SNMP Agent in Python with custom MIB

Question

I implimanted SNMP agent in python with pysnmp lib. This is the code that I used to response an custom OID.


from datetime import datetime
from pysnmp import debug
from pysnmp.carrier.asyncore.dgram import udp
from pysnmp.entity import engine, config
from pysnmp.entity.rfc3413 import cmdrsp, context
from pysnmp.proto.api import v2c
from pysnmp.smi import builder, instrum, exval

# Uncomment this to turn pysnmp debugging on
#debug.setLogger(debug.Debug('all'))

formatting = '[%(asctime)s-%(levelname)s]-(%(module)s) %(message)s'
logging.basicConfig(level=logging.DEBUG, format=formatting, )

logging.info("Starting....")

# Create SNMP engine
snmpEngine = engine.SnmpEngine()

# Transport setup

# UDP over IPv4
config.addTransport(
    snmpEngine,
    udp.domainName,
    udp.UdpTransport().openServerMode(('0.0.0.0', 12345))
)

# SNMPv2c setup

# SecurityName <-> CommunityName mapping.
config.addV1System(snmpEngine, 'my-area', 'public')

# Allow read MIB access for this user / securityModels at VACM
# Limit access to just the custom MIB. Widen if need be
config.addVacmUser(snmpEngine,
                   2,
                   'my-area',
                   'noAuthNoPriv',
                   (1, 3, 6, 4),
                   (1, 3, 6, 4))


# Create an SNMP context and ensure the custom MIB is loaded
# Your system must have this MIB installed otherwise pysnmp
# can't load it!
snmpContext = context.SnmpContext(snmpEngine)
logging.debug('Loading __EXAMPLE-MIB module...'),
mibBuilder = snmpContext.getMibInstrum().getMibBuilder()
(MibTable,
 MibTableRow,
 MibTableColumn,
 MibScalarInstance) = mibBuilder.importSymbols(
    'SNMPv2-SMI',
    'MibTable',
    'MibTableRow',
    'MibTableColumn',
    'MibScalarInstance'
)
logging.debug('done')


RowStatus, = mibBuilder.importSymbols('SNMPv2-TC', 'RowStatus')

mibBuilder.exportSymbols(
    '__EXAMPLE-MIB',
    # table object
    exampleTable=MibTable((1, 3, 6, 4, 1)).setMaxAccess('readcreate'),
    # table row object, also carries references to table indices
    exampleTableEntry=MibTableRow((1, 3, 6, 4, 1, 5)).setMaxAccess('readcreate').setIndexNames((0, '__EXAMPLE-MIB', 'exampleTableColumn1')),
    # table column: string index
    exampleTableColumn1=MibTableColumn((1, 3, 6, 4, 1, 5, 1), v2c.OctetString()).setMaxAccess('readcreate'),
    # table column: string value
    exampleTableColumn2=MibTableColumn((1, 3, 6, 4, 1, 5, 2), v2c.OctetString()).setMaxAccess('readcreate'),
    # table column: integer value with default
    exampleTableColumn3=MibTableColumn((1, 3, 6, 4, 1, 5, 3), v2c.Integer32(123)).setMaxAccess('readcreate'),
    # table column: row status
    exampleTableStatus=MibTableColumn((1, 3, 6, 4, 1, 5, 4), RowStatus('notExists')).setMaxAccess('readcreate')
)
logging.debug('done')

(exampleTableEntry,
 exampleTableColumn2,
 exampleTableColumn3,
 exampleTableStatus) = mibBuilder.importSymbols(
    '__EXAMPLE-MIB',
    'exampleTableEntry',
    'exampleTableColumn2',
    'exampleTableColumn3',
    'exampleTableStatus'
)

rowInstanceId = exampleTableEntry.getInstIdFromIndices('example record one')
mibInstrumentation = snmpContext.getMibInstrum()
mibInstrumentation.writeVars(
    ((exampleTableColumn2.name + rowInstanceId, 'hello'),
     (exampleTableColumn3.name + rowInstanceId, 123456),
     (exampleTableStatus.name + rowInstanceId, 'createAndGo'))
)

logging.debug('done')
logging.debug('Snmp Agent Start')

# Register SNMP Applications at the SNMP engine for particular SNMP context
cmdrsp.GetCommandResponder(snmpEngine, snmpContext)
cmdrsp.SetCommandResponder(snmpEngine, snmpContext)
cmdrsp.NextCommandResponder(snmpEngine, snmpContext)
cmdrsp.BulkCommandResponder(snmpEngine, snmpContext)

# Register an imaginary never-ending job to keep I/O dispatcher running forever
snmpEngine.transportDispatcher.jobStarted(1)

# Run I/O dispatcher which would receive queries and send responses
try:
    snmpEngine.transportDispatcher.runDispatcher()
except:
    snmpEngine.transportDispatcher.closeDispatcher()
    raise

I use iReasoning MIB Browser to sending snmp request.
When I send Get request of .1.3.6.4.1.5.1 oid, an error with this explaination appear: "No acces error."
Finally, I send Get Subtree request of .1.3.6.4.1.5 oid and MIB Browser show this result below:

As you see, OID result is: .1.3.6.4.1.5.1.18.101.120.97.109.112.108.101.32.114.101.99.111.114.100.32.111.110.101

This is not short and have some added values.

What's wrong?

Ilya Etingof Ilya Etingof · Accepted Answer · 2019-06-05T06:33:18

What you observe seems valid.

You can't operate on the 1.3.6.4.1.5.1 managed object because it's not a value object, but rather sort of a "type" defining the properties of the "variables" that can derive from it. The former is called managed object, the latter is managed object instance in SNMP.

To address particular managed object instance you need to append one or more sub-OIDs to the managed object OID. This trailing part is known as managed object instance ID in SNMP.

In your implementation, managed object instance ID is derived from the "example record one" string. It takes at least as many elements in the trailing part of the OID as there are characters in the string. That explains why the resulting OID is that long.

SNMP Agent in Python with custom MIB

2 Answers