Listing 2. Sample Callback Function That Will Dump a Packet in Hex

// Sample NF_QUEUE callback function.
static int nfqueue_cb(struct nfq_q_handle *qh,
                      struct nfgenmsg *nfmsg,
                      struct nfq_data *nfa,
                      void *data) {

    struct nfqnl_msg_packet_hdr *ph;
    int id = 0;
    int size = 0;
    int i;
    unsigned char *full_packet;
    unsigned char * c;
    struct in_addr ipa;
    char src_ip_str[20];
    char dst_ip_str[20];

ph = nfq_get_msg_packet_hdr(nfa);

if (ph) {

  // Print out metatdata.
  id = ntohl(ph->packet_id);
  fprintf(stdout,
        "hw_protocol = 0x%04x hook = %u id = %u\n",
         ntohs(ph->hw_protocol), ph->hook, id);

  // Retrieve packet payload.
  size = nfq_get_payload(nfa, &full_packet);

  // Get IP addresses in char form.
  ip = (struct iphdr *) full_packet;
  ipa.s_addr=ip->saddr
  strcpy (src_ip_str, inet_ntoa(ipa));
  ipa.s_addr=ip->daddr;
  strcpy (dst_ip_str, inet_ntoa(ipa));

  fprintf(stdout,
         "Source IP: %s   Destination IP: %s\n",
          src_ip_str, dst_ip_str);

  // Print out packet in hex.
  c = (unsigned char *)payload;
  for (i=0; i<size; ++i,++c) {
    fprintf (stdout, "%02x", (unsigned int)*c);
  }
  fprintf (stdout, "\n");

  // Done with packet, accept it.
  nfq_set_verdict(qh, id, NF_ACCEPT, 0, NULL);
  }
}