Diff: core/dns.c

Revision:

0:51ac1d130fd4

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/core/dns.c	Fri Jun 22 09:25:39 2012 +0000
@@ -0,0 +1,970 @@
+/**
+ * @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/memp.h"
+#include "lwip/dns.h"
+
+#include <string.h>
+
+/** DNS server IP address */
+#ifndef DNS_SERVER_ADDRESS
+#define DNS_SERVER_ADDRESS(ipaddr)        (ip4_addr_set_u32(ipaddr, ipaddr_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
+
+/** DNS query message structure.
+    No packing needed: only used locally on the stack. */
+struct dns_query {
+  /* DNS query record starts with either a domain name or a pointer
+     to a name already present somewhere in the packet. */
+  u16_t type;
+  u16_t cls;
+};
+#define SIZEOF_DNS_QUERY 4
+
+/** DNS answer message structure.
+    No packing needed: only used locally on the stack. */
+struct dns_answer {
+  /* DNS answer record starts with either a domain name or a pointer
+     to a name already present somewhere in the packet. */
+  u16_t type;
+  u16_t cls;
+  u32_t ttl;
+  u16_t len;
+};
+#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];
+  ip_addr_t ipaddr;
+  /* pointer to callback on DNS query done */
+  dns_found_callback found;
+  void *arg;
+};
+
+#if DNS_LOCAL_HOSTLIST
+
+#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, ip_addr_t *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 ip_addr_t              dns_servers[DNS_MAX_SERVERS];
+/** Contiguous buffer for processing responses */
+static u8_t                   dns_payload_buffer[LWIP_MEM_ALIGN_BUFFER(DNS_MSG_SIZE)];
+static u8_t*                  dns_payload;
+
+/**
+ * Initialize the resolver: set up the UDP pcb and configure the default server
+ * (DNS_SERVER_ADDRESS).
+ */
+void
+dns_init()
+{
+  ip_addr_t dnsserver;
+
+  dns_payload = (u8_t *)LWIP_MEM_ALIGN(dns_payload_buffer);
+  
+  /* initialize default DNS server address */
+  DNS_SERVER_ADDRESS(&dnsserver);
+
+  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, ip_addr_t *dnsserver)
+{
+  if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
+      (dnsserver != NULL) && !ip_addr_isany(dnsserver)) {
+    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.
+ */
+ip_addr_t
+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;
+  size_t namelen;
+  for (i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++) {
+    struct local_hostlist_entry *init_entry = &local_hostlist_init[i];
+    LWIP_ASSERT("invalid host name (NULL)", init_entry->name != NULL);
+    namelen = strlen(init_entry->name);
+    LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
+    entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST);
+    LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);
+    if (entry != NULL) {
+      entry->name = (char*)entry + sizeof(struct local_hostlist_entry);
+      MEMCPY((char*)entry->name, init_entry->name, namelen);
+      ((char*)entry->name)[namelen] = 0;
+      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
+ *         IPADDR_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 ip4_addr_get_u32(&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 ip4_addr_get_u32(&local_hostlist_static[i].addr);
+    }
+  }
+#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
+  return IPADDR_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 ip_addr_t *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) || ip_addr_cmp(&entry->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;
+      memp_free(MEMP_LOCALHOSTLIST, 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 ip_addr_t *addr)
+{
+  struct local_hostlist_entry *entry;
+  size_t namelen;
+  LWIP_ASSERT("invalid host name (NULL)", hostname != NULL);
+  namelen = strlen(hostname);
+  LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
+  entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST);
+  if (entry == NULL) {
+    return ERR_MEM;
+  }
+  entry->name = (char*)entry + sizeof(struct local_hostlist_entry);
+  MEMCPY((char*)entry->name, hostname, namelen);
+  ((char*)entry->name)[namelen] = 0;
+  ip_addr_copy(entry->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 ip_addr_t to
+ *         better check for failure: != IPADDR_NONE) or IPADDR_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)) != IPADDR_NONE) {
+    return addr;
+  }
+#endif /* DNS_LOCAL_HOSTLIST */
+#ifdef DNS_LOOKUP_LOCAL_EXTERN
+  if((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != IPADDR_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 ip4_addr_get_u32(&dns_table[i].ipaddr);
+    }
+  }
+
+  return IPADDR_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", !ip_addr_isany(&dns_servers[numdns]));
+
+  /* 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 = PP_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 = PP_HTONS(DNS_RRTYPE_A);
+    qry.cls = PP_HTONS(DNS_RRCLASS_IN);
+    SMEMCPY(query, &qry, SIZEOF_DNS_QUERY);
+
+    /* resize pbuf to the exact dns query */
+    pbuf_realloc(p, (u16_t)((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)
+{
+  err_t err;
+  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 */
+      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 */