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Configuration Overloading

The feature to specify multiple config files for check_multi can be used to provide flexible configuration. Especially in large environments this could save lots of efforts to generate a configuration.

Parser secrets

To understand this configuration overloading we have to take a look into the implementation of the configuration file parser. Whenever this parser reads a 'command' line like the following

# foo.cmd
command [ foo ] = /bin/echo is foo

it looks into its repository and searches for any command with name 'foo'. If there isn't any it stores the command into the repository. But if there is already a command called 'foo' the parser overloads the former command with the current command.

We now will see how we can use this behaviour to simplify our configuration.

Overloading

If we have a second file called bar.cmd which contains the following definition

# bar.cmd
command [ foo ] = /bin/echo is bar

and we load this file just after the previous command file foo.cmd then the parser finds the already stored foo command (/bin/echo is foo) and overwrites it with the command /bin/echo is bar.

Small debug session

nagios ~> libexec/check_multi -f foo.cmd -r 0
 OK - 1 plugins checked, 0 critical, 0 warning, 0 unknown, 1 ok
[ 1] foo is foo
nagios ~> libexec/check_multi -f bar.cmd -r 0
 OK - 1 plugins checked, 0 critical, 0 warning, 0 unknown, 1 ok
[ 1] foo is bar
nagios ~> libexec/check_multi -f foo.cmd -f bar.cmd -r 0
 OK - 1 plugins checked, 0 critical, 0 warning, 0 unknown, 1 ok
[ 1] foo is bar
  1. check_multi reads foo.cmd and executes echo foo
  2. bar.cmd is executed and shows bar
  3. foo.cmd contents are overloaded by bar.cmd: check_multi shows bar

Overloading benefits

Now all Linux specific checks can go into Linux.cmd and all Solaris specific checks into SunOS.cmd. No problem to split also release dependent configuration. And if some special cases for particular hosts have to be configured: do it in hostxyz.cmd.

Example: Syslog should be checked if at least once a day there is some movement in it. A Linux.cmd for example would contain the following line:

command [ syslog ] = check_file_age -c 86400 /var/log/messages

and Solaris.cmd would of course search the syslog in the directory /var/adm.

command [ syslog ] = check_file_age -c 86400 /var/adm/messages

And last not least our hostxyz.cmd should not monitor any syslog:

command [ syslog ] = /bin/echo "Check disabled, Matthias Flacke, 19.11.2007"

See below the appropriate lines in the Nagios configuration. The most important line in this context is

check_command check_multi!-f $_HOSTOS$.cmd!-f $HOSTNAME.cmd

where the OS specific part is evaluated first and maybe it gets overloaded by the host specific part.

# overload.cfg
define host{
   host_name linuxhost
   _OS       Linux
}
define host{
   host_name solarishost
   _OS       SunOS
}
define host
   host_name specialhost
   _OS       SunOS
}
define command {
   command_name check_multi
   command_line $USER1$/check_multi $ARG1$ $ARG2$ $ARG3$ $ARG4$
}
define service
   name          system_checks
   check_command check_multi!-f $_HOSTOS$.cmd!-f $HOSTNAME$.cmd
}
projects/check_multi/configuration/overload.txt · Last modified: 2009/10/03 22:38 by flackem
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