[collectd] Changing and "non-standard" OIDs

Florian Forster octo at verplant.org
Mon Mar 22 09:38:02 CET 2010

Hi Stian,

On Tue, Mar 16, 2010 at 05:53:46PM +0100, Stian Øvrevåge wrote:
> SNMPv2-SMI::enterprises. = STRING: "l"
> SNMPv2-SMI::enterprises. = STRING: "l"
> SNMPv2-SMI::enterprises. = STRING: "l"
> One of the things I want to monitor is the clients data rate, given in
> the OID SNMPv2-SMI::enterprises. Followed by
> some numbers I take are random to each AP. Followed either
> by a missing number for the first SSID configured and then by 50 for
> the second, 51 for third configured SSID and so on. Then followed by
> the client mac-address encoded octally or something.

I'm not sure I understand the layout of the data. So "..." is
the common prefix? But two out of three lines have a ".5" there instead
of a ".4"..? My guess is that ".4" is the "main" ESSID and those ".5"
(with ".50", ".51", ... later on) are something like "virtual routers"
or something. The last six IDs look like a MAC address (but printed in
decimal notation).

> 1. I want to collect the number of clients using a given data-rate at
> the current moment, for each SSID.

For that you would have to group all the data by its *value* and then
submit the number of sets in each group. That's not possible to do with
the SNMP plugin. Since this sort of processing doesn't quite fit in to
the plugin either, I don't think we'll ever add that.

Your best bet is probably to do the SNMP communication and the
aggregation yourself and then use the Exec plugin or the UnixSock plugin
to submit the data to collectd.

> 2. I may also want to collect the data-rates for each client over
> time.

The problem with that is that you've stumbled upon a typical bug in
Cisco's SNMP MIBs: They encode data into the OID, namely the client's
MAC address.

It would be nice to be able to handle this in the SNMP plugin - the only
change required would be to allow the instance to be set from multiple
IDs. Currently, only the "Sub-ID" (the last number in the full OID path)
is used.

Florian octo Forster
Hacker in training
GnuPG: 0x91523C3D
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