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The Network Interface (Net if) process delivers SNMP PDUs to the manager server, and receives SNMP PDUs from the manager server. The most common behaviour of a Net if process is that is receives request PDU from the manager server, encodes the PDU into bytes and transmits the bytes onto the network to an agent. When the reply from the agent is received by the Net if process, which it decodes into an SNMP PDU, which it sends to the manager server.
However, that simple behaviour can be modified in numerous ways. For example, the Net if process can apply some kind of encrypting/decrypting scheme on the bytes.
It is also possible to write your own Net if process. The default
Net if process is implemented in the module snmpm_net_if and
it uses UDP as the transport protocol.
This section describes how to write a Net if process.
A Net if process must implement the SNMP manager network interface behaviour.
The section Messages describes mandatory messages, which Net if must send to the manager server process.
Net if must send the following message when it receives an SNMP PDU from the network that is aimed for the MasterAgent:
Server ! {snmp_pdu, Pdu, Addr, Port}
Pdu is an SNMP PDU record, as defined in
snmp_types.hrl, with the SNMP request.
Addr is the source address.
Port is port number of the sender.
Server ! {snmp_trap, Trap, Addr, Port}
Trap is aither an SNMP pdu record or an trappdu record,
as defined in snmp_types.hrl, with the SNMP request.
Addr is the source address.
Port is port number of the sender.
Server ! {snmp_inform, Ref, Pdu, PduMS, Addr, Port}
Ref is either the atom ignore or something
that can be used to identify the inform-request (e.g. request-id).
ignore is used if the response (acknowledgement) to the
inform-request has already been sent (this means that the server
will not make the call to the
inform_response
function). See the
inform request behaviour
configuration option for more info.
Pdu is an SNMP PDU record, as defined in
snmp_types.hrl, with the SNMP request.
Addr is the source address.
Port is port number of the sender.
Server ! {snmp_report, Data, Addr, Port}
Data is either {ok, Pdu} or
{error, ReqId, ReasonInfo, Pdu}. Which one is used depends
on the return value from the MPD
process_msg function. If the MsgData is ok,
the first is used, and if it is {error, ReqId, Reason}
the latter is used.
Pdu is an SNMP PDU record, as defined in
snmp_types.hrl, with the SNMP request.
ReqId is an integer.
ReasonInfo is a term().
Addr is the source address.
Port is port number of the sender.
Since the Net if process is responsible for encoding and
decoding of SNMP messages, it must also update the relevant
counters in the SNMP group in MIB-II. It can use the functions
in the module snmpm_mpd for this purpose (refer to the
Reference Manual, section snmp, module snmpm_mpd
for more details).
There are also some useful functions for encoding and
decoding of SNMP messages in the module snmp_pdus.