"""
Observe SNMP engine operations
++++++++++++++++++++++++++++++

Listen and respond to SNMP GET/SET/GETNEXT/GETBULK queries with
the following options:

* SNMPv3
* with USM user 'usr-md5-des', auth: MD5, priv DES or
* allow access to SNMPv2-MIB objects (1.3.6.1.2.1)
* over IPv4/UDP, listening at 127.0.0.1:161
* registers its own execution observer to snmpEngine

The following Net-SNMP command will walk this Agent:

| $ snmpwalk -v3 -u usr-md5-des -l authPriv -A authkey1 -X privkey1 localhost .1.3.6

This script will report some details on request processing as seen
by rfc3412.receiveMessage() and rfc3412.returnResponsePdu()
abstract interfaces.

"""#
from pysnmp.entity import engine, config
from pysnmp.entity.rfc3413 import cmdrsp, context
from pysnmp.carrier.asyncore.dgram import udp

# Create SNMP engine
snmpEngine = engine.SnmpEngine()

# Execution point observer setup

# Register a callback to be invoked at specified execution point of 
# SNMP Engine and passed local variables at code point's local scope
def requestObserver(snmpEngine, execpoint, variables, cbCtx):
    print('Execution point: %s' % execpoint)
    print('* transportDomain: %s' % '.'.join([str(x) for x in variables['transportDomain']]))
    print('* transportAddress: %s (local %s)' % ('@'.join([str(x) for x in variables['transportAddress']]), '@'.join([str(x) for x in variables['transportAddress'].getLocalAddress()])))
    print('* securityModel: %s' % variables['securityModel'])
    print('* securityName: %s' % variables['securityName'])
    print('* securityLevel: %s' % variables['securityLevel'])
    print('* contextEngineId: %s' % variables['contextEngineId'].prettyPrint())
    print('* contextName: %s' % variables['contextName'].prettyPrint())
    print('* PDU: %s' % variables['pdu'].prettyPrint())

snmpEngine.observer.registerObserver(
    requestObserver,
    'rfc3412.receiveMessage:request',
    'rfc3412.returnResponsePdu'
)

# Transport setup

# UDP over IPv4
config.addTransport(
    snmpEngine,
    udp.domainName,
    udp.UdpTransport().openServerMode(('127.0.0.1', 161))
)

# SNMPv3/USM setup

# user: usr-md5-des, auth: MD5, priv DES
config.addV3User(
    snmpEngine, 'usr-md5-des',
    config.usmHMACMD5AuthProtocol, 'authkey1',
    config.usmDESPrivProtocol, 'privkey1'
)

# Allow full MIB access for each user at VACM
config.addVacmUser(snmpEngine, 3, 'usr-md5-des', 'authPriv', (1,3,6,1,2,1), (1,3,6,1,2,1)) 

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

# 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.observer.unregisterObserver()
    snmpEngine.transportDispatcher.closeDispatcher()
    raise
