Often times, a sytems administrator will want to trend data available to them via SNMP with a utility such as MRTG or Cacti. Although creating the graphs with MRTG can sometimes be a bit tedious. Somix technologies offers various MIB templates for various devices such as Netapp MIB’s, Netscreen (Juniper) Network Devices, and a wide variety of Cisco routing and switching devices just to name a few.

Plixer MRTG Repository

Often times, an administrator wants to extend the capabilities of the standard net-snmp installation. The UCD-SNMP MIB offers a wide variety of system performance metrics that can be retireived with SNMP and graphed with a enterprise monitoring tool such as Cacti, Zenos, GroundWork, Hyperic, MRTG, or many other open-source trending / monitoring utilities. The UCD SNMP-MIB offers a wealth of information about CPU, Disk Usage and Performance, Network Metrics, and Memory Metrics natively for most Unix-based hosts.

But how does one collect and trend data through SNMP for custom metrics, such as a custom built application? In scenarios where custom data must be collected from an SNMP agent there are a number of different ways the information can be collected:

Here I present some of those options and their applications and difficulty levels:

If only a few values are needed, a simple ‘extend’ in snmpd.conf could be sufficient. The snmpd.conf man page offers the information necessary to accomplish this task, but I have explained it in terms of a practical example (those pesky man pages sometimes can be difficult to interpret and need translation).

Here is the man page excerpt for extending the snmpd agent:

What this means in a practical sense, is that you can define a process that executes on the machine that returns some value, and map it to a custom OID that is defined in snmpd.conf.

For example, if I have a custom shell script that tells me how many java processes are running on my systems, I can extend the agent to provide this information by modifying my snmpd.conf as follows:

One can use perl extensions as well.

If you want something more complex, there are several possibilities, how to extend the agent. All are described at http://www.net-snmp.org/wiki/index.php/Tutorials, and search for “Agent Coding Tutorials”

And the last (but not least) option is to implement a binary module, usually in C and using mib2c. You can choose if you want to have a sub-agent (i.e. standalone process, communicating with the snmpd agent using AgentX protocol over unix domain socket or TCP) or an extension module (i.e. shared object, dynamically linked into snmpd process). The advantage of using this method is much more robust, as using the extend method will only allow string values to be returned. This method is much more involved and requires some C programming knowledge. Using a MIB ensures that the data on the agent side and the client side match up, and are used when data integrity is much more important and to be processed in an automated fashion.

The net-snmp source code can be obtained from http://net-snmp.sourceforge.net