#
# Command Generator
#
# Send SNMP GET request using the following scenario and options:
#
# * try to communicate with a SNMPv3 Engine using:
# ** a non-existing user
# ** over IPv4/UDP
# ** to an Agent at demo.snmplabs.com:161
# * if remote SNMP Engine ID is discovered, send SNMP GET request:
# ** with SNMPv3, user 'usr-md5-none', MD5 authentication, no privacy
#    at discovered securityEngineId
# ** to the same SNMP Engine ID
# ** for an OID in text form
#
from pysnmp.entity import engine
from pysnmp.entity.rfc3413.oneliner import cmdgen

snmpEngine = engine.SnmpEngine()

cmdGen = cmdgen.CommandGenerator(snmpEngine)

transportTarget = cmdgen.UdpTransportTarget(('demo.snmplabs.com', 161))

#
# To discover remote SNMP EngineID we will tap on SNMP engine inner workings
# by setting up execution point observer setup on INTERNAL class PDU processing
#

observerContext = {}

# Register a callback to be invoked at specified execution point of 
# SNMP Engine and passed local variables at execution point's local scope
snmpEngine.observer.registerObserver(
    lambda e,p,v,c: c.update(securityEngineId=v['securityEngineId']),
    'rfc3412.prepareDataElements:internal',
    cbCtx=observerContext
)

# Send probe SNMP request with invalid credentials

authData = cmdgen.UsmUserData('non-existing-user')

errorIndication, errorStatus, errorIndex, varBinds = cmdGen.getCmd(
    authData, transportTarget
)

# See if our SNMP engine received REPORT PDU containing securityEngineId

if 'securityEngineId' not in observerContext:
    print("Can't discover peer EngineID, errorIndication: %s" % errorIndication)
    raise Exception()

securityEngineId = observerContext.pop('securityEngineId')

print('Remote securityEngineId = %s' % securityEngineId.prettyPrint())

#
# Query remote SNMP Engine using usmUserTable entry configured for it
#

authData = cmdgen.UsmUserData('usr-md5-none', 'authkey1', 
                               securityEngineId=securityEngineId)

errorIndication, errorStatus, errorIndex, varBinds = cmdGen.getCmd(
    authData, transportTarget, '1.3.6.1.2.1.1.1.0'
)

# Check for errors and print out results
if errorIndication:
    print(errorIndication)
else:
    if errorStatus:
        print('%s at %s' % (
            errorStatus.prettyPrint(),
            errorIndex and varBinds[int(errorIndex)-1][0] or '?'
            )
        )
    else:
        for name, val in varBinds:
            print('%s = %s' % (name.prettyPrint(), val.prettyPrint()))