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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, 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 or 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 snmpa_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 agent network interface behaviour.
The section Messages describes mandatory messages, which Net if must send and be able to receive.
Net if must send the following message when it receives an SNMP PDU from the network that is aimed for the MasterAgent:
MasterAgent ! {snmp_pdu, Vsn, Pdu, PduMS, ACMData, From, Extra}
Vsn is either 'version-1',
'version-2', or 'version-3'.
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
snmpa_mpd:process_packet (see Reference Manual).
ACMData is data used by the Access Control Module
in use. Normally this is returned from
snmpa_mpd:process_packet (see Reference Manual).
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', 'version-2', or
'version-3'.
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.
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',
'version-2', or 'version-3'.
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
snmpa_mpd:generate_response_message (see Reference Manual).
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',
'version-2', or 'version-3'.
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
snmpa_mpd:generate_response_message (see Reference Manual).
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, MsgData, To}
This message is sent from a master agent when a trap is
to be sent.
Vsn is either 'version-1',
'version-2', or 'version-3'.
Pdu is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response.
MsgData is the message specific data used in
the SNMP message. This value is normally sent to
snmpa_mpd:generate_message/4. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information.
To is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to snmpa_mpd:generate_message/4.
{send_pdu_req, Vsn, Pdu, MsgData, 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',
'version-2', or 'version-3'.
Pdu is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response.
MsgData is the message specific data used in
the SNMP message. This value is normally sent to
snmpa_mpd:generate_message/4. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information.
To is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to snmpa_mpd:generate_message/4.
Pid is a process identifier.
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 snmpa_mpd for this purpose (refer to the
Reference Manual, section snmp,
module snmpa_mpd
for more details.)
There are also some useful functions for encoding and decoding of SNMP messages in the module snmp_pdus.