[mrtg] How to make an external script feed unknowns into a target (RRD+MRTG)

[Jakob Ilves]

Hello everyone!

(Here I'm discussing the case when MRTG is used in conjunction with
RRDtool)

Problem:  How can an external script target (those using backticks in
the Target[] directive you know) return an "Unknown" value an a way that
MRTG accepts?  Returning two empty lines do not make "unknown" to be
written into the RRDtool file but zeroes instead which isn't what I
want.  The reason as that the zeroes are taken into account when data is
consolidated and when calculating averages while the unknown values are
disregarded and do not affect the averaging/consolidation.  Another
thing is that the empty lines are making MRTG unhappy with error
messages being output everytime the target is "polled" which is bad as
well.

The reason I ask for this is that I have a script for doing measurements
and sometimes this script fails to do it's measure and in my opinion the
script should provide "unknown" back for both of it's values.

I didn't find anything in the docs (maybe I'm blind) and I prefer to ask
you folks who KNOW about things before I go down the path of digging
deep into the code, GUESSING how things work and then perform "trial and
error tango".

(or should I write "trial and horror tango" instead...?  Just kidding,
it's Friday. :-)

Best regards

/IlvJa



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