Getting ready

You should have a Nagios Core 3.0 or newer server running with a few hosts and services configured already. You should also already be familiar with the relationship between services, commands, and plugins.

How to do it...

We can customize an existing command definition as follows:

If the validation passed and the server restarted successfully, we should now be able to use the check_http_altport command, which is based on the original check_http command, in a service definition.

How it works...

The configuration we added to the commands.cfg file reproduces the command definition for check_http, but changes it in two ways:

The check_http_alt command can now be used as a check command in the same way as a check_http command. For example, a service definition that checks whether the sparta.naginet host is running an HTTP daemon on port 8080 might look similar to the following code snippet:

define service {
    use                  generic-service
    host_name            sparta.naginet
 service_description HTTP_8080
 check_command check_http_alt
}

There's more...

This recipe's title implies that we should customize the existing commands by editing them in place, and indeed, this works fine if we really do want to do things this way. Instead of copying the command definition, we could just add the -p 8080 or another customization to the command line and change the original command.

However, this is bad practice in most cases, mostly because it could break the existing monitoring and be potentially confusing to other administrators of the Nagios Core server. If we have a special case for monitoring—in this case, checking a non-standard port for HTTP—then it's wise to create a whole new command based on the existing one with the customizations we need.

There is no limit to the number of commands you can define, so you can be very liberal in defining as many alternative commands as you need. It's a good idea to give them instructive names that say something about what they do, as well as to add explanatory comments to the configuration file. You can add a comment to the file by prefixing it with a # character:

#
# 'check_http_altport' command_definition. This is to keep track of
# servers that have panels running on alternative ports.
#
define command {
 command_name check_http_altport
 command_line $USER1$/check_http -H $HOSTADDRESS$ -p 8080 $ARG1$
}

See also