pEntry->tmr     = 1;
      pEntry->retries = 0;
      
      /* send DNS packet for this entry */
      err = dns_send(pEntry->numdns, pEntry->name, i);
      if (err != ERR_OK) {
        LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
                    ("dns_send returned error: %s\n", lwip_strerr(err)));
      }
      break;
    }

    case DNS_STATE_ASKING: {
      if (--pEntry->tmr == 0) {
        if (++pEntry->retries == DNS_MAX_RETRIES) {
          if ((pEntry->numdns+1<DNS_MAX_SERVERS) && !ip_addr_isany(&dns_servers[pEntry->numdns+1])) {
            /* change of server */
            pEntry->numdns++;
            pEntry->tmr     = 1;
            pEntry->retries = 0;
            break;
          } else {
            LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", pEntry->name));
            /* call specified callback function if provided */
            if (pEntry->found)
              (*pEntry->found)(pEntry->name, NULL, pEntry->arg);
            /* flush this entry */
            pEntry->state   = DNS_STATE_UNUSED;
            pEntry->found   = NULL;
            break;
          }
        }

        /* wait longer for the next retry */
        pEntry->tmr = pEntry->retries;

        /* send DNS packet for this entry */
        err = dns_send(pEntry->numdns, pEntry->name, i);
        if (err != ERR_OK) {
          LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
                      ("dns_send returned error: %s\n", lwip_strerr(err)));
        }
      }
      break;
    }

    case DNS_STATE_DONE: {
      /* if the time to live is nul */
      if (--pEntry->ttl == 0) {
        LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
        /* flush this entry */
        pEntry->state = DNS_STATE_UNUSED;
        pEntry->found = NULL;
      }
      break;
    }
    case DNS_STATE_UNUSED:
      /* nothing to do */
      break;
    default:
      LWIP_ASSERT("unknown dns_table entry state:", 0);
      break;
  }
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void
dns_check_entries(void)
{
  u8_t i;

  for (i = 0; i < DNS_TABLE_SIZE; ++i) {
    dns_check_entry(i);
  }
}

/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 *
 * @params see udp.h
 */
static void
dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *addr, u16_t port)
{
  u16_t i;
  char *pHostname;
  struct dns_hdr *hdr;
  struct dns_answer ans;
  struct dns_table_entry *pEntry;
  u16_t nquestions, nanswers;

  LWIP_UNUSED_ARG(arg);
  LWIP_UNUSED_ARG(pcb);
  LWIP_UNUSED_ARG(addr);
  LWIP_UNUSED_ARG(port);

  /* is the dns message too big ? */
  if (p->tot_len > DNS_MSG_SIZE) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
    /* free pbuf and return */
    goto memerr;
  }

  /* is the dns message big enough ? */
  if (p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_DNS_ANSWER)) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
    /* free pbuf and return */
    goto memerr;
  }

  /* copy dns payload inside static buffer for processing */ 
  if (pbuf_copy_partial(p, dns_payload, p->tot_len, 0) == p->tot_len) {
    /* The ID in the DNS header should be our entry into the name table. */
    hdr = (struct dns_hdr*)dns_payload;
    i = htons(hdr->id);
    if (i < DNS_TABLE_SIZE) {
      pEntry = &dns_table[i];
      if(pEntry->state == DNS_STATE_ASKING) {
        /* This entry is now completed. */
        pEntry->state = DNS_STATE_DONE;
        pEntry->err   = hdr->flags2 & DNS_FLAG2_ERR_MASK;

        /* We only care about the question(s) and the answers. The authrr
           and the extrarr are simply discarded. */
        nquestions = htons(hdr->numquestions);
        nanswers   = htons(hdr->numanswers);

        /* Check for error. If so, call callback to inform. */
        if (((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0) || (pEntry->err != 0) || (nquestions != 1)) {
          LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", pEntry->name));
          /* call callback to indicate error, clean up memory and return */
          goto responseerr;
        }

#if DNS_DOES_NAME_CHECK
        /* Check if the name in the "question" part match with the name in the entry. */
        if (dns_compare_name((unsigned char *)(pEntry->name), (unsigned char *)dns_payload + SIZEOF_DNS_HDR) != 0) {
          LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
          /* call callback to indicate error, clean up memory and return */
          goto responseerr;
        }
#endif /* DNS_DOES_NAME_CHECK */

        /* Skip the name in the "question" part */
        pHostname = (char *) dns_parse_name((unsigned char *)dns_payload + SIZEOF_DNS_HDR) + SIZEOF_DNS_QUERY;

        while (nanswers > 0) {
          /* skip answer resource record's host name */
          pHostname = (char *) dns_parse_name((unsigned char *)pHostname);

          /* Check for IP address type and Internet class. Others are discarded. */
          SMEMCPY(&ans, pHostname, SIZEOF_DNS_ANSWER);
          if((ans.type == PP_HTONS(DNS_RRTYPE_A)) && (ans.cls == PP_HTONS(DNS_RRCLASS_IN)) &&
             (ans.len == PP_HTONS(sizeof(ip_addr_t))) ) {
            /* read the answer resource record's TTL, and maximize it if needed */
            pEntry->ttl = ntohl(ans.ttl);
            if (pEntry->ttl > DNS_MAX_TTL) {
              pEntry->ttl = DNS_MAX_TTL;
            }
            /* read the IP address after answer resource record's header */
            SMEMCPY(&(pEntry->ipaddr), (pHostname+SIZEOF_DNS_ANSWER), sizeof(ip_addr_t));
            LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", pEntry->name));
            ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
            LWIP_DEBUGF(DNS_DEBUG, ("\n"));
            /* call specified callback function if provided */
            if (pEntry->found) {
              (*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg);
            }
            /* deallocate memory and return */
            goto memerr;
          } else {
            pHostname = pHostname + SIZEOF_DNS_ANSWER + htons(ans.len);
          }
          --nanswers;
        }
        LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", pEntry->name));
        /* call callback to indicate error, clean up memory and return */
        goto responseerr;
      }
    }
  }

  /* deallocate memory and return */
  goto memerr;

responseerr:
  /* ERROR: call specified callback function with NULL as name to indicate an error */
  if (pEntry->found) {
    (*pEntry->found)(pEntry->name, NULL, pEntry->arg);
  }
  /* flush this entry */
  pEntry->state = DNS_STATE_UNUSED;
  pEntry->found = NULL;

memerr:
  /* free pbuf */
  pbuf_free(p);
  return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param found a callback founction to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return @return a err_t return code.
 */
static err_t
dns_enqueue(const char *name, dns_found_callback found, void *callback_arg)
{
  u8_t i;
  u8_t lseq, lseqi;
  struct dns_table_entry *pEntry = NULL;
  size_t namelen;

  /* search an unused entry, or the oldest one */
  lseq = lseqi = 0;
  for (i = 0; i < DNS_TABLE_SIZE; ++i) {
    pEntry = &dns_table[i];
    /* is it an unused entry ? */
    if (pEntry->state == DNS_STATE_UNUSED)
      break;

    /* check if this is the oldest completed entry */
    if (pEntry->state == DNS_STATE_DONE) {
      if ((dns_seqno - pEntry->seqno) > lseq) {
        lseq = dns_seqno - pEntry->seqno;
        lseqi = i;
      }
    }
  }

  /* if we don't have found an unused entry, use the oldest completed one */
  if (i == DNS_TABLE_SIZE) {
    if ((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE)) {
      /* no entry can't be used now, table is full */
      LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
      return ERR_MEM;
    } else {
      /* use the oldest completed one */
      i = lseqi;
      pEntry = &dns_table[i];
    }
  }

  /* use this entry */
  LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %"U16_F"\n", name, (u16_t)(i)));

  /* fill the entry */
  pEntry->state = DNS_STATE_NEW;
  pEntry->seqno = dns_seqno++;
  pEntry->found = found;
  pEntry->arg   = callback_arg;
  namelen = LWIP_MIN(strlen(name), DNS_MAX_NAME_LENGTH-1);
  MEMCPY(pEntry->name, name, namelen);
  pEntry->name[namelen] = 0;

  /* force to send query without waiting timer */
  dns_check_entry(i);

  /* dns query is enqueued */
  return ERR_INPROGRESS;
}

/**
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 * - ERR_ARG: dns client not initialized or invalid hostname
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a ip_addr_t where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t
dns_gethostbyname(const char *hostname, ip_addr_t *addr, dns_found_callback found,
                  void *callback_arg)
{
  u32_t ipaddr;
  /* not initialized or no valid server yet, or invalid addr pointer
   * or invalid hostname or invalid hostname length */
  if ((dns_pcb == NULL) || (addr == NULL) ||
      (!hostname) || (!hostname[0]) ||
      (strlen(hostname) >= DNS_MAX_NAME_LENGTH)) {
    return ERR_ARG;
  }

#if LWIP_HAVE_LOOPIF
  if (strcmp(hostname, "localhost")==0) {
    ip_addr_set_loopback(addr);
    return ERR_OK;
  }
#endif /* LWIP_HAVE_LOOPIF */

  /* host name already in octet notation? set ip addr and return ERR_OK */
  ipaddr = ipaddr_addr(hostname);
  if (ipaddr == IPADDR_NONE) {
    /* already have this address cached? */
    ipaddr = dns_lookup(hostname);
  }
  if (ipaddr != IPADDR_NONE) {
    ip4_addr_set_u32(addr, ipaddr);
    return ERR_OK;
  }

  /* queue query with specified callback */
  return dns_enqueue(hostname, found, callback_arg);
}

#endif /* LWIP_DNS */