Where to begin with SNMP agent implementation?

Question

before I start I realise there are a few SNMP related questions here already but not many seem to have been answered - that could mean I'm asking in the wrong place but I don't know where else to go at the moment.

I've been reading up as best I can on SNMP for a couple of days but am finding it difficult to get my head around what is meant to be happening. The idea is eventually we will integrate SNMP into our Java application server which will allow the end users to incorporate it into their pre-existing Network Management Systems(NMS).

Unfortunately I'm feeling entirely confused by what is meant to be going on. From what I understood from talking to the end users (which was unfortunately before any research) was that the monitoring allows their existing NMS to give their admin guys a view of the vital statistics in a tree type display, giving them feedback regarding different parts of the system at a high level and allowing them to dig down into specific subsystems.

From reading around we would implement an 'Agent' which has several defined interfaces allowing for GET requests etc to be processed and responded to. That makes sense but I am at a loss to work out what the format of the communication is - there don't seem to be any specific examples of what any of the messages look like, how the information is encoded.

More of my confusion though is regarding Management Information Base(MIB). I had, wrongly, assumed that the interface of the agent would allow for the monitored attributes to be requested and then in turn the values for those attributes requested. Allowing any new Agent to be started and detected without any configuration on the NMS end (with the exception of authentication in v3). This, if I understand correctly, is not the case and the Agent must instead define MIBs which can be used by the NMS to determine those attributes. My confusion is increased when people start referring to thousands of existing MIBs and that they can be reused which I don't understand. Is the intention that a single MIB definition can be used to say describe how a particular attribute of a network device (something simple like internet connected on a router:yes/no) for many different devices? If so I don't believe that our software would allow the monitoring of anything common to any other device/system but should we be looking for already exising MIBs? At the moment I don't really see any good rational for such a system, surely it would be easier for the Agent to export that information - so I'd appreciate it if someone could enlighten me!

I think it would help if I was able to setup a simple SNMP agent and some sort of client, I could begin to see the process and eventually inspect the communication between the two but am finding it difficult to find anywhere that provides any information on doing such a thing. Nagios has been recommended to us as a test 'client'/NMS but their 'get started quick' section recommends downloading a 600Mb virtual machine - surely there is a quicker way to get started?

Any help or suggestions will be appreciated, I have been through the Wiki page but it doesn't seem to go into much detail about the MIBs and the having not had to deal with anything like the referenced RFCs before, while they may contain all of the information they seem completely impenetrable to me at the moment. Or if there are any books that can be recommended for an overview and implementation of v3?

Thanks for reading and even more thanks if you think you can help!

Answer 1

It seems to me that you read all SNMP information piece by piece in an disorganized way. This is highly not recommended and of course lead you to confusion.

What about forgetting what you have learnt so far and dive into a good book such as Essential SNMP?

http://shop.oreilly.com/product/9780596008406.do

Click the Google Preview icon to preview it please.

You could not depend on a network forum to tell you the ABCs, as that's impractical I find out.

Answer 2

The communications interface is SNMP. That's the protocol used for transmission (usually on top of UDP). The thing that services information requests is an SNMP Agent. The thing that sends information requests is an SNMP Manager.

The definition of what information should be made available by the Agent, and requested by the Manager, goes in a MIB. A MIB is the "glue", a directory of what sort of things any particular system can/should offer. It maps numeric codes to names and types that allow us to make sense of the data, much like how a phone directory maps phone numbers to people's names and addresses.

Generally you would create and ship and use your own MIBs that can describe aspects specific to your own product, but you are supposed to service some standard information requests as well, which are defined in existing MIBs. Yes there are thousands of other pre-existing MIBs and the likelihood that you need more than one or two of these is remote. They are typically published versions of MIBs for existing products.

The conventional way to "toy around" is to install Net-SNMP (a software suite that includes an agent implementation and allows you to "bolt on" your own logic and your own MIBs fairly easily) then examine the results using a packet capturer like Wireshark.

For a fuller implementation in production you may stick with Net-SNMP, or write your own Agent software, or do what I did and create a hybrid of the two that's a little more flexible and performant but uses Net-SNMP's backend for handling all the low-level SNMP stuff.

Your first step, though, is to read a book or some other teaching material that can clear all your misconceptions, because guesswork won't cut it.

Answer 3

I had success using the samples from this page. Both the shell and Perl NetSNMP code was very straightforward to implement and query.