Definition of Manager Net if

## 13 Definition of Manager Net if

The Purpose of Manager Net if

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.

### 13.1 Mandatory Functions

A Net if process must implement the SNMP manager network interface behaviour.

### 13.2 Messages

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.

#### 13.2.1 Notes

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`.

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