[mrtg] Update: monitoring traffic in & out of Linux box

Leigh Orf orf at mailbag.com
Sun Nov 5 21:11:51 MET 2000

First of all thanks to those who responded to my query about monitoring
traffic in & out of a Linux box. I was succesful in finding the port on
the switch which handles my machine's traffic. I also managed to get
snmpd to give me access to the interfaces.ifTable.ifEntry.ifOutOctets.2
directive which mrtg uses to determine outgoing bandwidth on eth0.
However, the value which that directive is returning is roughly 6 times
smaller than what is being seen at the switch. The switch is returning
the correct values. I am running an FTP server with a per-user 20 kB/s
download bandwidth cap times about 20 users. I have mrtg running on the
switch and on eth0 on my Linux machine. To view both, see

http://orf.cx/mrtg/  (the switch port)
http://orf.cx/mrtg/dryas.atms.unca.edu.html (eth0)

(Note the flip-flop in incoming/ougoing which occurred at 16:00 was
my putting a hyphen before the Target directive since I was on the
"wrong side" of the switch).  Using snmpget, I monitored the value of
interfaces.ifTable.ifEntry.ifOutOctets.2 for my Linux box in a one
minute interval and calculated the alleged outgoing bandwidth dividing
the difference by 60, and MRTG is faithfully reporting the alleged
bandwidth at eth0 from the returned values. So it appears for whatever
reason snmpd is returning a number which isn't representative of the
actual outgoing (and incoming) bandwidth. The values seems to
be off by a factor of about 6.

Any clues??


Leigh Orf

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