[C][example code] snmp get

Makefile

#
# Warning: you may need more libraries than are included here on the
# build line.  The agent frequently needs various libraries in order
# to compile pieces of it, but is OS dependent and we can't list all
# the combinations here.  Instead, look at the libraries that were
# used when linking the snmpd master agent and copy those to this
# file.
#

CC=gcc

OBJS1=snmpdemoapp.o
OBJS2=example-demon.o nstAgentSubagentObject.o
OBJS3=asyncapp.o
TARGETS=example-demon snmpdemoapp asyncapp

CFLAGS=-I. `net-snmp-config --cflags`
BUILDLIBS=`net-snmp-config --libs`
BUILDAGENTLIBS=`net-snmp-config --agent-libs`

# shared library flags (assumes gcc)
DLFLAGS=-fPIC -shared

all: $(TARGETS)

snmpdemoapp: $(OBJS1)
    $(CC) -o snmpdemoapp $(OBJS1) $(BUILDLIBS)

asyncapp: $(OBJS3)
    $(CC) -o asyncapp $(OBJS3) $(BUILDLIBS)

example-demon: $(OBJS2)
    $(CC) -o example-demon $(OBJS2)  $(BUILDAGENTLIBS)

clean:
    rm $(OBJS2) $(OBJS2) $(TARGETS)

nstAgentPluginObject.so: nstAgentPluginObject.c Makefile
    $(CC) $(CFLAGS) $(DLFLAGS) -c -o nstAgentPluginObject.o nstAgentPluginObject.c
    $(CC) $(CFLAGS) $(DLFLAGS) -o nstAgentPluginObject.so nstAgentPluginObject.o

C code

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

/* change the word "define" to "undef" to try the (insecure) SNMPv1 version */
#define DEMO_USE_SNMP_VERSION_3

#ifdef DEMO_USE_SNMP_VERSION_3
const char *our_v3_passphrase = "The Net-SNMP Demo Password";
#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;

    /*
     * Initialize the SNMP library
     */
    init_snmp("snmpdemoapp");

    /*
     * Initialize a "session" that defines who we're going to talk to
     */
    snmp_sess_init( &session );                   /* set up defaults */
    session.peername = strdup("test.net-snmp.org");  /* Remote IP Address */

    /* set up the authentication parameters for talking to the server */

#ifdef DEMO_USE_SNMP_VERSION_3

    /* Use SNMPv3 to talk to the experimental server */

    /* set the SNMP version number */
    session.version=SNMP_VERSION_3;

    /* set the SNMPv3 user name */
    session.securityName = strdup("MD5User");
    session.securityNameLen = strlen(session.securityName);

    /* set the security level to authenticated, but not encrypted */
    session.securityLevel = SNMP_SEC_LEVEL_AUTHNOPRIV;

    /* set the authentication method to MD5 */
    session.securityAuthProto = usmHMACMD5AuthProtocol;
    session.securityAuthProtoLen = sizeof(usmHMACMD5AuthProtocol)/sizeof(oid);
    session.securityAuthKeyLen = USM_AUTH_KU_LEN;

    /* set the authentication key to a MD5 hashed version of our
       passphrase "The Net-SNMP Demo Password" (which must be at least 8
       characters long) */
    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 */

    /* set the SNMP version number */
    session.version = SNMP_VERSION_1;

    /* set the SNMPv1 community name used for authentication */
    session.community = "demopublic";
    session.community_len = strlen(session.community);

#endif /* SNMPv1 */

    /*
     * Open the session
     */
    SOCK_STARTUP;
    ss = snmp_open(&session);                     /* establish the session */

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

    /*
     * Create the PDU for the data for our request.
     *   1) We're going to GET the system.sysDescr.0 node.
     */
    pdu = snmp_pdu_create(SNMP_MSG_GET);
    anOID_len = MAX_OID_LEN;
    if (!snmp_parse_oid(".1.3.6.1.2.1.1.1.0", anOID, &anOID_len)) {
      snmp_perror(".1.3.6.1.2.1.1.1.0");
      SOCK_CLEANUP;
      exit(1);
    }
#if OTHER_METHODS
    /*
     *  These are alternatives to the 'snmp_parse_oid' call above,
     *    e.g. specifying the OID by name rather than numerically.
     */
    read_objid(".1.3.6.1.2.1.1.1.0", anOID, &anOID_len);
    get_node("sysDescr.0", anOID, &anOID_len);
    read_objid("system.sysDescr.0", anOID, &anOID_len);
#endif

    snmp_add_null_var(pdu, anOID, anOID_len);

    /*
     * Send the Request out.
     */
    status = snmp_synch_response(ss, pdu, &response);

    /*
     * Process the response.
     */
    if (status == STAT_SUCCESS && response->errstat == SNMP_ERR_NOERROR) {
      /*
       * SUCCESS: Print the result variables
       */

      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);
      free(sp);
    }
        else
          printf("value #%d is NOT a string! Ack!\n", count++);
      }
    } else {
      /*
       * FAILURE: print what went wrong!
       */

      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);

    }

    /*
     * Clean up:
     *  1) free the response.
     *  2) close the session.
     */
    if (response)
      snmp_free_pdu(response);
    snmp_close(ss);

    SOCK_CLEANUP;
    return (0);
} /* main() */

Compile command as following:
[happy@localhost 3]$ make snmpdemoapp
gcc -I. net-snmp-config --cflags -c -o snmpdemoapp.o snmpdemoapp.c
gcc -o snmpdemoapp snmpdemoapp.o net-snmp-config --libs


Execute command as following:
[happy@localhost 3]$ ./snmpdemoapp
SNMPv2-MIB::sysDescr.0 = STRING: test.net-snmp.org
value #1 is a string: test.net-snmp.org


Reference:

• Net-SNMP Tutorial – Demo Application