C++ SNMP Extension Agent - how to work with AsnSequence?

Question

What I need is to create SNMP agent for monitoring our software systems. I’m using "winsnmp" for writing my SNMP Extension Agent in Visual C++. The output is x64 DLL, which is registered in Windows registry, loaded by "SNMP Service" and properly executed. My SNMP agent is able to handle "SNMP_PDU_GET", "SNMP_PDU_GETNEXT", "SNMP_PDU_SET" requests as well as to generate SNMP Traps. Things are great so far, everything works fine.

But now I need to handle complex data type "sequence" which is presented as "AsnSequence" type or "ASN_SEQUENCE" type constant in "winsnmp". The requirement is to reply to SNMP manager with table, containing multiple records. The table has specific structure, here’s the sample MIB fragment:

...
      hrTestTable OBJECT-TYPE
        SYNTAX     SEQUENCE OF HrTestEntry
        MAX-ACCESS not-accessible
        STATUS     current
        DESCRIPTION
            "sequence"
        ::= { BMS_ibm_wsmq 3 }
      hrTestEntry OBJECT-TYPE
        SYNTAX     HrTestEntry
        MAX-ACCESS not-accessible
        STATUS     current
        DESCRIPTION
            "Table row"
        INDEX { hrTestIndex }
        ::= { hrTestTable 1 }
      HrTestEntry ::= SEQUENCE {
        hrTestIndex Integer32,
        hrTestType AutonomousType,
        hrTestDescr DisplayString
      }
      hrTestIndex OBJECT-TYPE
        SYNTAX     Integer32 (1..2147483647)
        MAX-ACCESS read-only
        STATUS     current
        DESCRIPTION
            "indexColumn1"
        ::= { hrTestEntry 1 }
      hrTestType OBJECT-TYPE
        SYNTAX     AutonomousType
        MAX-ACCESS read-only
        STATUS     current
        DESCRIPTION
            "column1"
        ::= { hrTestEntry 2 }
      hrTestDescr OBJECT-TYPE
        SYNTAX     DisplayString
        MAX-ACCESS read-only
        STATUS     current
        DESCRIPTION
            "column2"
        ::= { hrTestEntry 3 }
...

Once entire table is requested by SNMP manager, the agent receives "SNMP_PDU_GETNEXT" request:

BOOL SNMP_FUNC_TYPE SnmpExtensionQuery(BYTE operation, SnmpVarBindList *variableBindings, AsnInteger32 *errorStatus, AsnInteger32 *errorIndex) {
 …
 for (unsigned int index = 0; index < variableBindings->len; index++) {
  *errorStatus = SNMP_ERRORSTATUS_NOERROR;
  switch (operation) {
   …
   case SNMP_PDU_GETNEXT:
    …
    *errorStatus = GET_SEQUENCE(&variableBindings->list[index]);
    …
   …
  };
…

Variable

"&variableBindings->list[index].name" points to "hrTestIndex" at "index=0"
"&variableBindings->list[index].name" points to "hrTestType" at "index=1"
"&variableBindings->list[index].name" points to "hrTestDescr" at "index=2"

For all these cases "&variableBindings->list[index].value.asnType" is "ASN_NULL".

I suspect, that producing response "&variableBindings->list[index].value.asnType" has to initialized with "ASN_SEQUENCE" and "&variableBindings->list[index].value.asnValue.sequence" should be initialized with "AsnSequence".

So, am I right?

How do I initialize instance of "AsnSequence" and fill it with data then?

Thanks.

C++ SNMP Extension Agent - how to work with AsnSequence?

0 Answers0