Sysadmins Nightly Mental Pain (SNMP)

And quickly discovered that when you try to reproduce the example from "perldoc NetSNMP::agent" you get a crash.



This, dear diary, looks quite familiar. It seems that Suse in their desire to make Unix a more secure operating system have been linking libnetsnmpagent.so.5 against libwrap, but forgot to somehow make libnetsnmpagent to actually load libwrap. Annoying, but once you have identified the symbol and its source it is easily fixed. The first patch, and I haven't even started.



Now SNMP tools are enterprisey. They won't even give you the current uptime without a MIB. So for "SHOW GLOBAL STATUS" we need a MIB file as well. That is why I like perl so much. It gets things done. In this case, it is making me a MIB file out of a MySQL connection.



No complaints! Everything is going well! Let's check!



Wow! I am so cool, I could be Domas! Now let us go root and write something to the specification called Agent X! SuSE, still inclined on world domination, keeps close tabs on it's agents. If it detects one on your system, it automatically creates a headquarter and becomes the agentx master. We can check:



So the remaining task is just to write a SNMP agent to the MIBs specification. Let's import all the toys and play.



Now we do the MySQL dance and ask nicely for some status information. But not too often - we do not want to be too intrusive on the server.



All that is pretty standard fare and contains no surprises for the seasoned MySQL+Perl developer.

Now we need to write the agent. The agent will register itself for a subtree of Object Identifiers - we have already seen our tree in the snmptranslate above - so we will register for the mysqlMIB tree and snmpd will be kind enought to ask agentX for all the secret information about our mysql.



Now, isn't that simple?

We create one of these nicely prepackaged NetSNMP::agent objects and register the callback function mysql_snmp_handler for our mysqlMIB OID. Then we go into a loop and whenever the agent feels like it it will cycle through the loop and fetch new status data or process a request. Should a signal arrive during that time, we will catch it in our signal handler and $running will become false. That will shut down the agent cleanly.

Now the dirty business.
The handler:

We get called.
We get a bunch of parameters.
We detect the currently en vogue OID.
We check what we should do to it.
We do.
We are finished.

Easy?



Now SNMP is a pretty crappy protocol. It has one operation to get a value for a OID, called GET. When we get a GET, we deliver the value. It is that easy.

But there also is GETNEXT. When we get a GETNEXT, we find the next value in sequence and deliver the OID of that value and the actual value associated with it. This is used to walk the SNMP OID tree without knowledge of the actual tree as if you had no copy of the MIB - the primary reason for the strange value/next construction in our @global_status array.

To debug, we start the snmpd, kill the perl, and restart the agent manually with $debugging = 1. Then we ask nicely for data.



The debug output is the result of running the following command elsewhere:



We can also get all status variables step by step using the dreaded GETNEXT:



Now, dear diary, the way I am telling you this everything sounds nice and clear. But that is only because I left out so many things I have seen on my way to get here, things that have no names and others whose name shall not be mentioned.

The one thing to remember is this: If somebody is telling you about SNMP tables and the NetSNMP table API - don't go there. It is The King in Yellow, like gazing into the Abyss. Especially if you try to pair it with perl.

Now let me close this entry, dear diary, and try to clean my mind of the things that I have seen and which keep me awake at night. I can hear this nights rain drumming at the window from the outside, and other, darker darker things move in it... A peated whisky might be the right thing tonight to let me sleep and not have dreams. Maybe this entry in my diary will keep others straight on the path and away from Those Things That No Man Should Ever Know.