Figure 13.1: 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.
The snmp application provides two different modules, snmpm_net_if (the default) and snmpm_net_if_mt, both uses UDP as the transport protocol i.e the transport domains transportDomainUdpIpv4 and/or transportDomainUdpIpv6. The difference between the two modules is that the latter is "multi-threaded", i.e. for each message/request a new process is created that processes the message/request and then exits.
It is also possible to write your own Net if process and this section describes how to do that.
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.
In this section a Domain field is the transport domain i.e one of transportDomainUdpIpv4 or transportDomainUdpIpv6, and an Addr field is an {IpAddr,IpPort} tuple.
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, Domain, Addr}
Pdu is an SNMP PDU record, as defined in snmp_types.hrl, with the SNMP request.
Domain is the source transport domain.
Addr is the source address.
Server ! {snmp_trap, Trap, Domain, Addr}
Trap is either an SNMP pdu record or an trappdu record, as defined in snmp_types.hrl, with the SNMP request.
Domain is the source transport domain.
Addr is the source address.
Server ! {snmp_inform, Ref, Pdu, PduMS, Domain, Addr}
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 (acknowledgment) 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.
Domain is the source transport domain.
Addr is the source address.
Server ! {snmp_report, Data, Domain, Addr}
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().
Domain is the source transport domain.
Addr is the source address.
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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