[mrtg] bizarre off-by-one writing of logs

Antonio Querubin tony at lava.net
Thu Sep 9 20:05:22 MEST 1999

Has anyone noticed over the past two days an unusual off-by-one error in
MRTG where the wrong log/gif file gets written?

Here are the symptoms.  We've got a lot of ADSL customers coming in on
Frame Relay to one of our routers.  The MRTG config file has the following
among other things (just two of many ports):

Target[pueo.252]: 252:xxxxxxxxxxxxx at pueo
MaxBytes[pueo.252]: 48000
Title[pueo.252]: pueo.lava.net (): Serial5/4.25

Target[pueo.253]: 253:xxxxxxxxxxxxx at pueo
MaxBytes[pueo.253]: 32000
Title[pueo.253]: pueo.lava.net (): Serial5/4.26
On September 7, the two sets of graphs, pueo.252*gif and pueo.253*gif,
reflected accurate data for the two interfaces.  On the morning of
September 8, pueo.252*gif are now showing what used to be in pueo.253*gif.  
In fact as far as I can tell, for every sub-interface on the router,
pueo.(n-1)*gif now shows what used to be in pueo.(n)*gif, where n are just
successive ports numbers.

The mrtg.cfg file has not been changed over the two day period.  The
router has not crashed or rebooted in that period either.

Interestingly enough, the problem is consistently reproducible.  If I
restore the log and gif files from a backup tape made on Sept 7, I can see
all the correct graphs in the right place in the right files.  As soon as
mrtg processes the files, many graphs get shifted by one port number.  
The shift happens once and mrtg continues on its merry way with no error
message or warning.

It seems like it's some kind of date/time rollover problem.  But I haven't
been able to find the underlying cause.  Anyone else noticed this?  And if
you have, is there a workaround?

Antonio Querubin
tony at lava.net / ah6bw at hawaii.ampr.org

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