When refactoring a check_memory I wrote a few years ago I bumped into the feared
NRPE: Unable to read output
error message on our nagios instance.
When looking for a solution I went through most possible debug steps I could think of and which are nicely described by nagios support but didn't found any solution.
I almost grabbed to some anti depressants when I thought of the thing I always forget about.
When crawling through the audit log it became clear I forgot to configure the proper selinux context type for the new script.
type=PATH msg=audit(1518702310 …