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 /**
  * @file
  * DNS - host name to IP address resolver.
  *
  */
 
 /**
 
  * This file implements a DNS host name to IP address resolver.
 
  * Port to lwIP from uIP
  * by Jim Pettinato April 2007
 
  * uIP version Copyright (c) 2002-2003, Adam Dunkels.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  * 3. The name of the author may not be used to endorse or promote
  *    products derived from this software without specific prior
  *    written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
  * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
  * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
  * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  *
  * DNS.C
  *
  * The lwIP DNS resolver functions are used to lookup a host name and
  * map it to a numerical IP address. It maintains a list of resolved
  * hostnames that can be queried with the dns_lookup() function.
  * New hostnames can be resolved using the dns_query() function.
  *
  * The lwIP version of the resolver also adds a non-blocking version of
  * gethostbyname() that will work with a raw API application. This function
  * checks for an IP address string first and converts it if it is valid.
  * gethostbyname() then does a dns_lookup() to see if the name is 
  * already in the table. If so, the IP is returned. If not, a query is 
  * issued and the function returns with a ERR_INPROGRESS status. The app
  * using the dns client must then go into a waiting state.
  *
  * Once a hostname has been resolved (or found to be non-existent),
  * the resolver code calls a specified callback function (which 
  * must be implemented by the module that uses the resolver).
  */
 
 /*-----------------------------------------------------------------------------
  * RFC 1035 - Domain names - implementation and specification
  * RFC 2181 - Clarifications to the DNS Specification
  *----------------------------------------------------------------------------*/
 
 /** @todo: define good default values (rfc compliance) */
 /** @todo: improve answer parsing, more checkings... */
 /** @todo: check RFC1035 - 7.3. Processing responses */
 
 /*-----------------------------------------------------------------------------
  * Includes
  *----------------------------------------------------------------------------*/
 
 #include "lwip/opt.h"
 
 #if LWIP_DNS /* don't build if not configured for use in lwipopts.h */
 
 #include "lwip/udp.h"
 #include "lwip/mem.h"
 #include "lwip/dns.h"
 
 #include <string.h>
 
 /** DNS server IP address */
 #ifndef DNS_SERVER_ADDRESS
 #define DNS_SERVER_ADDRESS        inet_addr("208.67.222.222") /* resolver1.opendns.com */
 #endif
 
 /** DNS server port address */
 #ifndef DNS_SERVER_PORT
 #define DNS_SERVER_PORT           53
 #endif
 
 /** DNS maximum number of retries when asking for a name, before "timeout". */
 #ifndef DNS_MAX_RETRIES
 #define DNS_MAX_RETRIES           4
 #endif
 
 /** DNS resource record max. TTL (one week as default) */
 #ifndef DNS_MAX_TTL
 #define DNS_MAX_TTL               604800
 #endif
 
 /* DNS protocol flags */
 #define DNS_FLAG1_RESPONSE        0x80
 #define DNS_FLAG1_OPCODE_STATUS   0x10
 #define DNS_FLAG1_OPCODE_INVERSE  0x08
 #define DNS_FLAG1_OPCODE_STANDARD 0x00
 #define DNS_FLAG1_AUTHORATIVE     0x04
 #define DNS_FLAG1_TRUNC           0x02
 #define DNS_FLAG1_RD              0x01
 #define DNS_FLAG2_RA              0x80
 #define DNS_FLAG2_ERR_MASK        0x0f
 #define DNS_FLAG2_ERR_NONE        0x00
 #define DNS_FLAG2_ERR_NAME        0x03
 
 /* DNS protocol states */
 #define DNS_STATE_UNUSED          0
 #define DNS_STATE_NEW             1
 #define DNS_STATE_ASKING          2
 #define DNS_STATE_DONE            3
 
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/bpstruct.h"
 #endif
 PACK_STRUCT_BEGIN
 /** DNS message header */
 struct dns_hdr {
   PACK_STRUCT_FIELD(u16_t id);
   PACK_STRUCT_FIELD(u8_t flags1);
   PACK_STRUCT_FIELD(u8_t flags2);
   PACK_STRUCT_FIELD(u16_t numquestions);
   PACK_STRUCT_FIELD(u16_t numanswers);
   PACK_STRUCT_FIELD(u16_t numauthrr);
   PACK_STRUCT_FIELD(u16_t numextrarr);
 } PACK_STRUCT_STRUCT;
 PACK_STRUCT_END
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/epstruct.h"
 #endif
 #define SIZEOF_DNS_HDR 12
 
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/bpstruct.h"
 #endif
 PACK_STRUCT_BEGIN
 /** DNS query message structure */
 struct dns_query {
   /* DNS query record starts with either a domain name or a pointer
      to a name already present somewhere in the packet. */
   PACK_STRUCT_FIELD(u16_t type);
   PACK_STRUCT_FIELD(u16_t class);
 } PACK_STRUCT_STRUCT;
 PACK_STRUCT_END
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/epstruct.h"
 #endif
 #define SIZEOF_DNS_QUERY 4
 
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/bpstruct.h"
 #endif
 PACK_STRUCT_BEGIN
 /** DNS answer message structure */
 struct dns_answer {
   /* DNS answer record starts with either a domain name or a pointer
      to a name already present somewhere in the packet. */
   PACK_STRUCT_FIELD(u16_t type);
   PACK_STRUCT_FIELD(u16_t class);
   PACK_STRUCT_FIELD(u32_t ttl);
   PACK_STRUCT_FIELD(u16_t len);
 } PACK_STRUCT_STRUCT;
 PACK_STRUCT_END
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/epstruct.h"
 #endif
 #define SIZEOF_DNS_ANSWER 10
 
 /** DNS table entry */
 struct dns_table_entry {
   u8_t  state;
   u8_t  numdns;
   u8_t  tmr;
   u8_t  retries;
   u8_t  seqno;
   u8_t  err;
   u32_t ttl;
   char name[DNS_MAX_NAME_LENGTH];
   struct ip_addr ipaddr;
   /* pointer to callback on DNS query done */
   dns_found_callback found;
   void *arg;
 };
 
 #if DNS_LOCAL_HOSTLIST
 /** struct used for local host-list */
 struct local_hostlist_entry {
   /** static hostname */
   const char *name;
   /** static host address in network byteorder */
   u32_t addr;
   struct local_hostlist_entry *next;
 };
 
 #if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
 /** Local host-list. For hostnames in this list, no
  *  external name resolution is performed */
 static struct local_hostlist_entry *local_hostlist_dynamic;
 #else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
 
 /** Defining this allows the local_hostlist_static to be placed in a different
  * linker section (e.g. FLASH) */
 #ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
 #define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
 #endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
 /** Defining this allows the local_hostlist_static to be placed in a different
  * linker section (e.g. FLASH) */
 #ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
 #define DNS_LOCAL_HOSTLIST_STORAGE_POST
 #endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
 DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[]
   DNS_LOCAL_HOSTLIST_STORAGE_POST = DNS_LOCAL_HOSTLIST_INIT;
 
 #endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
 
 static void dns_init_local();
 #endif /* DNS_LOCAL_HOSTLIST */
 
 
 /* forward declarations */
 static void dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port);
 static void dns_check_entries(void);
 
 /*-----------------------------------------------------------------------------
  * Globales
  *----------------------------------------------------------------------------*/
 
 /* DNS variables */
 static struct udp_pcb        *dns_pcb;
 static u8_t                   dns_seqno;
 static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
 static struct ip_addr         dns_servers[DNS_MAX_SERVERS];
 
 #if (DNS_USES_STATIC_BUF == 1)
 static u8_t                   dns_payload[DNS_MSG_SIZE];
 #endif /* (DNS_USES_STATIC_BUF == 1) */
 
 /**
  * Initialize the resolver: set up the UDP pcb and configure the default server
  * (DNS_SERVER_ADDRESS).
  */
 void
 dns_init()
 {
   struct ip_addr dnsserver;
   
   /* initialize default DNS server address */
   dnsserver.addr = DNS_SERVER_ADDRESS;
 
   LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));
 
   /* if dns client not yet initialized... */
   if (dns_pcb == NULL) {
     dns_pcb = udp_new();
 
     if (dns_pcb != NULL) {
       /* initialize DNS table not needed (initialized to zero since it is a
        * global variable) */
       LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
         DNS_STATE_UNUSED == 0);
 
       /* initialize DNS client */
       udp_bind(dns_pcb, IP_ADDR_ANY, 0);
       udp_recv(dns_pcb, dns_recv, NULL);
 
       /* initialize default DNS primary server */
       dns_setserver(0, &dnsserver);
     }
   }
 #if DNS_LOCAL_HOSTLIST
   dns_init_local();
 #endif
 }
 
 /**
  * Initialize one of the DNS servers.
  *
  * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
  * @param dnsserver IP address of the DNS server to set
  */
 void
 dns_setserver(u8_t numdns, struct ip_addr *dnsserver)
 {
   if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
       (dnsserver != NULL) && (dnsserver->addr !=0 )) {
     dns_servers[numdns] = (*dnsserver);
   }
 }
 
 /**
  * Obtain one of the currently configured DNS server.
  *
  * @param numdns the index of the DNS server
  * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
  *         server has not been configured.
  */
 struct ip_addr
 dns_getserver(u8_t numdns)
 {
   if (numdns < DNS_MAX_SERVERS) {
     return dns_servers[numdns];
   } else {
     return *IP_ADDR_ANY;
   }
 }
 
 /**
  * The DNS resolver client timer - handle retries and timeouts and should
  * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
  */
 void
 dns_tmr(void)
 {
   if (dns_pcb != NULL) {
     LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
     dns_check_entries();
   }
 }
 
 #if DNS_LOCAL_HOSTLIST
 static void
 dns_init_local()
 {
 #if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT)
   int i;
   struct local_hostlist_entry *entry;
   /* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
   struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;
   for (i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++) {
     entry = mem_malloc(sizeof(struct local_hostlist_entry));
     LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);
     if (entry != NULL) {
       struct local_hostlist_entry *init_entry = &local_hostlist_init[i];
       entry->name = init_entry->name;
       entry->addr = init_entry->addr;
       entry->next = local_hostlist_dynamic;
       local_hostlist_dynamic = entry;
     }
   }
 #endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
 }
 
 /**
  * Scans the local host-list for a hostname.
  *
  * @param hostname Hostname to look for in the local host-list
  * @return The first IP address for the hostname in the local host-list or
  *         INADDR_NONE if not found.
  */
 static u32_t
 dns_lookup_local(const char *hostname)
 {
 #if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
   struct local_hostlist_entry *entry = local_hostlist_dynamic;
   while(entry != NULL) {
     if(strcmp(entry->name, hostname) == 0) {
       return entry->addr;
     }
     entry = entry->next;
   }
 #else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
   int i;
   for (i = 0; i < sizeof(local_hostlist_static) / sizeof(struct local_hostlist_entry); i++) {
     if(strcmp(local_hostlist_static[i].name, hostname) == 0) {
       return local_hostlist_static[i].addr;
     }
   }
 #endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
   return INADDR_NONE;
 }
 
 #if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
 /** Remove all entries from the local host-list for a specific hostname
  * and/or IP addess
  *
  * @param hostname hostname for which entries shall be removed from the local
  *                 host-list
  * @param addr address for which entries shall be removed from the local host-list
  * @return the number of removed entries
  */
 int
 dns_local_removehost(const char *hostname, const struct ip_addr *addr)
 {
   int removed = 0;
   struct local_hostlist_entry *entry = local_hostlist_dynamic;
   struct local_hostlist_entry *last_entry = NULL;
   while (entry != NULL) {
     if (((hostname == NULL) || !strcmp(entry->name, hostname)) &&
         ((addr == NULL) || (entry->addr == addr->addr))) {
       struct local_hostlist_entry *free_entry;
       if (last_entry != NULL) {
         last_entry->next = entry->next;
       } else {
         local_hostlist_dynamic = entry->next;
       }
       free_entry = entry;
       entry = entry->next;
       mem_free(free_entry);
       removed++;
     } else {
       last_entry = entry;
       entry = entry->next;
     }
   }
   return removed;
 }
 
 /**
  * Add a hostname/IP address pair to the local host-list.
  * Duplicates are not checked.
  *
  * @param hostname hostname of the new entry
  * @param addr IP address of the new entry
  * @return ERR_OK if succeeded or ERR_MEM on memory error
  */
 err_t
 dns_local_addhost(const char *hostname, const struct ip_addr *addr)
 {
   struct local_hostlist_entry *entry;
   entry = mem_malloc(sizeof(struct local_hostlist_entry));
   if (entry == NULL) {
     return ERR_MEM;
   }
   entry->name = hostname;
   entry->addr = addr->addr;
   entry->next = local_hostlist_dynamic;
   local_hostlist_dynamic = entry;
   return ERR_OK;
 }
 #endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
 #endif /* DNS_LOCAL_HOSTLIST */
 
 /**
  * Look up a hostname in the array of known hostnames.
  *
  * @note This function only looks in the internal array of known
  * hostnames, it does not send out a query for the hostname if none
  * was found. The function dns_enqueue() can be used to send a query
  * for a hostname.
  *
  * @param name the hostname to look up
  * @return the hostname's IP address, as u32_t (instead of struct ip_addr to
  *         better check for failure: != INADDR_NONE) or INADDR_NONE if the hostname
  *         was not found in the cached dns_table.
  */
 static u32_t
 dns_lookup(const char *name)
 {
   u8_t i;
 #if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
   u32_t addr;
 #endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
 #if DNS_LOCAL_HOSTLIST
   if ((addr = dns_lookup_local(name)) != INADDR_NONE) {
     return addr;
   }
 #endif /* DNS_LOCAL_HOSTLIST */
 #ifdef DNS_LOOKUP_LOCAL_EXTERN
   if((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != INADDR_NONE) {
     return addr;
   }
 #endif /* DNS_LOOKUP_LOCAL_EXTERN */
 
   /* Walk through name list, return entry if found. If not, return NULL. */
   for (i = 0; i < DNS_TABLE_SIZE; ++i) {
     if ((dns_table[i].state == DNS_STATE_DONE) &&
         (strcmp(name, dns_table[i].name) == 0)) {
       LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
       ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
       LWIP_DEBUGF(DNS_DEBUG, ("\n"));
       return dns_table[i].ipaddr.addr;
     }
   }
 
   return INADDR_NONE;
 }
 
 #if DNS_DOES_NAME_CHECK
 /**
  * Compare the "dotted" name "query" with the encoded name "response"
  * to make sure an answer from the DNS server matches the current dns_table
  * entry (otherwise, answers might arrive late for hostname not on the list
  * any more).
  *
  * @param query hostname (not encoded) from the dns_table
  * @param response encoded hostname in the DNS response
  * @return 0: names equal; 1: names differ
  */
 static u8_t
 dns_compare_name(unsigned char *query, unsigned char *response)
 {
   unsigned char n;
 
   do {
     n = *response++;
     /** @see RFC 1035 - 4.1.4. Message compression */
     if ((n & 0xc0) == 0xc0) {
       /* Compressed name */
       break;
     } else {
       /* Not compressed name */
       while (n > 0) {
         if ((*query) != (*response)) {
           return 1;
         }
         ++response;
         ++query;
         --n;
       };
       ++query;
     }
   } while (*response != 0);
 
   return 0;
 }
 #endif /* DNS_DOES_NAME_CHECK */
 
 /**
  * Walk through a compact encoded DNS name and return the end of the name.
  *
  * @param query encoded DNS name in the DNS server response
  * @return end of the name
  */
 static unsigned char *
 dns_parse_name(unsigned char *query)
 {
   unsigned char n;
 
   do {
     n = *query++;
     /** @see RFC 1035 - 4.1.4. Message compression */
     if ((n & 0xc0) == 0xc0) {
       /* Compressed name */
       break;
     } else {
       /* Not compressed name */
       while (n > 0) {
         ++query;
         --n;
       };
     }
   } while (*query != 0);
 
   return query + 1;
 }
 
 /**
  * Send a DNS query packet.
  *
  * @param numdns index of the DNS server in the dns_servers table
  * @param name hostname to query
  * @param id index of the hostname in dns_table, used as transaction ID in the
  *        DNS query packet
  * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
  */
 static err_t
 dns_send(u8_t numdns, const char* name, u8_t id)
 {
   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_DNS_HDR + DNS_MAX_NAME_LENGTH +
                  SIZEOF_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_DNS_HDR);
     hdr->id = htons(id);
     hdr->flags1 = DNS_FLAG1_RD;
     hdr->numquestions = htons(1);
     query = (char*)hdr + SIZEOF_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_DNS_QUERY);
 
     /* resize pbuf to the exact dns query */
     pbuf_realloc(p, (query + SIZEOF_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_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_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_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. */
           MEMCPY(&ans, pHostname, SIZEOF_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_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_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 = htonl(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)) != INADDR_NONE)) {
     return ERR_OK;
   }
 
   /* queue query with specified callback */
   return dns_enqueue(hostname, found, callback_arg);
 }
 
 #endif /* LWIP_DNS */