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SNMP: Mistake of Using Tabular OID in Practice

In General Linux on 20 March 2012 with no comments 827 views

Recently, I retrieve rectifier-related SNMP data from Eltek’s Smartpack Controller. This one has the following rear type where we can connect to the ethernet plug (see the purple area).

Eltek Smartpack Controller rear view (type that comes with SNMP support)

It is presumably in its default configuration and I can snmpwalk through its corporate specific OIDs (under 12148 tree) to find i.e. output voltage, current, battery temperature, etc. Doing it without translating against the MIB will retrieve all the values perfectly fine. However, I want to show you a mistake in practice related to how the MIB is written and put to place, which is a version 4 by the name of ELTEK-DISTRIBUTED-PLANTV4-MIB. It has tabular part as clipped in the following tree (see previous post on how to write such part in your own MIB):

            |     +--eltek(12148)
            |        |
            |        +--eltekDistributedPlantV4(9)
...
            |           |
            |           |     |
            |           |     +--rectifierStatusTable(2)
            |           |        |
            |           |        +--rectifierStatusEntry(1)
            |           |           |  Index: rectifierStatusID
            |           |           |
            |           |           +-- -R-- Integer32 rectifierStatusID(1)
            |           |           |        Range: 1..100
...
            |           |           +-- -R-- Integer32 rectifierStatusOutputVoltage(4)
            |           |           |        Range: 0..65535

Looking at the above rectifierStatusEntry branch, it marks a tabular entry expected to range from 1 to 100 as defined by rectifierStatusID. Hence, putting 0 as one of the output voltage index will confuse the OID reported by GET as

1. belong to the scalar type (a misconception that is)
2. belong to the tabular type but out of index range

GET against the MIB will return the following error

$ sudo snmpget -m +ELTEK-DISTRIBUTED-PLANTV4-MIB -v 1 -c public raddlex ELTEK-DISTRIBUTED-PLANTV4-MIB::rectifierStatusOutputCurrent.0
ELTEK-DISTRIBUTED-PLANTV4-MIB::rectifierStatusOutputCurrent.0: Unknown Object Identifier (Index out of range: 0 (rectifierStatusID))

while the item (output current) actually has value in it as found below

$ sudo snmpget -v 1 -c public raddlex .1.3.6.1.4.1.12148.9.5.5.2.1.3.0
SNMPv2-SMI::enterprises.12148.9.5.5.2.1.3.0 = INTEGER: 1

By this time the MIB is already the latest to be used with firmware version 4.3.