C NET-SNMP Get and Set specifically via MIB Name, Not OID

Question

I have written and am testing software for a generic SNMP client module in C as well as an implementation using this generic module. I am having trouble getting a get request to work by passing in a MIB name(e.g. sysDescr) instead of an OID(e.g. 1.3.6.1.2.1.1.1).

I am successful when I pass in a character array containing the OID to _snmp_parse_oid()_ but not the name.

I have checked the MIB file to make sure I am using the correct name. When I run the command line SNMP translate on the name it gives me the OID listed above:

$ snmptranslate -m +<MIB File> -IR -On <MIB Name>
.#.#.#.#.#.#.#####.#.#.#.#.#.#

(In the above command I replaced my actual mib file with <MIB File>, mib name with <MIB Name>, and OID numbers returned from the command with # characters)

The following is my code for my generic SNMP get function, assume returned values are #define numbers and I have removed some error handling for brevity:

/// @Synopsis        Function to send out get request since the 
///                  SNMPOidData object has been setup
///
/// @Param oid_name  String containing the OID to set
/// @Param value     Value to set
///
/// @Returns         Error
int snmpGet(SNMPAgent *this, char const * const oid_name, SNMPOidData * value)
{
    netsnmp_pdu           *pdu;
    netsnmp_pdu           *response;
    netsnmp_variable_list *vars;
    oid                   *retrieved_oid;
    oidStruct             oid_to_get;
    int                   status   = 0;
    int                   result   = ERROR_SUCCESS;

    // Create the PDU for the data for our request
    pdu = snmp_pdu_create(SNMP_MSG_GET);

    oid_to_get.OidLen = MAX_OID_LEN; // Set max length

    // Send out the request(s)
    retrieved_oid = snmp_parse_oid(oid_name, oid_to_get.Oid, &oid_to_get.OidLen);

    // Set the data
    snmp_add_null_var(pdu, oid_to_get.Oid, oid_to_get.OidLen))

    // Send the request out
    status = snmp_synch_response(this->port.snmp_session_handle, pdu, &response);
    if (STAT_SUCCESS == status)
    {
        if (SNMP_ERR_NOERROR == response->errstat)
        {
            vars = response->variables;
            value->type = vars->type;

            if (vars->next_variable != NULL)
            {
                // There are more values, set return type to null
                value->type = ASN_NULL;
            }
            else if (!(CHECK_END(vars->type))) // Exception
            {
                result = ERROR_NOT_PRESENT;
                fprintf(stderr, "Warning: OID=%s gets snmp exception %d \n",
                    oid_name, vars->type);
            }
            else if ((vars->type == ASN_INTEGER)
                  || (vars->type == ASN_COUNTER)
                  || (vars->type == ASN_UNSIGNED))
            {
                value->integer = *(vars->val.integer);
                value->str_len = sizeof(value->integer);
            }
            else
            {
                value->str_len = vars->val_len;

                if (value->str_len >= MAX_ASN_STR_LEN)
                    value->str_len = MAX_ASN_STR_LEN;

                if (value->str_len > 0)
                    memcpy(value->string, vars->val.string, value->str_len);

                // guarantee NULL terminated string
                value->string[value->str_len] = '\0';
            }
        }
    }

    this->freePDU(response); // Clean up: free the response

    return result;
}

The error I am getting:

oid_name: Unknown Object Identifier (Sub-id not found: (top) -> <MIB Name>)

Which comes from the following call:

retrieved_oid = snmp_parse_oid(oid_name, oid_to_get.Oid, &oid_to_get.OidLen);

I have made sure that the MIB files are on the machine in the configured place (snmptranslate wouldn't work if this weren't the case).

I have spent a good amount of time on Google results as well as directly searching here on Stack Overflow. The following is a good tutorial but does not address my issue (they directly reference the OID they want to get the value of): http://www.net-snmp.org/wiki/index.php/TUT:Simple_Application

Any help or insight would be much appreciated.

Some other info I can think of is that this is being compiled to run on an armv5tejl target running Linux communicating with an external device via ethernet.

Thanks,

Answer 1

When I call MIB variables by their string name I use the following net-snmp functions.

read_objid(OID, anOID, &anOID_len);
snmp_add_null_var(pdu, anOID, anOID_len);

Where:

oid anOID[MAX_OID_LEN];
size_t anOID_len = MAX_OID_LEN;

In my program I pack this all into a single function call.

void packSingleGetOID(const char *OID, struct snmp_pdu *pdu){
    // OID in / PDU out
    oid anOID[MAX_OID_LEN];
    size_t anOID_len = MAX_OID_LEN;

    read_objid(OID, anOID, &anOID_len);
    snmp_add_null_var(pdu, anOID, anOID_len);
}

I pass in the MIB OID string and the pointer to session pdu. Remember the OID string is MIB_Name::variable.