Adding support for a new environment probe device to NAV

This guide will document an example of how to implement support for a new type of environmental sensor probe in NAV.

The specific device used as an example is the AKCP sensorProbe8, described by the vendor as:

A High-Speed, Accurate and Intelligent Monitoring device. The sensorProbe8 is a completely embedded host with a proprietary Linux like OS which includes TCP/IP stack, a built in web-server and full Email and SNMP functionality.

The goal

We have a sensorProbe8 device with a range of sensors connected (temperature, humidity, etc.). We want each sensor value to be logged and graphed in NAV.

Sensors in NAV

NAV has a nav.models.manage.Sensor model, which maps arbitrary sensors to Netboxes. This model describes how to collect data from a sensor using SNMP, and NAV will automatically collect, log and graph data from all Sensor instances registered in the database.

Conceptually, to add support for a new type of device with (possibly) multiple sensors, you need to write a module that will discover the SNMP-available sensors on this type of device and insert each of them into the NAV database.

In practice, you don’t need to fiddle with the database at all, but just make a class with a standard API to report a list of sensor descriptions to the ipdevpoll sensors plugin.

Course of action

  1. We require AKCP’s MIB definition. This can be downloaded from

  2. The MIB file must be converted to a Python file, using the smidump program.

  3. A MibRetriever class to detect and report the relevant sensors to NAV using this MIB must be written.

  4. The ipdevpoll sensors plugin must be configured to use the new MibRetriever class for the appropriate devices.

Dumping the MIB

The downloaded akcp.mib file defines a MIB module named SPAGENT-MIB. Its definitions can be converted to a Python module thus:

smidump -k -f python akcp.mib > python/nav/smidumps/


The SPAGENT-MIB definitions are somewhat flawed and will cause smidump to output some parsing errors. The -k command line option is there to make it produce its output despite many of these errors.

It does not matter that the output file is invalid as a Python module name. It is loaded dynamically by NAV, and should be named verbatim after the MIB module it defines.

The nav.smidumps package is where NAV distributes Python versions of the MIB definitions its code uses.

Examining the MIB

Examining the MIB, reveals that it defines a number of tables; one for each type of sensor that can be connected to a sensorProbe device. The table rows typically define a sensor identifier, description, value readout, value unit description and a bunch of other more or less interesting metadata.

What NAV needs in a Sensor record is:

  • A unique identifier, that will not change when the sensor description changes.

  • A description of the sensor.

  • What base unit is used for the value readout.

  • The precision of the value readout (SNMP doesn’t support floating point numbers, so decimal precision is achieved by reporting a large integer and scaling it by a given factor).

  • The exact OID to use in an SNMP GET operation to read the sensor value.

Hopefully, the MIB provides us with enough information to record all of this. As an example, let’s get some data about the available temperature sensors:

$ ls
$ export MIBDIRS=/var/lib/mibs/ietf:.
$ snmpwalk -v1 -c public SPAGENT-MIB::sensorProbeTempTable
SPAGENT-MIB::sensorProbeTempDescription.0 = STRING: "Ambient temperature"
SPAGENT-MIB::sensorProbeTempDescription.1 = STRING: "Temperature2 Description"
SPAGENT-MIB::sensorProbeTempDescription.2 = STRING: "Temperature3 Description"
SPAGENT-MIB::sensorProbeTempDescription.3 = STRING: "Front of rack"
SPAGENT-MIB::sensorProbeTempDescription.4 = STRING: "Back of rack"
SPAGENT-MIB::sensorProbeTempDescription.5 = STRING: "Temperature6 Description"
SPAGENT-MIB::sensorProbeTempDescription.6 = STRING: "Temperature7 Description"
SPAGENT-MIB::sensorProbeTempDescription.7 = STRING: "Temperature8 Description"
SPAGENT-MIB::sensorProbeTempDegree.0 = INTEGER: 22
SPAGENT-MIB::sensorProbeTempDegree.1 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegree.2 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegree.3 = INTEGER: 17
SPAGENT-MIB::sensorProbeTempDegree.4 = INTEGER: 16
SPAGENT-MIB::sensorProbeTempDegree.5 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegree.6 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegree.7 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempOnline.0 = INTEGER: online(1)
SPAGENT-MIB::sensorProbeTempOnline.1 = INTEGER: offline(2)
SPAGENT-MIB::sensorProbeTempOnline.2 = INTEGER: offline(2)
SPAGENT-MIB::sensorProbeTempOnline.3 = INTEGER: online(1)
SPAGENT-MIB::sensorProbeTempOnline.4 = INTEGER: online(1)
SPAGENT-MIB::sensorProbeTempOnline.5 = INTEGER: offline(2)
SPAGENT-MIB::sensorProbeTempOnline.6 = INTEGER: offline(2)
SPAGENT-MIB::sensorProbeTempOnline.7 = INTEGER: offline(2)
SPAGENT-MIB::sensorProbeTempDegreeType.0 = INTEGER: celsius(1)
SPAGENT-MIB::sensorProbeTempDegreeType.1 = INTEGER: fahr(0)
SPAGENT-MIB::sensorProbeTempDegreeType.2 = INTEGER: fahr(0)
SPAGENT-MIB::sensorProbeTempDegreeType.3 = INTEGER: celsius(1)
SPAGENT-MIB::sensorProbeTempDegreeType.4 = INTEGER: celsius(1)
SPAGENT-MIB::sensorProbeTempDegreeType.5 = INTEGER: fahr(0)
SPAGENT-MIB::sensorProbeTempDegreeType.6 = INTEGER: fahr(0)
SPAGENT-MIB::sensorProbeTempDegreeType.7 = INTEGER: fahr(0)
SPAGENT-MIB::sensorProbeTempDegreeRaw.0 = INTEGER: 223
SPAGENT-MIB::sensorProbeTempDegreeRaw.1 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegreeRaw.2 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegreeRaw.3 = INTEGER: 170
SPAGENT-MIB::sensorProbeTempDegreeRaw.4 = INTEGER: 161
SPAGENT-MIB::sensorProbeTempDegreeRaw.5 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegreeRaw.6 = INTEGER: 0
SPAGENT-MIB::sensorProbeTempDegreeRaw.7 = INTEGER: 0

From the MIB’s description of the sensorProbeTempTable object, and from this output, we can surmise the following:

  • A total of 8 temperature sensors can be slotted in. All slots are reported in the table, but only the slots with an sensorProbeTempOnline value of online actually have an active temperature sensor connected.

  • If we want decimal precision in our temperature readouts, we should use the sensorProbeTempDegreeRaw value. Unfortunately, the MIB definition says nothing about the exact resolution of this number, only that it is «higher» resolution than the sensorProbeTempDegree value. The snmpwalk output seems to suggest it provides a precision of a single decimal digit (i.e. divide the readout value by 10).

  • The readout value unit is given by sensorProbeTempDegreeType (and we are given to suppose that a value of fahr means degrees fahrenheit).

Writing a MibRetriever

NAV provides the nav.mibs.mibretriever.MibRetriever base class, which provides the basis for implementing classes with knowledge of specific MIBs.

First, we will need a class skeleton to start with. Create a python/nav/mibs/ containing the following skeleton code:

from twisted.internet import defer
from nav.mibs import reduce_index
from nav.mibs.mibretriever import MibRetriever
from nav.smidumps import get_mib

class SPAgentMib(MibRetriever):
    mib = get_mib('SPAGENT-MIB')

The ipdevpoll plugin nav.ipdevpoll.plugins.sensors needs our MibRetriever to implement the get_all_sensors() method. This method should return a Twisted Deferred - a «promise» of a future result. The result must be a specific data structure describing a list of sensors discovered on a device.

Example using a single hardcoded sensor

Let’s hardcode an example result for a single temperature sensor, based on the snmpwalk from above:

class SPAgentMib(MibRetriever):
    mib = get_mib('SPAGENT-MIB')

    def get_all_sensors(self):
        result = [
                'oid': '.',
                'unit_of_measurement': 'celsius',
                'precision': 1,
                'scale': None,
                'description': "Ambient temperature",
                'name': "Ambient temperature",
                'internal_name': "Ambient temperature",
                'mib': 'SPAGENT-MIB',

This returns a list of a single item: A dictionary describing the first temperature sensor from the snmpwalk from above. The dictionary should contain the following keys:




The OID from which an SNMP-GET operation can extract the readout value. In this example, it corresponds to SPAGENT-MIB::sensorProbeTempDegreeRaw.0


The unit of measurement, used mostly for display purposes. It may also be used to discover which sensors actually measure temperature, when finding temperature sensors for a room-view in NAV.


The number of positions to move the decimal point of the readout value. In this example, a readout value of 223 will be registered as 22.3 degrees celsius.


The scale of the readout value. If the readout value was specified as a number of MegaWatts, the base unit of measurement would be Watts and the scale would be Mega.


A (preferably) human-readable description of the sensor.


A unique sensor name (can conceiveably be the same as the description).


An internal sensor name. If, for example, the actual readout value OID for a specific sensor can change over time, this should be an identifier that the sensor can be recognized by over time. This string is also used as part of the Graphite metric name when sensor readings are sent to its Carbon backend.


Should be the name of the MIB module that the sensor information was found in.

A note on standardizing unit names

Spelling and casing of unit names should be standardized throughout NAV. E.g., when a list of sensors is filtered to select only those that report temperature values, it’s much easier to write a filter if every temperature sensor reports either celsius or fahrenheit. If you register sensors that have units like C, F, fahr, °C or °F, it’s much harder to find all the relevant sensors.

For this reason, an attempt has been made to standardize on a set of unit names in the nav.models.manage.Sensor model class. It would be wise to import this model and use the relevant UNIT_* constants from this class when returning sensor dicts.

This is exactly what we will do in the next example.

Collecting actual sensors from the MIB

Let’s rewrite SPAgentMib to collect actual temperature sensors:

 1 from nav.models.manage import Sensor
 4 class SPAgentMib(MibRetriever):
 5     mib = get_mib('SPAGENT-MIB')
 7     @defer.inlineCallbacks
 8     def get_all_sensors(self):
 9         result = yield self.retrieve_columns([
10             'sensorProbeTempDescription',
11             'sensorProbeTempOnline',
12             'sensorProbeTempDegreeType',
13         ]).addCallback(self.translate_result).addCallback(reduce_index)
15         sensors = (self._temp_row_to_sensor(index, row)
16                    for index, row in result.iteritems())
18         defer.returnValue([s for s in sensors if s])
20     def _temp_row_to_sensor(self, index, row):
21         online = row.get('sensorProbeTempOnline', 'offline')
22         if online == 'offline':
23             return
25         number = index[-1]
26         internal_name = 'temperature%s' % number
27         descr = row.get('sensorProbeTempDescription', internal_name)
29         mibobject = self.nodes.get('sensorProbeTempDegreeRaw')
30         readout_oid = str(mibobject.oid + str(index))
32         unit = row.get("sensorProbeTempDegreeType", None)
33         if unit == 'fahr':
34             unit = Sensor.UNIT_FAHRENHEIT
36         return {
37             'oid': readout_oid,
38             'unit_of_measurement': unit,
39             'precision': 1,
40             'scale': None,
41             'description': descr,
42             'name': descr,
43             'internal_name': internal_name,
44             'mib': 'SPAGENT-MIB',
45         }

Lines 6 through 10 perform the actual SNMP query against a device. The get_all_sensors() method then delegates to the _temp_row_to_sensor() method the responsibility of translating each table row into a sensor dictionary that can be used by the ipdevpoll sensors plugin.

_temp_row_to_sensors() takes the index and row arguments. index is the row index in the SNMP table (it is an OID suffix, in this case a single-item tuple corresponding to the temperature sensor slot number). row is a dictionary containing the collected table columns, keyed by their names.

Expanding these code examples to include all the sensor types provided by the SPAGENT-MIB is left as an excercise to the reader.

Have the sensors plugin use our new MibRetriever

The sensors plugin employs the configuration sections sensors and sensors:vendormibs from ipdevpoll.conf to decide which MibRetriever classes to use for discovering sensors on a device. The plugins decides on a list of MIBs to query based on the type of the device under query (derived from the enterprise number in the device’s sysObjectID value).

AKCP’s enterprise number is 3854 (as assigned by IANA), so we will use that to select our MibRetriever in the ipdevpoll config.

3854 = SPAgentMib

Alternatively, if you want a potentially more readable vendormibs section:


Both versions will work equally well. The latter works because VENDOR_ID_KCP_INC is a registered constant mapped to AKCP’s enterprise number in the nav.enterprise.ids module, and our SPAgentMib MibRetriever has been mapped to the SPAGENT-MIB module by importing the smidump in its class definition.

If you implemented your MibRetriever outside of the NAV package tree, say in the module mynav.akcp, you can modify the loadmodules option:

loadmodules = nav.mibs.* mynav.akcp

The sensors plugin runs as part of ipdevpoll’s inventory job, normally every 6 hours. With these changes, adding an AKCP sensorProbe in SeedDB will cause the sensors plugin to discover and insert the temperature sensors of this device into NAV’s database. The ipdevpoll 1minstats job will retrieve the sensor readings once every minute and send them to Graphite.