Re: A very simple mistake about snmpget

On 11 March 2011 12:28, Jaime Lozano <[email protected]> wrote:
> This is very simple. This works:
>>snmpget -m all -v1 -c public localhost sysUptime.0 2>/dev/null
> DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (975551) 2:42:35.51

Yes - that reports the (single) value associated with the (instance) OID
    sysUpTime.0   (i.e. .1.3.6.1.2.1.1.3.0)

> But this doesnt work, and returns nothing:
>>snmpget -m all -v1 -c public localhost iso 2>/dev/null

That is correct.

You are asking for the value associated with the OID .1
There is no such value.

Two mistakes here:
   -  values are always associated with an *instance* of an object,
      rather than the object itself.   So there will *always* be at least
      once more subidentifier in the OID , compared to the OID of the object.
      (either the index value(s) for a table, or .0 for a scalar object)

   - values are only associated with (instances of) *leaf* objects,
      i.e. those defined using OBJECT-TYPE.
      Objects such as .iso are purely there to describe the internal
      structure of the information tree (and are defined using OBJECT
      IDENTIFIER)


The simplest way to think about this is to look at the output of
an 'snmpwalk' command.    The *only* OIDs that you can issue
a GET request on, are the ones that appear in that output.
If a particular OID doesn't appear in that list,  then trying to
GET it, will fail.

   snmpwalk  .... .iso

will report lots of OIDs that *start* with '.iso',
but nothing that is *just*    .iso = xxxx


Please see any book on SNMP to understand how MIBs and OIDs work.

Dave

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