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NAME

       snmpa_network_interface_filter  -  Behaviour  module  for the SNMP agent network-interface
       filter.

DESCRIPTION

       This  module  defines  the  behaviour  of  the   agent   network   interface   filter.   A
       snmpa_network_interface_filter compliant module must export the following functions:

         * accept_recv/2

         * accept_send/2

         * accept_recv_pdu/3

         * accept_send_pdu/2

       The semantics of them and their exact signatures are explained below.

       The  purpose of the network interface filter is to allow for filtering of messages (accept
       or reject) receive and send. This is done on two levels:

         * The first level is at the UDP entry / exit point, i.e. immediately after  the  receipt
           of  the  message,  before any message processing is done (accept_recv) and immediately
           before sending the message, after all message processing is done (accept_send).

         * The second level is at the MPD entry / exit point, i.e. immediately  after  the  basic
           message processing (accept_recv_pdu) / immediately before the basic message processing
           (accept_send_pdu).

       Note that the network interface filter is something which is used by the network interface
       implementation  provided by the application (snmpa_net_if). The default filter accepts all
       messages.

       A network interface filter can e.g. be used during testing or for load regulation. If  the
       intended    use    is   load   regulation,   see   also   req_limit   and   the   function
       register_notification_filter.

DATA TYPES

       port() = integer() > 0
       pdu_type() = 'get-request' | 'get-next-request' | 'get-response' |
                    'set-request' | trap | 'get-bulk-request' | 'inform-request' |
                    report

EXPORTS

       accept_recv(Ip, Port) -> boolean()

              Types:

                 Ip = ip_address()
                 Port = port()

              Called at the reception of a message (before any processing has been done).

              For the message to be discarded, the function must return false.

       accept_send(Ip, Port) -> boolean()

              Types:

                 Ip = ip_address()
                 Port = port()

              Called before the sending of a message (after all processing has been done).

              For the message to be discarded, the function must return false.

       accept_recv_pdu(Ip, Port, PduType) -> boolean()

              Types:

                 Ip = ip_address()
                 Port = port()
                 PduType = pdu_type()

              Called after the basic message processing (MPD) has been done, but before  the  pdu
              is handed over to the master-agent for primary processing.

              For the pdu to be discarded, the function must return false.

       accept_send_pdu(Targets, PduType) -> Reply

              Types:

                 Targets = targets()
                 targets() = [target()]
                 target() = {ip_address(), port()}
                 PduType = pdu_type() > 0
                 Reply = boolean() | NewTargets
                 NewTargets = targets()

              Called  before  the  basic  message  processing  (MPD) is done, when a pdu has been
              received from the master-agent.

              For the message to be discarded all together, the function must return false.

              Note that it is possible for this function to filter out targets (but not  add  its
              own) by returning an updated Targets list (NewTargets).