# Transport conversion

## IPv6-to-IPv4 conversion

Act as a local SNMPv1/v2c Agent listening on a UDP/IPv6 transport, relay messages to distant SNMPv1/2c Agent over UDP/IPv4 transport:

- with local SNMPv2c community ‘public’

- local Agent listening at [::1]:161

- remote SNMPv2c community ‘public’

- remote Agent listening at 104.236.166.95:161

This script can be queried with the following Net-SNMP command:

$ snmpget -v2c -c public udp6:[::1]:161 sysDescr.0

due to proxy, it is equivalent to

$ snmpget -v2c -c public 104.236.166.95:161 sysDescr.0

Warning: for production operation you would need to modify this script so that it will re-map possible duplicate request-ID values, coming in initial request PDUs from different Managers, into unique values to avoid sending duplicate request-IDs to Agents.

    from pysnmp.carrier.asyncore.dgram import udp, udp6
    from pysnmp.entity import engine, config
    from pysnmp.entity.rfc3413 import cmdrsp, cmdgen, context
    from pysnmp.proto.api import v2c
    from pysnmp import error

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

    #
    # Transport setup
    #

    # Agent section

    # UDP over IPv6
    config.addTransport(
        snmpEngine,
        udp6.domainName,
        udp6.Udp6Transport().openServerMode(('::1', 161))
    )

    # Manager section

    # UDP over IPv4
    config.addTransport(
        snmpEngine,
        udp.domainName,
        udp.UdpTransport().openClientMode()
    )

    #
    # SNMPv1/2c setup (Agent role)
    #

    # SecurityName <-> CommunityName mapping
    config.addV1System(snmpEngine, '1-my-area', 'public')

    #
    # SNMPv1/v2c setup (Manager role)
    #
    # Here we configure securityName lexicographically lesser than '1-my-area'
    # to let it match first in snmpCommunityTable on response processing.
    #

    config.addV1System(snmpEngine, '0-distant-area', 'public', transportTag='remote')

    #
    # Transport target used by Manager
    #

    config.addTargetParams(
        snmpEngine, 'distant-agent-auth', '0-distant-area', 'noAuthNoPriv', 1
    )
    config.addTargetAddr(
        snmpEngine, 'distant-agent',
        udp.domainName, ('104.236.166.95', 161),
        'distant-agent-auth', retryCount=0, tagList='remote'
    )

    # Default SNMP context
    config.addContext(snmpEngine, '')

    class CommandResponder(cmdrsp.CommandResponderBase):
        cmdGenMap = {
            v2c.GetRequestPDU.tagSet: cmdgen.GetCommandGenerator(),
            v2c.SetRequestPDU.tagSet: cmdgen.SetCommandGenerator(),
            v2c.GetNextRequestPDU.tagSet: cmdgen.NextCommandGeneratorSingleRun(),
            v2c.GetBulkRequestPDU.tagSet: cmdgen.BulkCommandGeneratorSingleRun()
        }
        pduTypes = cmdGenMap.keys()  # This app will handle these PDUs

        # SNMP request relay
        def handleMgmtOperation(self, snmpEngine, stateReference, contextName,
                                PDU, acInfo):
            cbCtx = stateReference, PDU
            contextEngineId = None  # address authoritative SNMP Engine
            try:
                self.cmdGenMap[PDU.tagSet].sendPdu(
                    snmpEngine, 'distant-agent',
                    contextEngineId, contextName,
                    PDU,
                    self.handleResponsePdu, cbCtx
                )
            except error.PySnmpError:
                self.handleResponsePdu(
                    snmpEngine, stateReference, 'error', None, cbCtx
                )

        # SNMP response relay
        # noinspection PyUnusedLocal
        def handleResponsePdu(self, snmpEngine, sendRequestHandle,
                              errorIndication, PDU, cbCtx):
            stateReference, reqPDU = cbCtx

            if errorIndication:
                PDU = v2c.apiPDU.getResponse(reqPDU)
                PDU.setErrorStatus(PDU, 5)

            self.sendPdu(
                snmpEngine, stateReference, PDU
            )

            self.releaseStateInformation(stateReference)

    CommandResponder(snmpEngine, context.SnmpContext(snmpEngine))

    snmpEngine.transportDispatcher.jobStarted(1)  # this job would never finish

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

`Download` script.

See also: library-reference.