10 Definition of Net if
The Purpose of Net ifThe 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 thesnmp_agent
process.10.1.1 Function
Module:start_link(MasterAgent)
10.1.2 Arguments
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 insnmp_types.hrl
, with the SNMP request.PduMS
is the Maximum Size of the response Pdu allowed. Normally this is returned fromsnmp_mpc:process_packet
(see further below).ACMData
is data used by the Access Control Module in use. Normally this is returned fromsnmp_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}
, whereIP
is a 4-tuple with the IP address, andUDPport
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 callingsnmp: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 thesend_pdu_req
message (see below).Vsn
is either'version-1'
or'version-2
.Pdu
is the SNMP Pdu receivedFrom
is the source address. If UDP over IP is used, this should be a 2-tuple{IP, UDPport}
, whereIP
is a 4-tuple with the IP address, andUDPport
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 tosnmp_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}
, whereIP
is a 4-tuple with the IP address, andUDPport
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 tosnmp_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.To
is 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 asnmp_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.To
is 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
.