/*
     This file is part of GNUnet.
     Copyright (C) 2007-2016 GNUnet e.V.

     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     or (at your option) any later version.

     GNUnet is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Affero General Public License for more details.

     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

     SPDX-License-Identifier: AGPL3.0-or-later
 */

/**
 * @file util/gnunet-service-resolver.c
 * @brief code to do DNS resolution
 * @author Christian Grothoff
 */
#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_protocols.h"
#include "gnunet_statistics_service.h"
#include "resolver.h"


/**
 * How long do we wait for DNS answers?
 */
#define DNS_TIMEOUT GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 30)

/**
 * Maximum number of hostnames we cache results for.
 */
#define MAX_CACHE 1024

/**
 * Entry in list of cached DNS records for a hostname.
 */
struct RecordListEntry
{
  /**
   * This is a doubly linked list.
   */
  struct RecordListEntry *next;

  /**
   * This is a doubly linked list.
   */
  struct RecordListEntry *prev;

  /**
   * Cached data.
   */
  struct GNUNET_DNSPARSER_Record *record;
};


/**
 * A cached DNS lookup result.
 */
struct ResolveCache
{
  /**
   * This is a doubly linked list.
   */
  struct ResolveCache *next;

  /**
   * This is a doubly linked list.
   */
  struct ResolveCache *prev;

  /**
   * Which hostname is this cache for?
   */
  char *hostname;

  /**
   * head of a double linked list containing the lookup results
   */
  struct RecordListEntry *records_head;

  /**
   * tail of a double linked list containing the lookup results
   */
  struct RecordListEntry *records_tail;
};


/**
 * Information about pending lookups.
 */
struct ActiveLookup
{
  /**
   * Stored in a DLL.
   */
  struct ActiveLookup *next;

  /**
   * Stored in a DLL.
   */
  struct ActiveLookup *prev;

  /**
   * The client that queried the records contained in this cache entry.
   */
  struct GNUNET_SERVICE_Client *client;

  /**
   * handle for cancelling a request
   */
  struct GNUNET_DNSSTUB_RequestSocket *resolve_handle;

  /**
   * handle for the resolution timeout task
   */
  struct GNUNET_SCHEDULER_Task *timeout_task;

  /**
   * Which hostname are we resolving?
   */
  char *hostname;

  /**
   * If @a record_type is #GNUNET_DNSPARSER_TYPE_ALL, did we go again
   * for the AAAA records yet?
   */
  int did_aaaa;

  /**
   * type of queried DNS record
   */
  uint16_t record_type;

  /**
   * Unique request ID of a client if a query for this hostname/record_type
   * is currently pending, undefined otherwise.
   */
  uint32_t client_request_id;

  /**
   * Unique DNS request ID of a client if a query for this hostname/record_type
   * is currently pending, undefined otherwise.
   */
  uint16_t dns_id;
};


/**
 * Start of the linked list of cached DNS lookup results.
 */
static struct ResolveCache *cache_head;

/**
 * Tail of the linked list of cached DNS lookup results.
 */
static struct ResolveCache *cache_tail;

/**
 * Head of the linked list of DNS lookup results from /etc/hosts.
 */
static struct ResolveCache *hosts_head;

/**
 * Tail of the linked list of DNS lookup results from /etc/hosts.
 */
static struct ResolveCache *hosts_tail;

/**
 * Start of the linked list of active DNS lookups.
 */
static struct ActiveLookup *lookup_head;

/**
 * Tail of the linked list of active DNS lookups.
 */
static struct ActiveLookup *lookup_tail;

/**
 * context of dnsstub library
 */
static struct GNUNET_DNSSTUB_Context *dnsstub_ctx;

/**
 * My domain, to be appended to the hostname to get a FQDN.
 */
static char *my_domain;

/**
 * How many entries do we have in #cache_head DLL?
 */
static unsigned int cache_size;


/**
 * Remove @a entry from cache.
 *
 * @param rc entry to free
 */
static void
free_cache_entry (struct ResolveCache *rc)
{
  struct RecordListEntry *pos;

  while (NULL != (pos = rc->records_head))
  {
    GNUNET_CONTAINER_DLL_remove (rc->records_head, rc->records_tail, pos);
    GNUNET_DNSPARSER_free_record (pos->record);
    GNUNET_free (pos->record);
    GNUNET_free (pos);
  }
  GNUNET_free (rc->hostname);
  GNUNET_CONTAINER_DLL_remove (cache_head, cache_tail, rc);
  cache_size--;
  GNUNET_free (rc);
}


/**
 * Remove @a entry from cache.
 *
 * @param rc entry to free
 */
static void
free_hosts_entry (struct ResolveCache *rc)
{
  struct RecordListEntry *pos;

  while (NULL != (pos = rc->records_head))
  {
    GNUNET_CONTAINER_DLL_remove (rc->records_head, rc->records_tail, pos);
    GNUNET_DNSPARSER_free_record (pos->record);
    GNUNET_free (pos->record);
    GNUNET_free (pos);
  }
  GNUNET_free (rc->hostname);
  GNUNET_CONTAINER_DLL_remove (hosts_head, hosts_tail, rc);
  cache_size--;
  GNUNET_free (rc);
}


/**
 * Release resources associated with @a al
 *
 * @param al an active lookup
 */
static void
free_active_lookup (struct ActiveLookup *al)
{
  GNUNET_CONTAINER_DLL_remove (lookup_head, lookup_tail, al);
  if (NULL != al->resolve_handle)
  {
    GNUNET_DNSSTUB_resolve_cancel (al->resolve_handle);
    al->resolve_handle = NULL;
  }
  if (NULL != al->timeout_task)
  {
    GNUNET_SCHEDULER_cancel (al->timeout_task);
    al->timeout_task = NULL;
  }
  GNUNET_free (al->hostname);
  GNUNET_free (al);
}


/**
 * Find out if the configuration file line contains a string
 * starting with "nameserver ", and if so, return a copy of
 * the nameserver's IP.
 *
 * @param line line to parse
 * @param line_len number of characters in @a line
 * @return NULL if no nameserver is configured in this @a line
 */
static char *
extract_dns_server (const char *line, size_t line_len)
{
  if (0 == strncmp (line, "nameserver ", strlen ("nameserver ")))
    return GNUNET_strndup (line + strlen ("nameserver "),
                           line_len - strlen ("nameserver "));
  return NULL;
}


/**
 * Find out if the configuration file line contains a string
 * starting with "search ", and if so, return a copy of
 * the machine's search domain.
 *
 * @param line line to parse
 * @param line_len number of characters in @a line
 * @return NULL if no nameserver is configured in this @a line
 */
static char *
extract_search_domain (const char *line, size_t line_len)
{
  if (0 == strncmp (line, "search ", strlen ("search ")))
    return GNUNET_strndup (line + strlen ("search "),
                           line_len - strlen ("search "));
  return NULL;
}


/**
 * Reads the list of nameservers from /etc/resolve.conf
 *
 * @param server_addrs[out] a list of null-terminated server address strings
 * @return the number of server addresses in @server_addrs, -1 on error
 */
static int
lookup_dns_servers (char ***server_addrs)
{
  struct GNUNET_DISK_FileHandle *fh;
  struct GNUNET_DISK_MapHandle *mh;
  off_t bytes_read;
  const char *buf;
  size_t read_offset;
  unsigned int num_dns_servers;

  fh = GNUNET_DISK_file_open ("/etc/resolv.conf",
                              GNUNET_DISK_OPEN_READ,
                              GNUNET_DISK_PERM_NONE);
  if (NULL == fh)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Could not open /etc/resolv.conf. "
                "DNS resolution will not be possible.\n");
    return -1;
  }
  if (GNUNET_OK != GNUNET_DISK_file_handle_size (fh, &bytes_read))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Could not determine size of /etc/resolv.conf. "
                "DNS resolution will not be possible.\n");
    GNUNET_DISK_file_close (fh);
    return -1;
  }
  if (((unsigned long long) bytes_read) > (unsigned long long) SIZE_MAX)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "/etc/resolv.conf file too large to mmap. "
                "DNS resolution will not be possible.\n");
    GNUNET_DISK_file_close (fh);
    return -1;
  }
  buf = GNUNET_DISK_file_map (fh,
                              &mh,
                              GNUNET_DISK_MAP_TYPE_READ,
                              (size_t) bytes_read);
  *server_addrs = NULL;
  read_offset = 0;
  num_dns_servers = 0;
  while (read_offset < (size_t) bytes_read)
  {
    const char *newline;
    size_t line_len;
    char *dns_server;

    newline = strchr (buf + read_offset, '\n');
    if (NULL == newline)
      break;
    line_len = newline - buf - read_offset;
    dns_server = extract_dns_server (buf + read_offset, line_len);
    if (NULL != dns_server)
    {
      GNUNET_array_append (*server_addrs, num_dns_servers, dns_server);
    }
    else if (NULL == my_domain)
    {
      my_domain = extract_search_domain (buf + read_offset, line_len);
    }
    read_offset += line_len + 1;
  }
  GNUNET_DISK_file_unmap (mh);
  GNUNET_DISK_file_close (fh);
  return (int) num_dns_servers;
}


/**
 * Compute name to use for DNS reverse lookups from @a ip.
 *
 * @param ip IP address to resolve, in binary format, network byte order
 * @param af address family of @a ip, AF_INET or AF_INET6
 */
static char *
make_reverse_hostname (const void *ip, int af)
{
  char *buf = GNUNET_new_array (80, char);
  int pos = 0;

  if (AF_INET == af)
  {
    struct in_addr *addr = (struct in_addr *) ip;
    uint32_t ip_int = addr->s_addr;

    for (int i = 3; i >= 0; i--)
    {
      int n =
        GNUNET_snprintf (buf + pos, 80 - pos, "%u.", ((uint8_t *) &ip_int)[i]);
      if (n < 0)
      {
        GNUNET_free (buf);
        return NULL;
      }
      pos += n;
    }
    pos += GNUNET_snprintf (buf + pos, 80 - pos, "in-addr.arpa");
  }
  else if (AF_INET6 == af)
  {
    struct in6_addr *addr = (struct in6_addr *) ip;
    for (int i = 15; i >= 0; i--)
    {
      int n =
        GNUNET_snprintf (buf + pos, 80 - pos, "%x.", addr->s6_addr[i] & 0xf);
      if (n < 0)
      {
        GNUNET_free (buf);
        return NULL;
      }
      pos += n;
      n = GNUNET_snprintf (buf + pos, 80 - pos, "%x.", addr->s6_addr[i] >> 4);
      if (n < 0)
      {
        GNUNET_free (buf);
        return NULL;
      }
      pos += n;
    }
    pos += GNUNET_snprintf (buf + pos, 80 - pos, "ip6.arpa");
  }
  buf[pos] = '\0';
  return buf;
}


/**
 * Send DNS @a record back to our @a client.
 *
 * @param record information to transmit
 * @param record_type requested record type from client
 * @param client_request_id to which request are we responding
 * @param client where to send @a record
 * @return #GNUNET_YES if we sent a reply,
 *         #GNUNET_NO if the record type is not understood or
 *         does not match @a record_type
 */
static int
send_reply (struct GNUNET_DNSPARSER_Record *record,
            uint16_t record_type,
            uint32_t client_request_id,
            struct GNUNET_SERVICE_Client *client)
{
  struct GNUNET_RESOLVER_ResponseMessage *msg;
  struct GNUNET_MQ_Envelope *env;
  const void *payload;
  size_t payload_len;

  switch (record->type)
  {
  case GNUNET_DNSPARSER_TYPE_CNAME:
    if (GNUNET_DNSPARSER_TYPE_CNAME != record_type)
      return GNUNET_NO;
    payload = record->data.hostname;
    payload_len = strlen (record->data.hostname) + 1;
    break;

  case GNUNET_DNSPARSER_TYPE_PTR:
    if (GNUNET_DNSPARSER_TYPE_PTR != record_type)
      return GNUNET_NO;
    payload = record->data.hostname;
    payload_len = strlen (record->data.hostname) + 1;
    break;

  case GNUNET_DNSPARSER_TYPE_A:
    if ((GNUNET_DNSPARSER_TYPE_A != record_type) &&
        (GNUNET_DNSPARSER_TYPE_ALL != record_type))
      return GNUNET_NO;
    payload = record->data.raw.data;
    payload_len = record->data.raw.data_len;
    break;

  case GNUNET_DNSPARSER_TYPE_AAAA:
    if ((GNUNET_DNSPARSER_TYPE_AAAA != record_type) &&
        (GNUNET_DNSPARSER_TYPE_ALL != record_type))
      return GNUNET_NO;
    payload = record->data.raw.data;
    payload_len = record->data.raw.data_len;
    break;

  default:
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Cannot handle DNS response type %u: not supported here\n",
                record->type);
    return GNUNET_NO;
  }
  env = GNUNET_MQ_msg_extra (msg,
                             payload_len,
                             GNUNET_MESSAGE_TYPE_RESOLVER_RESPONSE);
  msg->client_id = client_request_id;
  GNUNET_memcpy (&msg[1], payload, payload_len);
  GNUNET_MQ_send (GNUNET_SERVICE_client_get_mq (client), env);
  return GNUNET_YES;
}


/**
 * Send message to @a client that we transmitted all
 * responses for @a client_request_id
 *
 * @param client_request_id to which request are we responding
 * @param client where to send @a record
 */
static void
send_end_msg (uint32_t client_request_id, struct GNUNET_SERVICE_Client *client)
{
  struct GNUNET_RESOLVER_ResponseMessage *msg;
  struct GNUNET_MQ_Envelope *env;

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "Sending END message\n");
  env = GNUNET_MQ_msg (msg, GNUNET_MESSAGE_TYPE_RESOLVER_RESPONSE);
  msg->client_id = client_request_id;
  GNUNET_MQ_send (GNUNET_SERVICE_client_get_mq (client), env);
}


/**
 * Remove expired entries from @a rc
 *
 * @param rc entry in resolver cache
 * @return #GNUNET_YES if @a rc was completely expired
 *         #GNUNET_NO if some entries are left
 */
static int
remove_expired (struct ResolveCache *rc)
{
  struct GNUNET_TIME_Absolute now = GNUNET_TIME_absolute_get ();
  struct RecordListEntry *n;

  for (struct RecordListEntry *pos = rc->records_head; NULL != pos; pos = n)
  {
    n = pos->next;
    if (now.abs_value_us > pos->record->expiration_time.abs_value_us)
    {
      GNUNET_CONTAINER_DLL_remove (rc->records_head, rc->records_tail, pos);
      GNUNET_DNSPARSER_free_record (pos->record);
      GNUNET_free (pos->record);
      GNUNET_free (pos);
    }
  }
  if (NULL == rc->records_head)
  {
    free_cache_entry (rc);
    return GNUNET_YES;
  }
  return GNUNET_NO;
}


/**
 * Process DNS request for @a hostname with request ID @a request_id
 * from @a client demanding records of type @a record_type.
 *
 * @param hostname DNS name to resolve
 * @param record_type desired record type
 * @param client_request_id client's request ID
 * @param client who should get the result?
 */
static void
process_get (const char *hostname,
             uint16_t record_type,
             uint32_t client_request_id,
             struct GNUNET_SERVICE_Client *client);


/**
 * Get an IP address as a string (works for both IPv4 and IPv6).  Note
 * that the resolution happens asynchronously and that the first call
 * may not immediately result in the FQN (but instead in a
 * human-readable IP address).
 *
 * @param hostname what hostname was to be resolved
 * @param record_type what type of record was requested
 * @param client_request_id unique identification of the client's request
 * @param client handle to the client making the request (for sending the reply)
 */
static int
try_cache (const char *hostname,
           uint16_t record_type,
           uint32_t client_request_id,
           struct GNUNET_SERVICE_Client *client)
{
  struct ResolveCache *pos;
  struct ResolveCache *next;
  int found;
  int in_hosts;

  in_hosts = GNUNET_NO;
  for (pos = hosts_head; NULL != pos; pos = pos->next)
    if (0 == strcmp (pos->hostname, hostname))
    {
      in_hosts = GNUNET_YES;
      break;
    }
  if (NULL == pos)
  {
    next = cache_head;
    for (pos = next; NULL != pos; pos = next)
    {
      next = pos->next;
      if (GNUNET_YES == remove_expired (pos))
        continue;
      if (0 == strcmp (pos->hostname, hostname))
        break;
    }
  }
  if (NULL == pos)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "No cache entry for '%s'\n", hostname);
    return GNUNET_NO;
  }
  if ((GNUNET_NO == in_hosts) && (cache_head != pos))
  {
    /* move result to head to achieve LRU for cache eviction */
    GNUNET_CONTAINER_DLL_remove (cache_head, cache_tail, pos);
    GNUNET_CONTAINER_DLL_insert (cache_head, cache_tail, pos);
  }
  found = GNUNET_NO;
  for (struct RecordListEntry *rle = pos->records_head; NULL != rle;
       rle = rle->next)
  {
    const struct GNUNET_DNSPARSER_Record *record = rle->record;

    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Checking cache entry for '%s', record is for '%s'\n",
                hostname,
                record->name);
    if ((GNUNET_DNSPARSER_TYPE_CNAME == record->type) &&
        (GNUNET_DNSPARSER_TYPE_CNAME != record_type) && (GNUNET_NO == found))
    {
      const char *hostname = record->data.hostname;

      process_get (hostname, record_type, client_request_id, client);
      return GNUNET_YES;     /* counts as a cache "hit" */
    }
    if (0 == strcmp (record->name, hostname))
      found |= send_reply (rle->record, record_type, client_request_id, client);
  }
  if (GNUNET_NO == found)
    return GNUNET_NO; /* had records, but none matched! */
  send_end_msg (client_request_id, client);
  return GNUNET_YES;
}


/**
 * Create DNS query for @a hostname of type @a type
 * with DNS request ID @a dns_id.
 *
 * @param hostname DNS name to query
 * @param type requested DNS record type
 * @param dns_id what should be the DNS request ID
 * @param packet_buf[out] where to write the request packet
 * @param packet_size[out] set to size of @a packet_buf on success
 * @return #GNUNET_OK on success
 */
static int
pack (const char *hostname,
      uint16_t type,
      uint16_t dns_id,
      char **packet_buf,
      size_t *packet_size)
{
  struct GNUNET_DNSPARSER_Query query;
  struct GNUNET_DNSPARSER_Packet packet;

  query.name = (char *) hostname;
  query.type = type;
  query.dns_traffic_class = GNUNET_TUN_DNS_CLASS_INTERNET;
  memset (&packet, 0, sizeof(packet));
  packet.num_queries = 1;
  packet.queries = &query;
  packet.id = htons (dns_id);
  packet.flags.recursion_desired = 1;
  if (GNUNET_OK !=
      GNUNET_DNSPARSER_pack (&packet, UINT16_MAX, packet_buf, packet_size))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to pack query for hostname `%s'\n",
                hostname);
    packet_buf = NULL;
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


static void
cache_answers (const char *name,
               struct GNUNET_DNSPARSER_Record *records,
               unsigned int num_records)
{
  struct ResolveCache *rc;
  struct GNUNET_DNSPARSER_Record *record;
  struct RecordListEntry *rle;

  for (unsigned int i = 0; i != num_records; i++)
  {
    record = &records[i];

    for (rc = cache_head; NULL != rc; rc = rc->next)
      if (0 == strcasecmp (rc->hostname, name))
        break;
    if (NULL == rc)
    {
      rc = GNUNET_new (struct ResolveCache);
      rc->hostname = GNUNET_strdup (name);
      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                  "Caching record for name %s under %s\n",
                  record->name, name);
      GNUNET_CONTAINER_DLL_insert (cache_head, cache_tail, rc);
      cache_size++;
    }
    /* TODO: ought to check first if we have this exact record
       already in the cache! */
    rle = GNUNET_new (struct RecordListEntry);
    rle->record = GNUNET_DNSPARSER_duplicate_record (record);
    GNUNET_CONTAINER_DLL_insert (rc->records_head, rc->records_tail, rle);
  }
}


/**
 * We got a result from DNS. Add it to the cache and
 * see if we can make our client happy...
 *
 * @param cls the `struct ActiveLookup`
 * @param dns the DNS response
 * @param dns_len number of bytes in @a dns
 */
static void
handle_resolve_result (void *cls,
                       const struct GNUNET_TUN_DnsHeader *dns,
                       size_t dns_len)
{
  struct ActiveLookup *al = cls;
  struct GNUNET_DNSPARSER_Packet *parsed;

  parsed = GNUNET_DNSPARSER_parse ((const char *) dns, dns_len);
  if (NULL == parsed)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to parse DNS reply (hostname %s, request ID %u)\n",
                al->hostname,
                al->dns_id);
    return;
  }
  if (al->dns_id != ntohs (parsed->id))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Request ID in DNS reply does not match\n");
    GNUNET_DNSPARSER_free_packet (parsed);
    return;
  }
  if (0 == parsed->num_answers + parsed->num_authority_records
      + parsed->num_additional_records)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "DNS reply (hostname %s, request ID %u) contains no answers\n",
                al->hostname,
                (unsigned int) al->client_request_id);
    /* resume by trying again from cache */
    if (GNUNET_NO == try_cache (al->hostname,
                                al->record_type,
                                al->client_request_id,
                                al->client))
    /* cache failed, tell client we could not get an answer */
    {
      send_end_msg (al->client_request_id, al->client);
    }
    GNUNET_DNSPARSER_free_packet (parsed);
    free_active_lookup (al);
    return;
  }
  /* LRU-based cache eviction: we remove from tail */
  while (cache_size > MAX_CACHE)
    free_cache_entry (cache_tail);

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Got reply for hostname %s and request ID %u\n",
              al->hostname,
              (unsigned int) al->client_request_id);
  /* add to cache */
  cache_answers (al->hostname, parsed->answers, parsed->num_answers);
  cache_answers (al->hostname,
                 parsed->authority_records,
                 parsed->num_authority_records);
  cache_answers (al->hostname,
                 parsed->additional_records,
                 parsed->num_additional_records);

  /* see if we need to do the 2nd request for AAAA records */
  if ((GNUNET_DNSPARSER_TYPE_ALL == al->record_type) &&
      (GNUNET_NO == al->did_aaaa))
  {
    char *packet_buf;
    size_t packet_size;
    uint16_t dns_id;

    dns_id = (uint16_t) GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_NONCE,
                                                  UINT16_MAX);
    if (GNUNET_OK == pack (al->hostname,
                           GNUNET_DNSPARSER_TYPE_AAAA,
                           dns_id,
                           &packet_buf,
                           &packet_size))
    {
      al->did_aaaa = GNUNET_YES;
      al->dns_id = dns_id;
      GNUNET_DNSSTUB_resolve_cancel (al->resolve_handle);
      al->resolve_handle = GNUNET_DNSSTUB_resolve (dnsstub_ctx,
                                                   packet_buf,
                                                   packet_size,
                                                   &handle_resolve_result,
                                                   al);
      GNUNET_free (packet_buf);
      GNUNET_DNSPARSER_free_packet (parsed);
      return;
    }
  }

  /* resume by trying again from cache */
  if (GNUNET_NO == try_cache (al->hostname,
                              al->record_type,
                              al->client_request_id,
                              al->client))
  /* cache failed, tell client we could not get an answer */
  {
    send_end_msg (al->client_request_id, al->client);
  }
  free_active_lookup (al);
  GNUNET_DNSPARSER_free_packet (parsed);
}


/**
 * We encountered a timeout trying to perform a
 * DNS lookup.
 *
 * @param cls a `struct ActiveLookup`
 */
static void
handle_resolve_timeout (void *cls)
{
  struct ActiveLookup *al = cls;

  al->timeout_task = NULL;
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "DNS lookup timeout!\n");
  send_end_msg (al->client_request_id, al->client);
  free_active_lookup (al);
}


/**
 * Initiate an active lookup, then cache the result and
 * try to then complete the resolution.
 *
 * @param hostname DNS name to resolve
 * @param record_type record type to locate
 * @param client_request_id client request ID
 * @param client handle to the client
 * @return #GNUNET_OK if the DNS query is now pending
 */
static int
resolve_and_cache (const char *hostname,
                   uint16_t record_type,
                   uint32_t client_request_id,
                   struct GNUNET_SERVICE_Client *client)
{
  char *packet_buf;
  size_t packet_size;
  struct ActiveLookup *al;
  uint16_t dns_id;
  uint16_t type;

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "resolve_and_cache `%s'\n", hostname);
  dns_id = (uint16_t) GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_NONCE,
                                                UINT16_MAX);

  if (GNUNET_DNSPARSER_TYPE_ALL == record_type)
    type = GNUNET_DNSPARSER_TYPE_A;
  else
    type = record_type;
  if (GNUNET_OK != pack (hostname, type, dns_id, &packet_buf, &packet_size))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to pack query for hostname `%s'\n",
                hostname);
    return GNUNET_SYSERR;
  }

  al = GNUNET_new (struct ActiveLookup);
  al->hostname = GNUNET_strdup (hostname);
  al->record_type = record_type;
  al->client_request_id = client_request_id;
  al->dns_id = dns_id;
  al->client = client;
  al->timeout_task =
    GNUNET_SCHEDULER_add_delayed (DNS_TIMEOUT, &handle_resolve_timeout, al);
  al->resolve_handle = GNUNET_DNSSTUB_resolve (dnsstub_ctx,
                                               packet_buf,
                                               packet_size,
                                               &handle_resolve_result,
                                               al);
  GNUNET_free (packet_buf);
  GNUNET_CONTAINER_DLL_insert (lookup_head, lookup_tail, al);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Resolving %s, client_request_id = %u, dns_id = %u\n",
              hostname,
              (unsigned int) client_request_id,
              (unsigned int) dns_id);
  return GNUNET_OK;
}


/**
 * Process DNS request for @a hostname with request ID @a client_request_id
 * from @a client demanding records of type @a record_type.
 *
 * @param hostname DNS name to resolve
 * @param record_type desired record type
 * @param client_request_id client's request ID
 * @param client who should get the result?
 */
static void
process_get (const char *hostname,
             uint16_t record_type,
             uint32_t client_request_id,
             struct GNUNET_SERVICE_Client *client)
{
  char fqdn[255];

  if (GNUNET_NO != try_cache (hostname, record_type, client_request_id, client))
    return;
  if ((NULL != my_domain) && (NULL == strchr (hostname, (unsigned char) '.')) &&
      (strlen (hostname) + strlen (my_domain) <= 253))
  {
    GNUNET_snprintf (fqdn, sizeof(fqdn), "%s.%s", hostname, my_domain);
  }
  else if (strlen (hostname) < 255)
  {
    GNUNET_snprintf (fqdn, sizeof(fqdn), "%s", hostname);
  }
  else
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (client);
    return;
  }
  if (GNUNET_NO == try_cache (fqdn, record_type, client_request_id, client))
  {
    if (GNUNET_OK !=
        resolve_and_cache (fqdn, record_type, client_request_id, client))
    {
      send_end_msg (client_request_id, client);
    }
  }
}


/**
 * Verify well-formedness of GET-message.
 *
 * @param cls closure, unused
 * @param get the actual message
 * @return #GNUNET_OK if @a get is well-formed
 */
static int
check_get (void *cls, const struct GNUNET_RESOLVER_GetMessage *get)
{
  uint16_t size;
  int direction;
  int af;

  (void) cls;
  size = ntohs (get->header.size) - sizeof(*get);
  direction = ntohl (get->direction);
  if (GNUNET_NO == direction)
  {
    GNUNET_MQ_check_zero_termination (get);
    return GNUNET_OK;
  }
  af = ntohl (get->af);
  switch (af)
  {
  case AF_INET:
    if (size != sizeof(struct in_addr))
    {
      GNUNET_break (0);
      return GNUNET_SYSERR;
    }
    break;

  case AF_INET6:
    if (size != sizeof(struct in6_addr))
    {
      GNUNET_break (0);
      return GNUNET_SYSERR;
    }
    break;

  default:
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Handle GET-message.
 *
 * @param cls identification of the client
 * @param msg the actual message
 */
static void
handle_get (void *cls, const struct GNUNET_RESOLVER_GetMessage *msg)
{
  struct GNUNET_SERVICE_Client *client = cls;
  int direction;
  int af;
  uint32_t client_request_id;
  char *hostname;

  direction = ntohl (msg->direction);
  af = ntohl (msg->af);
  client_request_id = msg->client_id;
  GNUNET_SERVICE_client_continue (client);
  if (GNUNET_NO == direction)
  {
    /* IP from hostname */
    hostname = GNUNET_strdup ((const char *) &msg[1]);
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Client asks to resolve `%s'\n",
                hostname);
    switch (af)
    {
    case AF_UNSPEC: {
        process_get (hostname,
                     GNUNET_DNSPARSER_TYPE_ALL,
                     client_request_id,
                     client);
        break;
      }

    case AF_INET: {
        process_get (hostname,
                     GNUNET_DNSPARSER_TYPE_A,
                     client_request_id,
                     client);
        break;
      }

    case AF_INET6: {
        process_get (hostname,
                     GNUNET_DNSPARSER_TYPE_AAAA,
                     client_request_id,
                     client);
        break;
      }

    default: {
        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "got invalid af: %d\n", af);
        GNUNET_assert (0);
      }
    }
  }
  else
  {
    /* hostname from IP */
    hostname = make_reverse_hostname (&msg[1], af);
    process_get (hostname,
                 GNUNET_DNSPARSER_TYPE_PTR,
                 client_request_id,
                 client);
  }
  GNUNET_free (hostname);
}


/**
 * Service is shutting down, clean up.
 *
 * @param cls NULL, unused
 */
static void
shutdown_task (void *cls)
{
  (void) cls;

  while (NULL != lookup_head)
    free_active_lookup (lookup_head);
  while (NULL != cache_head)
    free_cache_entry (cache_head);
  while (NULL != hosts_head)
    free_hosts_entry (hosts_head);
  GNUNET_DNSSTUB_stop (dnsstub_ctx);
  GNUNET_free (my_domain);
}


/**
 * Add information about a host from /etc/hosts
 * to our cache.
 *
 * @param hostname the name of the host
 * @param rec_type DNS record type to use
 * @param data payload
 * @param data_size number of bytes in @a data
 */
static void
add_host (const char *hostname,
          uint16_t rec_type,
          const void *data,
          size_t data_size)
{
  struct ResolveCache *rc;
  struct RecordListEntry *rle;
  struct GNUNET_DNSPARSER_Record *rec;

  rec = GNUNET_malloc (sizeof(struct GNUNET_DNSPARSER_Record));
  rec->expiration_time = GNUNET_TIME_UNIT_FOREVER_ABS;
  rec->type = rec_type;
  rec->dns_traffic_class = GNUNET_TUN_DNS_CLASS_INTERNET;
  rec->name = GNUNET_strdup (hostname);
  rec->data.raw.data = GNUNET_memdup (data, data_size);
  rec->data.raw.data_len = data_size;
  rle = GNUNET_new (struct RecordListEntry);
  rle->record = rec;
  rc = GNUNET_new (struct ResolveCache);
  rc->hostname = GNUNET_strdup (hostname);
  GNUNET_CONTAINER_DLL_insert (rc->records_head, rc->records_tail, rle);
  GNUNET_CONTAINER_DLL_insert (hosts_head, hosts_tail, rc);
}


/**
 * Extract host information from a line in /etc/hosts
 *
 * @param line the line to parse
 * @param line_len number of bytes in @a line
 */
static void
extract_hosts (const char *line, size_t line_len)
{
  const char *c;
  struct in_addr v4;
  struct in6_addr v6;
  char *tbuf;
  char *tok;

  /* ignore everything after '#' */
  c = memrchr (line, (unsigned char) '#', line_len);
  if (NULL != c)
    line_len = c - line;
  /* ignore leading whitespace */
  while ((0 < line_len) && isspace ((unsigned char) *line))
  {
    line++;
    line_len--;
  }
  tbuf = GNUNET_strndup (line, line_len);
  tok = strtok (tbuf, " \t");
  if (NULL == tok)
  {
    GNUNET_free (tbuf);
    return;
  }
  if (1 == inet_pton (AF_INET, tok, &v4))
  {
    while (NULL != (tok = strtok (NULL, " \t")))
      add_host (tok, GNUNET_DNSPARSER_TYPE_A, &v4, sizeof(struct in_addr));
  }
  else if (1 == inet_pton (AF_INET6, tok, &v6))
  {
    while (NULL != (tok = strtok (NULL, " \t")))
      add_host (tok, GNUNET_DNSPARSER_TYPE_AAAA, &v6, sizeof(struct in6_addr));
  }
  GNUNET_free (tbuf);
}


/**
 * Reads the list of hosts from /etc/hosts.
 */
static void
load_etc_hosts (void)
{
  struct GNUNET_DISK_FileHandle *fh;
  struct GNUNET_DISK_MapHandle *mh;
  off_t bytes_read;
  const char *buf;
  size_t read_offset;

  fh = GNUNET_DISK_file_open ("/etc/hosts",
                              GNUNET_DISK_OPEN_READ,
                              GNUNET_DISK_PERM_NONE);
  if (NULL == fh)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO, "Failed to open /etc/hosts");
    return;
  }
  if (GNUNET_OK != GNUNET_DISK_file_handle_size (fh, &bytes_read))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Could not determin size of /etc/hosts. "
                "DNS resolution will not be possible.\n");
    GNUNET_DISK_file_close (fh);
    return;
  }
  if (((unsigned long long) bytes_read) > (unsigned long long) SIZE_MAX)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "/etc/hosts file too large to mmap. "
                "DNS resolution will not be possible.\n");
    GNUNET_DISK_file_close (fh);
    return;
  }
  buf = GNUNET_DISK_file_map (fh,
                              &mh,
                              GNUNET_DISK_MAP_TYPE_READ,
                              (size_t) bytes_read);
  read_offset = 0;
  while (read_offset < (size_t) bytes_read)
  {
    const char *newline;
    size_t line_len;

    newline = strchr (buf + read_offset, '\n');
    if (NULL == newline)
      break;
    line_len = newline - buf - read_offset;
    extract_hosts (buf + read_offset, line_len);
    read_offset += line_len + 1;
  }
  GNUNET_DISK_file_unmap (mh);
  GNUNET_DISK_file_close (fh);
}


/**
 * Service is starting, initialize everything.
 *
 * @param cls NULL, unused
 * @param cfg our configuration
 * @param sh service handle
 */
static void
init_cb (void *cls,
         const struct GNUNET_CONFIGURATION_Handle *cfg,
         struct GNUNET_SERVICE_Handle *sh)
{
  char **dns_servers;
  int num_dns_servers;

  (void) cfg;
  (void) sh;
  load_etc_hosts ();
  GNUNET_SCHEDULER_add_shutdown (&shutdown_task, cls);
  dnsstub_ctx = GNUNET_DNSSTUB_start (128);
  dns_servers = NULL;
  num_dns_servers = lookup_dns_servers (&dns_servers);
  if (0 >= num_dns_servers)
  {
    GNUNET_log (
      GNUNET_ERROR_TYPE_ERROR,
      _ ("No DNS server available. DNS resolution will not be possible.\n"));
    return;
  }
  for (int i = 0; i < num_dns_servers; i++)
  {
    int result = GNUNET_DNSSTUB_add_dns_ip (dnsstub_ctx, dns_servers[i]);
    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
                "Adding DNS server '%s': %s\n",
                dns_servers[i],
                GNUNET_OK == result ? "success" : "failure");
    GNUNET_free (dns_servers[i]);
  }
  GNUNET_free (dns_servers);
}


/**
 * Callback called when a client connects to the service.
 *
 * @param cls closure for the service, unused
 * @param c the new client that connected to the service
 * @param mq the message queue used to send messages to the client
 * @return @a c
 */
static void *
connect_cb (void *cls,
            struct GNUNET_SERVICE_Client *c,
            struct GNUNET_MQ_Handle *mq)
{
  (void) cls;
  (void) mq;

  return c;
}


/**
 * Callback called when a client disconnected from the service
 *
 * @param cls closure for the service
 * @param c the client that disconnected
 * @param internal_cls should be equal to @a c
 */
static void
disconnect_cb (void *cls, struct GNUNET_SERVICE_Client *c, void *internal_cls)
{
  struct ActiveLookup *n;

  (void) cls;

  GNUNET_assert (c == internal_cls);
  n = lookup_head;
  for (struct ActiveLookup *al = n; NULL != al; al = n)
  {
    n = al->next;
    if (al->client == c)
      free_active_lookup (al);
  }
}


/**
 * Define "main" method using service macro.
 */
GNUNET_SERVICE_MAIN (
  "resolver",
  GNUNET_SERVICE_OPTION_NONE,
  &init_cb,
  &connect_cb,
  &disconnect_cb,
  NULL,
  GNUNET_MQ_hd_var_size (get,
                         GNUNET_MESSAGE_TYPE_RESOLVER_REQUEST,
                         struct GNUNET_RESOLVER_GetMessage,
                         NULL),
  GNUNET_MQ_handler_end ());


#if defined(__linux__) && defined(__GLIBC__)
#include <malloc.h>

/**
 * MINIMIZE heap size (way below 128k) since this process doesn't need much.
 */
void __attribute__ ((constructor))
GNUNET_RESOLVER_memory_init ()
{
  mallopt (M_TRIM_THRESHOLD, 4 * 1024);
  mallopt (M_TOP_PAD, 1 * 1024);
  malloc_trim (0);
}


#endif


/* end of gnunet-service-resolver.c */
