Hello, Nagios\n

Nagios is a generic monitoring tool, and I want to use it to monitor my own application. It has a lot of built-in and offical plugins. However, it is always good to try a Hello, World-type example.

Firstly, my very interesting application. It follows the Nagios Plugin guidelines: return a value in {0, 1, 2, 3} and a status message of less than 4k to standard out. And it gives you some random sysadmin-action:

(np.c, compiles into an executable np)

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(int argc, char* argv[]) {
 srand(time(NULL));

 int r = rand() % 10;

 int status = -1;
 char* message = NULL;

 switch(r){
  case 0:
  case 1:
  case 2:
  case 3:
   status = 0;
   message = "Everything is alright :-)";
   break;
  case 4:
  case 5:
  case 6:
   status = 1;
   message = "You have been warned.";
   break;
  case 7:
  case 8:
   status = 2;
   message = "She is gonna blow!";
   break;
  case 9:
   status = 3;
   message = "Oh shit...";
   break;
 }

 printf("%s\n", message);

 return status;
}

Nagios is easy to install with this quickstart guide. You just need to configure it:

1) In $NAGIOS_HOME/etc/nagios.cfg, add a line cfg_file=$NAGIOS_HOME/etc/objects/np.cfg

2) In $NAGIOS_HOME/etc/nagios.cfg, set interval_length to a suitably low number, e.g. 5 seconds, so you can see changes quickly

3) In $NAGIOS_HOME/etc/objects/np.cfg, make it look something like this:

define hostgroup {
  hostgroup_name        app_hosts
  alias                 Application Hosts
}

define host {
  use                   generic-host
  host_name             app_host
  alias                 Application Host
  address               localhost
  hostgroups            app_hosts
  max_check_attempts    10
}

define service {
  use                   local-service
  host_name             app_host
  service_description   My Application
  check_command         monitor_np
  normal_check_interval 1
  retry_check_interval  1
}

define command {
  command_name          monitor_np
  command_line          $APPLICATION_HOME/bin/np
}

The configuration is quite fidgety. Lots of required attributes, no sensible defaults. Luckily you can reuse example configuration that comes with Nagios. And there is a handy pre-flight-check feature, you just run /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg and it will give you errors and warnings against your configuration files, with line numbers.

Anyway, by now Bob is your uncle and you can restart nagios to pick up changes. Then point your browser to http://localhost/nagios and watch the action!