I as recently compiling an external script within Nagios XI to run using the NSClient where upon execution I was receiving the following message in the nsclient.log:
2013-11-21 17:02:10: error:modules\CheckExternalScripts\CheckExternalScripts.cpp:202: Request string contained illegal metachars!
This is due to the NRPE daemon by default not accepting the use of the the characters `&><‘”\[]{} in arguments.
In order to resolve the issue I had to enable the ‘allow_nasty_meta_chars’ setting for External Scripts and restart the NSClient++ service:
[External Script] allow_nasty_meta_chars=1
Deans Blog 