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The biggest reason for Nagios success is the ability to extend it with custom scripts which makes it one of the most powerful monitoring systems. Now Nagios is not the only place where you can extend your monitoring! NSClient++ provides many ways to extend it with scripts and since I have gotten many questions about how to use scripts with NSClient++ lately I have decided to write this tutorial to help sort out the concepts.

My script is better than yours!

First off there are two different types of script:

  1. Internal scripts
  2. External scripts

Internal scripts are running inside NSClient++ and have full access to the inner workings of NSClient++ they are very powerful but only works with NSClient++.

External scripts are much like how scripts work on Unix with Nagios. They are programs which are executed by the operating systems and the results are handed back to the caller.

For using internal scripts see previous blog posts as this will not be covered here. This post is about using external scripts.

External scripts

So what is an external script and where can you find one?

Answering the latter is easy there are two main sites where I tend to look for scripts and they are:

As for the first part it is more difficult. An external scripts is a program so technically it does not have to be a script (you can for instance use NSClient++ it self as an external “script” if you want). But usually it is (on Windows) vb-scripts or batch files and the more modern powershell scripting language. But you can just as easily create scripts in Perl, Python, Lua or whatever language you feel comfortable with just make sure the scripting environment is installed and working. Best way to do this is to run the script from a windows command line:

Here you can see the command to test script in red and the resulting message in blue.

To see the return status I use echo %ERRORLEVEL% which displays 0 (marked in green).

Now you might have noticed that when I run the batch script I add cmd /c “…” this is due to a windows oddity with closing the calling window when a batch script exits you can discard it but if your batch file exits with a status code it will terminate your calling shell if you do.

The last question to cover here is where to place your scripts?

I tend to put them under the scripts folder of NSClient++ which makes things simple. This is not strictly a requirement but for me it makes things simpler. If you are bundling scripts and distributing them to many node it might be a good idea to place them in a “local” subfolder to prevent accidental overwrite if I were to include more scripts in the future.

So now that we (presumably) have our scripts placed somewhere it is time to use them from NSClient++.


The first thing we need is the CheckExternalScritps module which provides us with the ability to run scripts. To bring in that module we use the following command:

nscp settings --activate-module CheckExternalScripts --add-defaults

This brings in 5 new section and about 50 or so or so new lines in our configuration file.

All sections are under /settings/external scripts so if your configuration file is bigger you need only care about sections starting with /settings/external scripts.

Lets run through the sections briefly:

  • /settings/external scripts Main section where you configure the actual module
  • /settings/external scripts/alias Section for aliases (see below), aliases are in short just another name for another command.
  • /settings/external scripts/scripts This is where we place the actual scripts!
  • /settings/external scripts/wrapped scripts Another macro based place to place scripts (see below).
  • /settings/external scripts/wrappings Section for defining macros for the wrapped scripts.

Writing a script

This is a very short section as writing scripts is first of all much the same as it is for Nagios but also a rather long and complicated topic (since there are hundreds of language you can write scripts in).

A quick check_ok.bat script looks like this:

@echo OK: Everything is going to be fine a
@exit 0

The @ signs prevents the line from being echoed (you can put @echo off to disable echoing as well). Anything after echo will be displayed on the console and you use exit to return a status. The statuses are the same as in Nagios i.e:

  • 0 = ok
  • 1 = warning
  • 2 = critical
  • 3 = unknown

So if instead we want to return a critical state we would rewrite the script like so:

@echo OK: Everything is not going to be fine a
@exit 2

Adding a script

So lets start by adding a script to the configuration.

The simplest way to add a script is to create a new key value pair under the scripts section like so:

[/settings/external scripts/scripts]
custom\_ok = "scripts\\\\custom\\\\check\_ok.bat"
custom\_crit = "scripts\\\\custom\\\\check\_crit.bat" "Hello World"
custom\_check\_perl = "C:\\strawberry\\\\perl\\\\bin\\\\perl.exe" scripts\\\\custom\\\\check\ "Argument 1" "Argument " "$ARG1$"

The first keyword "custom_ok" is the name of the command we just create and the latter part is the command to execute. I prefixed my scripts custom_again to avoid clashing if an internal commands comes along with your name it wont be overwritten. The only other ting to notice is that strings need to have \ escaped (and various other control characters").

Adding more complicated scripts such as perl is not much more difficult all you do is add the exact same thing you would run from the command line. See for instance above in the custom_check_perl example.


Arguments can be a rather confusing subject and the biggest issue is probably the various “allow arguments” flags you might encounter so lets cover that first.

The reason for the “allow arguments” options are to provide a security measure since NRPE lacks authentication (something I have added in NSClient++).

If you do not allow arguments to be injected it means that all commands have to be predefined (configured inside NSClient++) and thus are harmless (ish).

Where as this is fine on paper it creates a massive administrative burden so a lot of people want to allow arguments to make it easy to configure and change checks centrally. Now what usually confuses people is that this option exists in two places.

You can allow or disallow arguments for the NRPE server and you can do so for the CheckExternalScripts module.

  • Enabling them only for NRPE would allow arguments for all internal commands but not for scripts.
  • Enabling them only for CheckExternalScripts would not make much sense really
  • Enabling them for both would allow external scripts (and internal commands) to have arguments.

So if your script has command line arguments you have three options:

  1. Allow arguments from BOTH and configure the arguments from Nagios.
  2. Allow arguments from CheckExternalScripts and define an alias (see below)
  3. Don’t allow arguments and create the definition (with arguments) inside NSClient++ configuration.

The other issue is how to configure them.

[/settings/external scripts/scripts]
custom\_1 = "scripts\\\\custom\\\\check.bat"
custom\_2 = "scripts\\\\custom\\\\check.bat" --help
custom\_3 = "scripts\\\\custom\\\\check.bat" $ARG1$
custom\_4 = "scripts\\\\custom\\\\check.bat" --date $ARG1$ --foo $ARG2$ --bar $ARG3$

In this examples we have a few different ways to define arguments. We start off running the script with out them and then we have a hard coded argument.

Next up comes the more interesting $ARG1$ syntax which will take the argument from the incoming command (think check_nrpe … –a THIS). Lastly you can see that you can mix and match between $ARG?$ and hard coded options. The number of arguments are not limited so you can easily create $ARG1234567890$ but my guess is you would run out of space since NRPE only allows 1024 chars. If you argument has spaces in it it is probably a good idea to enclose it inside quotes like so “$ARG1$”.

Running a script (A full example)

As always I advocate starting your tests inside NSClient++ and not from the Nagios console. This saves you a lot of time and makes life easier. But before we get ahead or ourselves lets start with the configuration I will use here:

CheckExternalScripts = enabled
NRPEServer = enabled

allow arguments = true

[/settings/external scripts]
allow arguments = true

[/settings/external scripts/scripts]
test1 = scripts\\\\check\_test.bat
test2 = scripts\\\\check\_test.bat Hello $ARG1$ "$ARG2$"

Here we have enabled NRPE server and CheckExternalScripts we also allow arguments and define two scripts test1 and test2.

The first thing we need to test is that the scripts work so lets start NSClient++ in test mode (make sure you have stopped the service):

d:\\source\\build\\x64\\dist> nscp test
debug core NSClient++ - 0,4,2,1 2012-08-08 Started!
message client Enter command to inject or exit to terminate...

Next up we run the first script:

debug core Injecting: test1...
debug ext-script Arguments:
debug core Result test1: CRITICAL
message client CRITICAL:CRITICAL: Everything is not going to be ok! ( )

Not that exciting we can see the script worked and returned a critical state. To make things a bit more interesting lets run the second one which takes arguments instead:

test2 world foo
debug core Injecting: test2...
debug ext-script Arguments: Hello world foo
debug core Result test2: CRITICAL
message client CRITICAL:CRITICAL: Everything is not going to be ok!
(Hello world foo)

So arguments seems to work, now since we have enabled NRPE lets try locally from NRPE as well to that open up a new console window and run the following commands. If it does not work please go back to the first console and check the log which usually tells you want is wrong!

d:\\source\\nscp\\build\\x64>nscp nrpe -H -- -c test1
CRITICAL: Everything is not going to be ok! ( )
d:\\source\\nscp\\build\\x64>nscp nrpe -H -- -c test2 -a 1 2 3
CRITICAL: Everything is not going to be ok! (Hello 1 2)

Simplifying with wrapped scripts

Now as you saw above the Perl command line started to become a bit cumbersome so what if you have 30 Perl scripts? Not so fun, especially not once you change from Strawberry to activestate or something similar so the command changes for all of them.

To resolve this I added something called wrapped-scripts. The reside in the own section called: /settings/external scripts/wrapped scripts. Essentially a wrapped script will use a macro during instantiation splitting the script configuration from the runtime configuration.

The way it works is you define a template (macro) for a given extension and once you add (to the wrapped scripts section, not the normal scripts section) a script with that extension the template will be used.

The default wrappings are for batch, vba and powershell scripts. But you can easily define you own as I have done below for pl scripts.

; A list of wrappped scripts (ie. using the template mechanism)
[/settings/external scripts/wrapped scripts]
custom\_ok = check\_ok.bat
custom\_crit = check\_crit.bat "Hello World"
custom\_check\_perl = check\ "Argument 1" "Argument " "$ARG1$"

; A list of templates for wrapped scripts
[/settings/external scripts/wrappings]
; Perl scripts
pl = "C:\\\\strawberry\\\\perl\\\\bin\\\\perl.exe" scripts\\\\%SCRIPT% %ARGS%
bat = scripts\\\\%SCRIPT% %ARGS%

The way it works is that when our .pl script is found it looks up the pl template and replaces %SCRIPT% with the name of the script and %ARGS% with the list of arguments and then use the resulting command line instead. As you can see I usually also specify the folder of the script in the template which makes it very simple to add scripts.

Simplifying with aliases

Now first off I want to say that aliases should never have been included in the CheckExternalScripts module and in 0.4.2 they will be removed. Now don’t get excited they will instead be included in the core where they belong!

Aliases are extremely useful and the best way to kick start your monitoring.

But they are not really script related in fact I would say they are more useful for internal commands then they are for scripts.

So what are aliases then? Well they are a way to remove options you never want to change. Think of aliases as the feature with the same name in the Unix shell (at least in bash). You create a predefined command with you can run without having to specify all the options.

A good example: A standard CheckCPU check might look like this: “checkCPU warn=80 crit=90 time=5m time=1m time=30s” but that’s a lot of typing to do on the Nagios server since it is always the same you can create an alias like so and just call “alias_cpu”.

[/settings/external scripts/alias]
alias\_cpu = checkCPU warn=80 crit=90 time=5m time=1m time=30s


Well, that’s pretty much it.

Running external script is pretty simple and don’t forget to use wrapped script if you keep having to type a lot in your script command line. And remember alias is not just for scripts just as useful for internal commands as well.