[mrtg] mrtg vs. Linux

[Eric Seppanen]

Hi, just wanted to drop a line and get this into the list archives.  It
details some of the things I learned trying to get mrtg monitoring an i386
machine running Red Hat Linux 6.0.

The machine is mostly stock plus all current Red Hat updates, which sadly
only brings me to kernel 2.2.5.  I installed the ucd-snmp package, version
3.6.1, which is part of the Red Hat distribution.  I let mrtg run for a
week before noticing that the results graphed, while _appearing_ to be
valid traffic (lots during the day and not much at night), the numbers
were roughly 1/4 to 1/3 of what they should have been.  However, at times 
the traffic graphed seemed to be somewhat random.  Strange.

I upgraded to ucd-snmp 3.6.2 with no effect.  I found a patch on the
ucd-snmp homepage (http://ucd-snmp.ucdavis.edu) that was supposed to fix
the bug.  Apparently Linux 2.0 doesn't actually keep track of bytes
transmitted or received, so ucd-snmp reads the number of packets
transmitted and multiplies it by some constant to get a crude estimate of 
bytes transmitted.  This probably worked fine until Linux 2.2 changed the 
layout of /proc/net/dev and totally broke ucd-snmp.  Getting back on
track, the patch I found at ucdavis to fix this somehow managed to
make it worse- not only were the results still wrong, but the transmit
statistics now went to zero and stayed there.

I downloaded ucd-snmp 4.0.1 and things worked correctly from there.
Finally!

So, things to remember:
* ucd-snmp 3.6.x are all broken on linux 2.2, even with the patch.
* if you want perfect statistics don't bother with linux 2.0

Wishing you better luck than I had,
Eric Seppanen
eds at reric.net

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