[ First - *please* don't mail me privately, without copying
any responses to the mailing list. I don't have the time
or inclination to offer private, unpaid, SNMP consultancy.
Keep discussions to the list, where others can both learn
and offer advice. Thanks. ]
On 23 April 2012 18:11, Lawrence Widman <wid...@cecst.com> wrote:
> Dave,
>
> Many thanks for your reply. Please forgive my ignorance: I'm new at
> this. I am writing an agent that accepts the status of a network device
> and makes it available to a monitoring system. If counters cannot be set
> by any process, how are the values updated?
It's not that counters cannot be set "by any process".
Rather that they cannot be set via the SNMP SET request.
> snmpd has to be getting the
> information from someplace, I would think. I searched the RFC for every
> occurrence of "counter" and did not find any explanation of how counters
> get updated,
That doesn't really surprise me - the RFCs are purely describing
the operation of the SNMP protocol (and syntax of the MIB files).
They are concerned with how the management information is
used - not how it is obtained in the first place.
> ... and looked at google without result. One possibility is
> that another variable accepts a rate and the extended agent adds the
> rate to a given counter, but if that were the case one would think the
> standard would allow pairing of the two variables.
Take a step back (TM), and think for a moment about what problem
SNMP is trying to address. The network manager needs to know
what's happening on their network - to find out information about the
systems they are looking after. Counters are typically used to monitor
how many times something has happened - whether that's something
unusual or unexpected (how many times the building has caught fire),
or something perfectly normal (the number of packets sent on a given
network interface).
In both cases, this information will be stored somewhere (the safety
log, or an internal field in the relevant kernel driver). All that the agent
is doing is retrieving this value from the underlying subsystem, and
passing it on to the network administrator in a standard form.
So the counters will be maintained and updated by the underlying
subsystem. And similarly for all the other MIB variables.
This is an important point - the agent is *not* typically responsible
for maintaining the values that it reports. It's simply a middle-man.
It may keep a local cached copy of this data (e.g. network statistics
or the routing table), but this would be purely for performance reasons.
The source of the data would be the underlying subsystem, and it's
_this_ that is responsible for maintaining it, updating counters, etc.
OK?
> I understand if you do not have time for dumb questions. If so, please
> accept my thanks for the information you provided.
Don't worry about it. As I keep telling our students, I like easy questions.
I can answer easy questions. It's the hard ones that are more of a problem.
And the only dumb question is the one you don't ask.....
(together with the question you ask twice because you didn't
like the answer first time round. Now that *does* annoy me!)
Dave
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