your system supports it). You can check for support in your agent but looking at the output of the "snmpd -H" command for the "dlmod" token. If its listed, the compiled agent supports it.

Note: All command line options below assume you have an appropriately setup ~/.snmp/snmp.conf file that allows you to not have to specify a snmp version number, community name, username, or whatever else in order to talk to your agent. The agent, of course, must have a matching /usr/local/share/snmp/snmpd.conf file (or equivalent).

Here are the files discussed in this example so you can download them:

{{TUT:File-Table-Top-and-basics}}
|-
| [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/dlmod/nstAgentPluginObject.h nstAgentPluginObject.h] || The mib module's header file
|-
| [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/dlmod/nstAgentPluginObject.c nstAgentPluginObject.c] || The mib module's C source code
|}

The shared object loader calls the "init_nstAgentPluginObject()" function in the above code when it gets loaded, and calls the "deinit_nstAgentPluginObject()" when (and if) the module is unloaded. These function names to use are built by adding the shared object's module name (UCD-DLMOD-MIB::dlmodName) to the `init_' and `deinit_' prefixes.

=== Steps to build the shared object ===

# First off we must get the [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/demoapp/Makefile Makefile] file, the [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/dlmod/nstAgentPluginObject.h nstAgentPluginObject.h] file, and the [http://www.net-snmp.org/tutorial/tutorial-5/toolkit/dlmod/nstAgentPluginObject.c nstAgentPluginObject.c] file.
# '''make nstAgentPluginObject.so'''

=== Steps to test the shared object via runtime MIB configuration ===

# Start the snmpd and watch the dlmod and nstAgentPluginObject modules interact using the debugging flag (this assumes you already have access control set up properly for your agent):
#:% '''snmpd -f -L -DnstAgentPluginObject,dlmod'''
# In another window, test to make sure that the agent doesn't currently support the nstAgentPluginObject (if you get different results running this command you need to recompile the net-snmp agent without the nstAgentPluginObject mib module compiled in directly):
#:% '''snmpget localhost NET-SNMP-TUTORIAL-MIB::nstAgentPluginObject.0'''
#:nstAgentPluginObject.0 = No Such Object available on this agent at this OID
# Then, run snmpset to create a new row in the dlmod table:
#:% '''snmpset localhost UCD-DLMOD-MIB::dlmodStatus.1 i create'''
#:dlmodStatus.1 = create(6)
# See that the row was created:
#:% '''snmptable localhost UCD-DLMOD-MIB::dlmodTable '''
#:SNMP table: dlmodTable
#:
#:dlmodName dlmodPath dlmodError dlmodStatus
#: unloaded
#Then set the properties of the row up to point to our new object and to give it a name:
#:% '''snmpset localhost UCD-DLMOD-MIB::dlmodName.1 s "nstAgentPluginObject" UCD-DLMOD-MIB::dlmodPath.1 s "/path/to/nstAgentPluginObject.so" '''
#:dlmodName.1 = "nstAgentPluginObject"
#:dlmodName.1 = "/path/to/nstAgentPluginObject.so"
#:
#:% '''snmptable localhost UCD-DLMOD-MIB::dlmodTable '''
#: SNMP table: dlmodTable
#:
#: dlmodName dlmodPath dlmodError dlmodStatus
#: nstAgentPluginObject /path/to/nstAgentPluginObject.so unloaded
#Finally, load the shared object into the running agent:
#:% '''snmpset localhost UCD-DLMOD-MIB::dlmodStatus.1 i load '''
#:dlmodStatus.1 = loaded(1)
#:
#:% snmptable localhost UCD-DLMOD-MIB::dlmodTable
#:SNMP table: dlmodTable
#:
#: dlmodName dlmodPath dlmodError dlmodStatus
#: nstAgentPluginObject /path/to/nstAgentPluginObject.so loaded
#If everything above was done correctly, then the following command should work and will access the shared object's data:
#:% '''snmpget localhost NET-SNMP-TUTORIAL-MIB::nstAgentPluginObject.0 '''
#: nstAgentPluginObject.0 = INTEGER: 3

=== Loading via the snmpd.conf file ===
You can also load shared objects using the "dlmod" token in the snmpd.conf file by putting a line like this in your snmpd.conf file:
dlmod nstAgentPluginObject /path/to/nstAgentPluginObject.so

The first argument specifies the shared object's module name (UCD-DLMOD-MIB::dlmodName) and second argument specifies the full pathname of the shared object file.

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TUT:Writing a Dynamically Loadable Object