[mrtg] Target data not collected (RHEL 6.0, MRTG 2.16.2, Perl 5.10.1)

[Niall O'Reilly]

In my experience, MRTG "just works".
Just recently, some strange behaviour has come to my attention.

I'ld appreciate any brief indication of whether what I'm
seeing is a known problem, and if so, whether I can expect it
to disappear following an installation of MRTG from current source.

[ I have NO PLANS to attempt changes to the distro-supplied
  instance of Perl.  8-) ]

The initial symptom was that my display front-end showed
a number of units as unresponsive, on finding "NaN" in the RRD
database.  Investigation followed.  Snmpwalk was able to return
the counters which MRTG was not picking up.  Tcpdump appeared to
show that MRTG was not polling for some of the desired target OIDs.

Moreover, tcpdump showed different traffic patterns between the
same instance of MRTG and different probed units.

In the "normal" traffic pattern, a succession of GetRequest
PDUs are sent, each seeking four items of data from the probed
unit (typically ifName.n, ifInOctets.n, inName.n, ifOutOctets.), 
for each interface corresponding to a Target in the configuration.
For the units currently of interest, the count of associated 
802.11 clients is sometimes specified in the configuration, and
is duly indicated in the packets captured by netstat.  The 
corresponding response PDUs also appear, as expected.

When this "normal" traffic pattern is observed, the collected
numeric data is stored in the corresponding RRD file, and the
requested graphs are generated as expected. 

In the "abberrant" traffic pattern, some traffic is seen which
matches the description given above.  However, some expected
GetRequest PDUs do not appear.  In addition, a number of
GetNextRequest PDUs are shown, stepping through ifName, ifDescr,
ifType, ipAdEntIfIndex, ipAdEntNetMask, and ifPhysAddress.  Again,
corresponding responses appear.  It seems that MRTG is trying to
map the properties and status of the unit, as well as to collect
data corresponding to (only!) some of the configured Targets.

Data is stored as "NaN" for the configured targets which are 
omitted from the SNMP polling sequence.  Corresponding graphs
are, unsurprisingly, blank.

This behaviour has been observed on Red Hat Enterprise Linux
Server release 6.0, with the versions of Perl (5.10.1) and 
MRTG (2.16.2) provided in this distro.  Our preference is to
avoid (as far as is feasible) installation from source.

The same MRTG configuration is also running without problem on an
older system, due for retirement in the near future.  This system
runs Red Hat Enterprise Linux ES release 3.4, with Perl 5.8.0 and
MRTG 2.15.2.

Best regards and Happy Christmas!

Niall O'Reilly