Using free variables in nagios/icinga for snmp community settings

With icinga you can monitor your it environment. It's a fork of the well known nagios monitoring solution.

It's a dropin replacement of nagios with some good extensions/enhancements.

One of the pains to monitor services via SNMP is, that you usually have different SNMP communities per server. The simplest way is to define one service per service+host and there define the snmp community.

But after a your IT environment expands you will get many many service definition, all identical with the exception of the snmp community.

There is a simple way out of this: Use custom object variables
The original documentation can be found here.

The documentation is somewhat short, so here a complete example:

• On the host configuration, define a variable named _SNMP_COMMUNITY and assign it the value for the specified host.
• In the check command, specify the $_HOSTSNMP_COMMUNITY$ as the parameter for your command.

The trick is, that your variable names must always start with a underline character. (So it does not clash with other definitions).
The second trick is used when referring to the defined variable: Icinga/Nagios do prefix the variable name you defined with _HOST / _SERVICE or _CONTACT, depending on the place where the variable was specified.
The last trick is, that the leading _ of your variable definition is removed before the final name is built in the evaluation.

Example of such a command definition

define command {
        command_name check_snmp_disk_group
        command_line $USER1$/check_snmp_storage.pl -H $HOSTADDRESS$ -C $_HOSTSNMP_COMMUNITY$  -w $ARG1$ -c $ARG2$ -m $ARG3$ -r -T pl -f
        register                        1
}

The corresponding host definition has then to be similar to this:

define host {
        host_name                       servername
        alias                           serveralias
        address                         ipaddress
        _SNMP_COMMUNITY                 mysnmppassword
        register                        1
}

A great tool to manage your icinga/nagios configuration is NagiosQL.