Definition of Net if

10 Definition of Net if

The Purpose of Net if
The Network Interface (Net if) process delivers SNMP PDUs to a master agent, and receives SNMP PDUs from the master agent. The most common behaviour of a Net if process is that is receives bytes from a network, decodes them into an SNMP PDU which it sends to a master agent. When the master agent has processed the PDU, it sends a response PDU to the Net if process, which encodes the PDU into bytes and transmits the bytes onto the network.
However, this 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, or the Net if process can act as a proxy filter which sends some packets to a proxy agent and some packets to the master agent.
It is also possible to write your own Net if process. The default Net if process is implemented in the module snmp_net_if and it uses UDP as the transport protocol.
This section describes how to write a Net if process.
10.1 Mandatory Functions
A Net if process must be implemented in a module which exports the Module:start_link/1 function. This function starts a new Net if process. The name of the Net if module is passed as a start argument to the snmp_agent process.
10.1.1 Function
Module:start_link(MasterAgent)
10.1.2 Arguments
MasterAgent is a Pid.
10.1.3 Return values
The return values are:

{ok, Pid}, where Pid is a linked Pid of the Net if process.
{error, Reason} if the operation fails.

10.2 Messages
This section describes mandatory messages which Net if must send and be able to receive.
10.2.1 Outgoing Messages
Net if must send the following message when it receives an SNMP PDU from the network which is aimed for the MasterAgent:
MasterAgent ! {snmp_pdu, Vsn, Pdu, PduMS, ACMData, From, Extra}
      
Vsn is either 'version-1' or 'version-2.
Pdu is an SNMP PDU record, as defined in snmp_types.hrl, with the SNMP request.
PduMS is the Maximum Size of the response Pdu allowed. Normally this is returned from snmp_mpc:process_packet (see further below).
ACMData is data used by the Access Control Module in use. Normally this is returned from snmp_mpc:process_packet (see further below).
From is the source address. If UDP over IP is used, this should be a 2-tuple {IP, UDPport}, where IP is a 4-tuple with the IP address, and UDPport is an integer.
Extra is any term the Net if process wishes to send to the agent. This term can be retrieved by the instrumentation functions by calling snmp:current_net_if_data(). This data is also sent back to the Net if process when the agent generates a response to the request.

The following message is used to report that a response to a request has been received. The only request an agent can send is an Inform-Request.
Pid ! {snmp_response_received, Vsn, Pdu, From}
      
Pid is the Process that waits for the response for the request. The Pid was specified in the send_pdu_req message (see below).
Vsn is either 'version-1' or 'version-2.
Pdu is the SNMP Pdu received
From is the source address. If UDP over IP is used, this should be a 2-tuple {IP, UDPport}, where IP is a 4-tuple with the IP address, and UDPport is an integer.

10.2.2 Incoming Messages
This section describes the incoming messages which a Net if process must be able to receive.

{snmp_response, Vsn, Pdu, Type, ACMData, To, Extra} This message is sent to the Net if process from a master agent as a response to a previously received request.

Vsn is either 'version-1' or 'version-2.
Pdu is an SNMP PDU record (as defined in snmp_types.hrl) with the SNMP response.
Type is the #pdu.type of the original request.
ACMData is data used by the Access Control Module in use. Normally this is just sent to snmp_mpc:generate_response_message (see further below).
To is the destination address. If UDP over IP is used, this should be a 2-tuple {IP, UDPport}, where IP is a 4-tuple with the IP address, and UDPport is an integer.
Extra is the term that the Net if process sent to the agent when the request was sent to the agent.

{discarded_pdu, Vsn, ReqId, ACMData, Variable, Extra} This message is sent from a master agent if it for some reason decided to discard the pdu.

Vsn is either 'version-1' or 'version-2.
ReqId is the request id of the original request.
ACMData is data used by the Access Control Module in use. Normally this is just sent to snmp_mpc:generate_response_message (see further below).
Variable is the name of an snmp counter that represents the error, e.g. snmpInBadCommunityUses.
Extra is the term that the Net if process sent to the agent when the request was sent to the agent.

{send_pdu, Vsn, Pdu, SecData, To} This message is sent from a master agent when a trap is to be sent.

Vsn is either 'version-1' or 'version-2.
Pdu is an SNMP PDU record (as defined in snmp_types.hrl) with the SNMP response.
SecData is the security data used in the SNMP message. In SNMPv1 and SNMPv2c, this security data is the community string.
Tois a list of the destination addresses. If UDP over IP is used, each address should be a 4-tuple with the IP address.

{send_pdu_req, Vsn, Pdu, SecData, To, Pid} This message is sent from a master agent when a request is to be sent. The only request an agent can send is Inform-Request. The net if process needs to remember the request id and the Pid, and when a response is received for the request id, send it to Pid, using a snmp_response_received message.

Vsn is either 'version-1' or 'version-2.
Pdu is an SNMP PDU record (as defined in snmp_types.hrl) with the SNMP response.
SecData is the security data used in the SNMP message. In SNMPv1 and SNMPv2c, this security data is the community string.
Tois a list of the destination addresses. If UDP over IP is used, each address should be a 4-tuple with the IP address.
Pid is a process identifier.

10.2.3 Notes
As 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 snmp_mpc for this purpose.
There are also some useful functions for encoding and decoding of SNMP messages in the module snmp_pdus.