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 voiddns_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 voiddns_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 voiddns_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_tdns_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_tdns_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 */