# Advanced topics

## Specific SNMP Engine ID

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

- SNMPv3

- with SNMP EngineID: 8000000004030201

- with USM user ‘usr-md5-des’, auth: MD5, priv DES

- allow access to SNMPv2-MIB objects (1.3.6.1.2.1)

- over IPv4/UDP, listening at 127.0.0.1:161

The following Net-SNMP command will walk this Agent:

$ snmpwalk -v3 -u usr-md5-des -l authPriv -A authkey1 -X privkey1 -e 8000000004030201 localhost .1.3.6

    from pysnmp.entity import engine, config
    from pysnmp.entity.rfc3413 import cmdrsp, context
    from pysnmp.carrier.asyncore.dgram import udp
    from pysnmp.proto import rfc1902

    # Create SNMP engine
    snmpEngine = engine.SnmpEngine(rfc1902.OctetString(hexValue='8000000004030201'))

    # 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.transportDispatcher.closeDispatcher()
        raise

`Download` script.

## 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
    # noinspection PyUnusedLocal,PyUnusedLocal
    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

`Download` script.

## Multiple SNMP Engines

Run multiple SNMP Engines each with a complete Command Responder. Bind each SNMP Engine to a dedicated network transport endpoint:

- IPv4/UDP, listening at 127.0.0.1:161

- IPv4/UDP, listening at 127.0.0.2:161

Each Command Responder will respond to SNMP GET/SET/GETNEXT/GETBULK queries with the following options:

- SNMPv3

- with USM user ‘usr-md5-des’, auth: MD5, priv DES

- allow read access to SNMPv2-MIB objects (1.3.6)

- allow write access to SNMPv2-MIB objects (1.3.6.1.2.1)

The following Net-SNMP commands will walk the first and the second Agent respectively:

$ snmpwalk -Ob -v3 -u usr-md5-des -l authPriv -A authkey1 -X privkey1 127.0.0.1 usmUserEntry
$ snmpwalk -Ob -v3 -u usr-md5-des -l authPriv -A authkey1 -X privkey1 127.0.0.2 usmUserEntry

Notice differently configured snmpEngineId’s in usmUserEntry columns.

    from pysnmp.entity import engine, config
    from pysnmp.entity.rfc3413 import cmdrsp, context
    from pysnmp.proto import rfc1902
    from pysnmp.carrier.asyncore.dispatch import AsyncoreDispatcher
    from pysnmp.carrier.asyncore.dgram import udp

    # Configuration parameters for each of SNMP Engines
    snmpEngineInfo = (
        ('0102030405060708', udp.domainName + (0,), ('127.0.0.1', 161)),
        ('0807060504030201', udp.domainName + (1,), ('127.0.0.2', 161))
    )

    # Instantiate the single transport dispatcher object
    transportDispatcher = AsyncoreDispatcher()

    # Setup a custom data routing function to select snmpEngine by transportDomain
    transportDispatcher.registerRoutingCbFun(lambda td, t, d: td)

    # Instantiate and configure SNMP Engines
    for snmpEngineId, transportDomain, transportAddress in snmpEngineInfo:
        # Create SNMP engine with specific engineID
        snmpEngine = engine.SnmpEngine(rfc1902.OctetString(hexValue=snmpEngineId))

        # Register SNMP Engine object with transport dispatcher. Request incoming
        # data from specific transport endpoint to be funneled to this SNMP Engine.
        snmpEngine.registerTransportDispatcher(transportDispatcher, transportDomain)

        # Transport setup

        # UDP over IPv4
        config.addTransport(
            snmpEngine,
            transportDomain,
            udp.UdpTransport().openServerMode(transportAddress)
        )

        # 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 this user / securityModels at VACM
        config.addVacmUser(snmpEngine, 3, 'usr-md5-des', 'authPriv', (1, 3, 6), (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
    transportDispatcher.jobStarted(1)

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

`Download` script.

## Detailed VACM configuration

Serves MIB subtrees under different conditions:

- Respond to SNMPv2c commands

- with SNMP community “public”

- over IPv4/UDP, listening at 127.0.0.1:161

- Serve MIB under non-default contextName abcd

- Allow access to SNMPv2-MIB::system subtree

- Although deny access to SNMPv2-MIB::sysUpTime by a bit mask

- Use partial context name matching (a)

This example demonstrates detailed VACM configuration performed via low-level VACM calls: addContext, addVacmGroup, addVacmAccess and addVacmView. Each function populates one of the tables defined in SNMP-VIEW-BASED-ACM-MIB and used strictly as described in the above mentioned MIB.

The following Net-SNMP’s commands will GET a value at this Agent:

$ snmpget -v2c -c public 127.0.0.1 SNMPv2-MIB::sysLocation.0

However this command will fail:

$ snmpget -v2c -c public 127.0.0.1 SNMPv2-MIB::sysUpTime.0

This command will not reveal SNMPv2-MIB::sysUpTime.0 among other objects:

$ snmpwalk -v2c -c public 127.0.0.1 SNMPv2-MIB::system

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

    # Create SNMP engine with autogenernated engineID and pre-bound
    # to socket transport dispatcher
    snmpEngine = engine.SnmpEngine()

    # Transport setup

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

    # Register default MIB instrumentation controller with a new SNMP context

    contextName = 'abcd'

    snmpContext = context.SnmpContext(snmpEngine)

    snmpContext.registerContextName(
        contextName, snmpEngine.msgAndPduDsp.mibInstrumController)

    # Add new SNMP community name, map it to a new security name and
    # SNMP context

    securityName = 'my-area'
    communityName = 'public'

    config.addV1System(
        snmpEngine, securityName, communityName,
        contextEngineId=snmpContext.contextEngineId,
        contextName=contextName)

    # VACM configuration settings

    securityModel = 2  # SNMPv2c
    securityLevel = 1  # noAuthNoPriv

    vacmGroup = 'my-group'
    readViewName = 'my-read-view'

    # We will match by context name prefix
    contextPrefix = contextName[:1]

    # Populate SNMP-VIEW-BASED-ACM-MIB::vacmContextTable
    config.addContext(snmpEngine, contextName)

    # Populate SNMP-VIEW-BASED-ACM-MIB::vacmSecurityToGroupTable
    config.addVacmGroup(
        snmpEngine, vacmGroup, securityModel, securityName)

    # Populate SNMP-VIEW-BASED-ACM-MIB::vacmAccessTable
    config.addVacmAccess(
        snmpEngine, vacmGroup, contextPrefix, securityModel, securityLevel,
        'prefix', readViewName, '', '')

    # Populate SNMP-VIEW-BASED-ACM-MIB::vacmViewTreeFamilyTable

    # Allow the whole system subtree
    config.addVacmView(
        snmpEngine, readViewName, 'included', '1.3.6.1.2.1.1.1', '1.1.1.1.1.1.1.0')

    # ...but exclude one sub-branch (just one scalar OID)
    config.addVacmView(
        snmpEngine, readViewName, 'excluded', '1.3.6.1.2.1.1.3', '1.1.1.1.1.1.1.1')

    # Register SNMP Applications at the SNMP engine for particular SNMP context
    cmdrsp.GetCommandResponder(snmpEngine, snmpContext)
    cmdrsp.SetCommandResponder(snmpEngine, snmpContext)
    cmdrsp.NextCommandResponder(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 Exception:
        snmpEngine.transportDispatcher.closeDispatcher()
        raise

`Download` script.

See also: library reference.