C: How to access value returned by Net-SNMP GET

Question

I apologize for the naive question, Iam new to Net-SNMP. I have tried using this simple SNMP demo app given in Net-SNMP website.

This code performs a SNMP-GET and manipulates the response to check if the value returned is a ASN_OCTET_STRING , and if yes, access the string using vars->val.string and assigned to a character pointer sp .

But Iam unable to figure out how to access this value if the type is anything other than ASN_OCTET_STRING . For example how do I take this value and, say, assign it to a variable if it is of type 'ASN_INTEGER' or 'ASN_OBJECT_ID' .

#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <string.h>
#define DEMO_USE_SNMP_VERSION_3

#ifdef DEMO_USE_SNMP_VERSION_3
const char *our_v3_passphrase = "MD5Password";
#endif

int main(int argc, char ** argv)
{
    netsnmp_session session, *ss;
    netsnmp_pdu *pdu;
    netsnmp_pdu *response;

    oid anOID[MAX_OID_LEN];
    size_t anOID_len;

    netsnmp_variable_list *vars;
    int status;
    int count=1;

    init_snmp("snmpdemoapp");

    snmp_sess_init( &session );                   
    session.peername = strdup("localhost:161");


#ifdef DEMO_USE_SNMP_VERSION_3

    session.version=SNMP_VERSION_3;

    session.securityName = strdup("user2");
    session.securityNameLen = strlen(session.securityName);

    session.securityLevel = SNMP_SEC_LEVEL_AUTHNOPRIV;

    session.securityAuthProto = usmHMACMD5AuthProtocol;
    session.securityAuthProtoLen = sizeof(usmHMACMD5AuthProtocol)/sizeof(oid);
    session.securityAuthKeyLen = USM_AUTH_KU_LEN;

    if (generate_Ku(session.securityAuthProto,
                    session.securityAuthProtoLen,
                    (u_char *) our_v3_passphrase, strlen(our_v3_passphrase),
                    session.securityAuthKey,
                    &session.securityAuthKeyLen) != SNMPERR_SUCCESS) {
        snmp_perror(argv[0]);
        snmp_log(LOG_ERR,
                 "Error generating Ku from authentication pass phrase. \n");
        exit(1);
    }

#else /* we'll use the insecure (but simplier) SNMPv1 */

    session.version = SNMP_VERSION_1;

    session.community = "demopublic";
    session.community_len = strlen(session.community);

#endif /* SNMPv1 */

    SOCK_STARTUP;
    ss = snmp_open(&session);                     
    if (!ss) {
      snmp_sess_perror("ack", &session);
      SOCK_CLEANUP;
      exit(1);
    }

    pdu = snmp_pdu_create(SNMP_MSG_GET);
    anOID_len = MAX_OID_LEN;
    if (!snmp_parse_oid("ip.21.1.8.xx.xx.xx.xx", anOID, &anOID_len)) {
      snmp_perror("ip.21.1.8.xx.xx.xx.xx");
      SOCK_CLEANUP;
      exit(1);
    }

    snmp_add_null_var(pdu, anOID, anOID_len);

    status = snmp_synch_response(ss, pdu, &response);

    if (status == STAT_SUCCESS && response->errstat == SNMP_ERR_NOERROR) {

      for(vars = response->variables; vars; vars = vars->next_variable)
        print_variable(vars->name, vars->name_length, vars);

      /* manipuate the information ourselves */
      for(vars = response->variables; vars; vars = vars->next_variable) {



        if (vars->type == ASN_OCTET_STR) {
      char *sp = (char *)malloc(1 + vars->val_len);
      memcpy(sp, vars->val.string, vars->val_len);
      sp[vars->val_len] = '\0';
          printf("value #%d is a string: %s\n", count++, sp); //Here sp now has the string - But this doesnt work when the string is for eg."HOST-RESOURCES-MIB::hrSWInstalledDate.1953 = STRING: 0-1-1,0:0:0.0"
      free(sp);
    }


        else if(vars->type == ASN_INTEGER) {
          printf("value is an Integer\n");
          int ObjVal;
          // How do I get the Integer value and assign it to 'ObjVal'
        }


        else if(vars->type == ASN_OBJECT_ID) {
          printf("value is an OID\n");
          // How do I get the OID and assign it to some variable
        }


        else if(vars->type == ASN_TIMETICKS) {
          printf("value is in Timeticks\n");
          // How do I get the Timeticks and assign it to some variable for further processing
        }



      }
    } else {
      if (status == STAT_SUCCESS)
        fprintf(stderr, "Error in packet\nReason: %s\n",
                snmp_errstring(response->errstat));
      else if (status == STAT_TIMEOUT)
        fprintf(stderr, "Timeout: No response from %s.\n",
                session.peername);
      else
        snmp_sess_perror("snmpdemoapp", ss);

    }

    if (response)
      snmp_free_pdu(response);
    snmp_close(ss);

    SOCK_CLEANUP;
    return (0);
} 

Tried vars->val.integer or vars->val.object_id , but that doesnot contain the value. What am I missing here?

My another question is, even when it is of type ASN_OCTET_STRING , when the GET reply is something like this,

HOST-RESOURCES-MIB::hrSWInstalledDate.1953 = STRING: 0-1-1,0:0:0.0

then vars->val.string doesnt have "0-1-1,0:0:0.0" as string.

Basically my question is How does the value get stored in the response structure from which I can retrieve the values?

Thanks in Advance!!

PS: Makefile link from Net-SNMP website.

Edit1: For Integers, i can read using *vars->val->string as pointed out by immibis . Any Ideas about how to access other datatypes?

Answer 1

As you can see in /usr/include/net-snmp/types.h file or similar on your system, net-snmp vars->val has the following union type:

typedef union {
   long           *integer;
   u_char         *string;
   oid            *objid;
   u_char         *bitstring;
   struct counter64 *counter64;
#ifdef NETSNMP_WITH_OPAQUE_SPECIAL_TYPES
   float          *floatVal;
   double         *doubleVal;
   /*
    * t_union *unionVal; 
    */
#endif                          /* NETSNMP_WITH_OPAQUE_SPECIAL_TYPES */
} netsnmp_vardata;

also *vars has val_len field, where the length of data stored. So you can access integer as *vars->val.integer , string as pointer to u_char vars->val.string with vars->val_len chars, oid as pointer to oid vars->val.objid with vars->val_len/sizeof(oid) oid elements and so on.