If you're looking at writing your own plugins for Nagios Core, please make sure to visit these other resources:
Scripts and executables must do two things (at a minimum) in order to function as Nagios plugins:
The inner workings of your plugin are unimportant to Nagios Core. Your plugin could check the status of a TCP port, run a database query, check disk free space, or do whatever else it needs to check something. The details will depend on what needs to be checked - that's up to you.
Nagios Core determines the status of a host or service by evaluating the return code from plugins. The following tables shows a list of valid return codes, along with their corresponding service or host states.
|Plugin Return Code||Service State||Host State|
|1||WARNING||UP or DOWN/UNREACHABLE*|
Note: If the use_aggressive_host_checking option is enabled, return codes of 1 will result in a host state of DOWN or UNREACHABLE. Otherwise return codes of 1 will result in a host state of UP. The process by which Nagios Core determines whether or not a host is DOWN or UNREACHABLE is discussed here.
At a minimum, plugins should return at least one line of text output. Beginning with Nagios Core 3, plugins can optionally return multiple lines of output. Plugins may also return optional performance data that can be processed by external applications. The basic format for plugin output is shown below:
TEXT OUTPUT | OPTIONAL PERFDATA
LONG TEXT LINE 1
LONG TEXT LINE 2
LONG TEXT LINE N | PERFDATA LINE 2
PERFDATA LINE 3
PERFDATA LINE N
The performance data (shown in orange) is optional. If a plugin returns performance data in its output, it must separate the performance data from the other text output using a pipe (|) symbol. Additional lines of long text output (shown in blue) are also optional.
Let's see some examples of possible plugin output.
Assume we have a plugin that returns one line of output that looks like this:
If this plugin was used to perform a service check, the entire line of output will be stored in the $SERVICEOUTPUT$ macro.
A plugin can return optional performance data for use by external applications. To do this, the performance data must be separated from the text output with a pipe (|) symbol like such:
A plugin optionally return multiple lines of both text output and perfdata, like such:
If this plugin was used to perform a service check, the red portion of first line of output (left of the pipe separator) will be stored in the $SERVICEOUTPUT$ macro. The orange portions of the first and subsequent lines are concatenated (with spaces) are stored in the $SERVICEPERFDATA$ macro. The blue portions of the 2nd - 5th lines of output will be concatenated (with escaped newlines) and stored in $LONGSERVICEOUTPUT$ the macro.
The final contents of each macro are listed below:
|$SERVICEOUTPUT$||DISK OK - free space: / 3326 MB (56%);|
|$SERVICEPERFDATA$||/=2643MB;5948;5958;0;5968 /boot=68MB;88;93;0;98 /home=69357MB;253404;253409;0;253414 /var/log=818MB;970;975;0;980|
|$LONGSERVICEOUTPUT$||/ 15272 MB (77%);\n/boot 68 MB (69%);\n/var/log 819 MB (84%);|
With regards to multiple lines of output, you have the following options for returning performance data:
Nagios Core will only read the first 4 KB of data that a plugin returns. This is done in order to prevent runaway plugins from dumping megs or gigs of data back to Nagios Core. This 4 KB output limit is fairly easy to change if you need. Simply edit the value of the MAX_PLUGIN_OUTPUT_LENGTH definition in the include/nagios.h.in file of the source code distribution and recompile Nagios Core. There's nothing else you need to change!
If you're looking for some example plugins to study, I would recommend that you download the official Nagios plugins and look through the code for various C, Perl, and shell script plugins. Information on obtaining the official Nagios plugins can be found here.