Perform a noAuthNoPriv SNMP-GET request

net_snmp_no_auth_no_priv.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/library/snmpv3.h>
#include <net-snmp/library/snmpusm.h>
#include <net-snmp/library/snmp_api.h>
#include <net-snmp/net-snmp-includes.h>

int main(void) {
	// Create some variables
   struct snmp_session session, *ss;
   struct snmp_pdu *pdu;
   struct snmp_pdu *response;

   oid anOID[MAX_OID_LEN];
   size_t anOID_len = MAX_OID_LEN;

   struct variable_list *vars;
   int status;

   // Startup the SNMP lib
   init_snmp("snmpapp");
   snmp_sess_init( &session );
   // Configure the session
   session.peername = "127.0.0.1";
   session.version=SNMP_VERSION_3;
   session.securityName = strdup("snmpnoauth");
   session.securityNameLen = strlen(session.securityName);
   session.securityLevel = SNMP_SEC_LEVEL_NOAUTH;

   // Set the defaults of the AuthProto since noAuthNoPriv doesn't care
   session.securityAuthProto = usmHMACSHA1AuthProtocol;
   session.securityAuthProtoLen = sizeof(usmHMACSHA1AuthProtocol) / sizeof(oid);;
   session.securityAuthKeyLen = USM_AUTH_KU_LEN;

   // Set the defaults of the Priv since noAuthNoPriv doesn't care
   session.securityPrivProto =  usmDESPrivProtocol;
   session.securityPrivProtoLen = USM_PRIV_PROTO_DES_LEN;
   session.securityPrivKeyLen = USM_PRIV_KU_LEN;

   // Print out some of these symbols
   printf("%d\n", (int) sizeof(usmHMACSHA1AuthProtocol));
   printf("%d\n", (int) sizeof(oid));
   printf("%d\n", (int) USM_PRIV_PROTO_DES_LEN);

   SOCK_STARTUP;
   ss = snmp_open(&session);

   pdu = snmp_pdu_create(SNMP_MSG_GET);
   anOID_len = MAX_OID_LEN;

   if (!snmp_parse_oid("1.3.6.1.2.1.2.2.1.2.2", anOID, &anOID_len)) {
                snmp_perror("1.3.6.1.2.1.2.2.1.2.2");
                SOCK_CLEANUP;
                exit(1);
  }

  snmp_add_null_var(pdu, anOID, anOID_len);
  status = snmp_synch_response(ss, pdu, &response);

  if (status == STAT_SUCCESS)
    for (vars = response->variables; vars; vars = vars->next_variable)
      print_variable(vars->name, vars->name_length, vars);
  else if (status == STAT_TIMEOUT)
    fprintf(stderr, "Timeout: No response from %s.\n", session.peername);
  else {
    snmp_sess_perror("unknown error\n", ss);
  }
}

You can then compile this and run it:
gcc gist.c -o snmpgist -I /usr/include/ -lnetsnmp

and run it:
./snmpgist

and see:
80 8 10 IF-MIB::ifDescr.2 = STRING: GigabitEthernet0/1