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Adding SNMP to anything with Redis and Python

Bolting an SNMP Server and stats collection onto your existing Python projects.

I’ve been adding SNMP support to an open source project I’ve been working on (PyHSS) to generate metrics / performance statistics from it, and this meant staring down SNMP again, but this time I’ve come up with a novel way to handle SNMP, that made it much less painful that normal.

The requirement was simple enough, I already had a piece of software I’d written in Python, but I had a need to add an SNMP server to get information about that bit of software.

For a little more detail – PyHSS handles Device Watchdog Requests already, but I needed a count of how many it had handled, made accessible via SNMP. So inside the logic that does this I just increment a counter in Redis;

#Device Watchdog Answer
    def Answer_280(self, packet_vars, avps):                                                      
        self.redis_store.incr('Answer_280_attempt_count')

In the code example above I just add 1 (increment) the Redis key ‘Answer_280_attempt_count’.

The beauty is that that this required minimal changes to the rest of my code – I just sprinkled in these statements to increment Redis keys throughout my code.

Now when that existing function is run, the Redis key “Answer_280_attempt_count” is incremented.

So I ran my software and the function I just added the increment to was called a few times, so I jumped into redis-cli to check on the values;

GIF showing using Redis-CLI to get a value

And just like that we’ve done all the heavy lifting to add SNMP to our software.

For anything else we want counters on, add the increment to your code to store a counter in Redis with that information.

So next up we need to expose our Redis keys via SNMP,

For this, I took a simple SNMP server example from Stackoverflow, to set the output of a MIB tree, and simply bolted in getting a bit of data from, code below:

#Pulled from https://stackoverflow.com/questions/58909285/how-to-add-variable-in-the-mib-tree

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


import redis
redis_store = redis.Redis(host='localhost', port=6379, db=0)
# Create SNMP engine
snmpEngine = engine.SnmpEngine()

# Transport setup

# UDP over IPv4
config.addTransport(
    snmpEngine,
    udp.domainName,
    udp.UdpTransport().openServerMode(('127.0.0.1', 1161))
)

# SNMPv3/USM setup

# user: usr-md5-none, auth: MD5, priv NONE
config.addV3User(
    snmpEngine, 'usr-md5-none',
    config.usmHMACMD5AuthProtocol, 'authkey1'
)
# Allow full MIB access for each user at VACM
config.addVacmUser(snmpEngine, 3, 'usr-md5-none', 'authNoPriv', (1, 3, 6, 1, 2, 1), (1, 3, 6, 1, 2, 1))


# SNMPv2c setup

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

# Allow full MIB access for this user / securityModels at VACM
config.addVacmUser(snmpEngine, 2, 'my-area', 'noAuthNoPriv', (1, 3, 6, 1, 2, 1), (1, 3, 6, 1, 2, 1))

# Get default SNMP context this SNMP engine serves
snmpContext = context.SnmpContext(snmpEngine)


# Create an SNMP context with default ContextEngineId (same as SNMP engine ID)
snmpContext = context.SnmpContext(snmpEngine)

# Create multiple independent trees of MIB managed objects (empty so far)
mibTreeA = instrum.MibInstrumController(builder.MibBuilder())
mibTreeB = instrum.MibInstrumController(builder.MibBuilder())

# Register MIB trees at distinct SNMP Context names
snmpContext.registerContextName(v2c.OctetString('context-a'), mibTreeA)
snmpContext.registerContextName(v2c.OctetString('context-b'), mibTreeB)

mibBuilder = snmpContext.getMibInstrum().getMibBuilder()

MibScalar, MibScalarInstance = mibBuilder.importSymbols(
    'SNMPv2-SMI', 'MibScalar', 'MibScalarInstance'
)
class MyStaticMibScalarInstance(MibScalarInstance):
    def getValue(self, name, idx):
        currentDT = datetime.datetime.now()
        return self.getSyntax().clone(
            'Hello World!! It\'s currently: ' + str(currentDT)
        )

class AnotherStaticMibScalarInstance(MibScalarInstance):
    def getValue(self, name, idx):
        return self.getSyntax().clone('Ahoy hoy?')

class Answer_280_attempt_count(MibScalarInstance):
    def getValue(self, name, idx):
        return self.getSyntax().clone(redis_store.get('Answer_280_attempt_count'))


mibBuilder.exportSymbols(
    '__MY_MIB', MibScalar((1, 3, 6, 1, 2, 1, 1, 1), v2c.OctetString()),
    MyStaticMibScalarInstance((1, 3, 6, 1, 2, 1, 1, 1), (0,), v2c.OctetString()),
    AnotherStaticMibScalarInstance((1, 3, 6, 1, 2, 1, 1, 1), (0,1), v2c.OctetString()),
    Answer_280_attempt_count((1, 3, 6, 1, 2, 1, 1, 1), (0,2), v2c.Integer32())
)

# 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

While PySNMP can be a bit much to wrap your head around, all you need to know:

V2 community string set in:

config.addV1System(snmpEngine, 'my-area', 'public')

Create an additional class from the template below for each of your Redis keys you wish to expose;

class something_else_from_Redis(MibScalarInstance):
    def getValue(self, name, idx):
        return self.getSyntax().clone(redis_store.get('something_else_from_Redis'))

Renaming the class and replacing the redis_store.get() value with the Redis key you want to pull,

And finally inside mibBuilder.exportSymbols() add each of the new classes you added and the OID for each;

    Answer_280_attempt_count((1, 3, 6, 1, 2, 1, 1, 1), (0,2), v2c.Integer32())
    something_else_from_Redis((1, 3, 6, 1, 2, 1, 1, 1), (0,3), v2c.Integer32())

Then when you run it, presto, you’re exposing that data via SNMP.

You can verify it through SNMP walk or start integrating it into your NMS, in the above example OID 1.3.6.1.2.1.1.1.0.2, contains the value of Answer_280_attempt_count from Redis, and with that, you’re exposing info via SNMP, all while not really having to think about SNMP.

*Ok, you still have to sort which OIDs you assign for what, but you get the idea.

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