err_t err;
  struct dns_hdr *hdr;
  struct dns_query qry;
  struct pbuf *p;
  char *query, *nptr;
  const char *pHostname;
  u8_t n;

  LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U16_F"] \"%s\": request\n",
              (u16_t)(numdns), name));
  LWIP_ASSERT("dns server out of array", numdns < DNS_MAX_SERVERS);
  LWIP_ASSERT("dns server has no IP address set", dns_servers[numdns].addr != 0);

  /* if here, we have either a new query or a retry on a previous query to process */
  p = pbuf_alloc(PBUF_TRANSPORT, sizeof(struct dns_hdr) + DNS_MAX_NAME_LENGTH +
                 sizeof(struct dns_query), PBUF_RAM);
  if (p != NULL) {
    LWIP_ASSERT("pbuf must be in one piece", p->next == NULL);
    /* fill dns header */
    hdr = (struct dns_hdr*)p->payload;
    memset(hdr, 0, sizeof(struct dns_hdr));
    hdr->id = htons(id);
    hdr->flags1 = DNS_FLAG1_RD;
    hdr->numquestions = htons(1);
    query = (char*)hdr + sizeof(struct dns_hdr);
    pHostname = name;
    --pHostname;

    /* convert hostname into suitable query format. */
    do {
      ++pHostname;
      nptr = query;
      ++query;
      for(n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname) {
        *query = *pHostname;
        ++query;
        ++n;
      }
      *nptr = n;
    } while(*pHostname != 0);
    *query++='\0';

    /* fill dns query */
    qry.type  = htons(DNS_RRTYPE_A);
    qry.class = htons(DNS_RRCLASS_IN);
    MEMCPY( query, &qry, sizeof(struct dns_query));

    /* resize pbuf to the exact dns query */
    pbuf_realloc(p, (query + sizeof(struct dns_query)) - ((char*)(p->payload)));

    /* connect to the server for faster receiving */
    udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
    /* send dns packet */
    err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);

    /* free pbuf */
    pbuf_free(p);
  } else {
    err = ERR_MEM;
  }

  return err;
}

/**
 * dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void
dns_check_entry(u8_t i)
{
  struct dns_table_entry *pEntry = &dns_table[i];

  LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);

  switch(pEntry->state) {

    case DNS_STATE_NEW: {
      /* initialize new entry */
      pEntry->state   = DNS_STATE_ASKING;
      pEntry->numdns  = 0;
      pEntry->tmr     = 1;
      pEntry->retries = 0;
      
      /* send DNS packet for this entry */
      dns_send(pEntry->numdns, pEntry->name, i);
      break;
    }

    case DNS_STATE_ASKING: {
      if (--pEntry->tmr == 0) {
        if (++pEntry->retries == DNS_MAX_RETRIES) {
          if ((pEntry->numdns+1<DNS_MAX_SERVERS) && (dns_servers[pEntry->numdns+1].addr!=0)) {
            /* 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 */
        dns_send(pEntry->numdns, pEntry->name, i);
      }
      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, struct ip_addr *addr, u16_t port)
{
  u8_t i;
  char *pHostname;
  struct dns_hdr *hdr;
  struct dns_answer ans;
  struct dns_table_entry *pEntry;
  u8_t nquestions, nanswers;
#if (DNS_USES_STATIC_BUF == 0)
  u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 0) */
#if (DNS_USES_STATIC_BUF == 2)
  u8_t* dns_payload;
#endif /* (DNS_USES_STATIC_BUF == 2) */

  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 memerr1;
  }

  /* is the dns message big enough ? */
  if (p->tot_len < (sizeof(struct dns_hdr) + sizeof(struct dns_query) + sizeof(struct dns_answer))) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
    /* free pbuf and return */
    goto memerr1;
  }

#if (DNS_USES_STATIC_BUF == 2)
  dns_payload = mem_malloc(p->tot_len);
  if (dns_payload == NULL) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: mem_malloc error\n"));
    /* free pbuf and return */
    goto memerr1;
  }
#endif /* (DNS_USES_STATIC_BUF == 2) */

  /* 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(struct 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(struct dns_hdr)) + sizeof(struct 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. */
          MEMCPY(&ans, pHostname, sizeof(struct dns_answer));
          if((ntohs(ans.type) == DNS_RRTYPE_A) && (ntohs(ans.class) == DNS_RRCLASS_IN) && (ntohs(ans.len) == sizeof(struct ip_addr)) ) {
            /* 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 */
            MEMCPY( &(pEntry->ipaddr), (pHostname+sizeof(struct dns_answer)), sizeof(struct ip_addr));
            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 memerr2;
          } else {
            pHostname = pHostname + sizeof(struct 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 memerr2;

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;

memerr2:
#if (DNS_USES_STATIC_BUF == 2)
  /* free dns buffer */
  mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */

memerr1:
  /* 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;

  /* 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;
  strcpy(pEntry->name, name);

  /* 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.
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a struct ip_addr 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, struct ip_addr *addr, dns_found_callback found,
                  void *callback_arg)
{
  /* 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_VAL;
  }

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

  /* host name already in octet notation? set ip addr and return ERR_OK
   * already have this address cached? */
  if (((addr->addr = inet_addr(hostname)) != INADDR_NONE) ||
      ((addr->addr = dns_lookup(hostname)) != 0)) {
    return ERR_OK;
  }

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

#endif /* LWIP_DNS */