Tuesday, December 8, 2009

Defining your own MIBs

Net-snmp has a flaw, at least on Linux. If you ask for anything from the IF-MIB at the rate of once a second, it chews up around 35% of the CPU processing power. This is not a good idea if what we want for the DDoS testbed is a discreet monitoring of each host. The cause is, according to this guy:

netsnmp_arch_interface_container_load() function from "if-mib/data_access/interface_linux.c" has a file pointer to "proc/net/dev" from it tries to get updates of each interface in a while loop, which takes too much of cpu time when we have many VLAN interfaces defined in our server this loads CPU 100%

So the way around it seems to be to write your own MIB. But how easy is that, exactly?

MIB files are defined using the arcane SMI syntax. But they can also be converted into C code, and then compiled. But you still need the textual MIB files to translate the even more arcance OIDs or dot-notation, into more meaningful textual identifiers. For example, in the Net-SNMP tutorial NET-SNMP-TUTORIAL-MIB::nstAgentModuleObject.0 represents the OID . Another advantage of using the text-form of an OID is that snmpset then works with a simple "=" sign. Otherwise you have to tell snmpset the data type of the thing you are setting.

So far I have managed to compile the example SNMP mib C file at http://www.net-snmp.org/tutorial/tutorial-5/toolkit/mib_module/nstAgentModuleObject.c. What they suggest is to include this as part of your snmpd demon, so when you launch it, the demon will automatically recognise the numeric OID. If you want it also to recognise the text-form you have to add the text MIB to .snmp/mibs. Also I had to de-install net-snmp before I could get it to work, and also launch it with supervisor privileges. Simply putting the MIB files into .snmp/mibs is sufficient - the snmpd agent does not have to be restarted.

The next step is to rewrite the sample MIB to do something useful for the testbed. Also, to allow it to be added dynamically instead of requiring recompilation of the agent.

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