/**
 * collectd - src/ntpd.c
 * Copyright (C) 2006-2012  Florian octo Forster
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 *
 * Authors:
 *   Florian octo Forster <octo at collectd.org>
 **/

#define _DEFAULT_SOURCE
#define _BSD_SOURCE /* For NI_MAXHOST */

#include "collectd.h"

#include "plugin.h"
#include "utils/common/common.h"

#if HAVE_NETDB_H
#include <netdb.h>
#endif
#if HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#if HAVE_ARPA_INET_H
#include <arpa/inet.h> /* inet_ntoa */
#endif
#if HAVE_NETINET_TCP_H
#include <netinet/tcp.h>
#endif
#if HAVE_POLL_H
#include <poll.h>
#endif

#ifndef STA_NANO
#define STA_NANO 0x2000
#endif

static const char *config_keys[] = {"Host", "Port", "ReverseLookups",
                                    "IncludeUnitID"};
static int config_keys_num = STATIC_ARRAY_SIZE(config_keys);

static bool do_reverse_lookups = true;

/* This option only exists for backward compatibility. If it is false and two
 * ntpd peers use the same refclock driver, the plugin will try to write
 * simultaneous measurements from both to the same type instance. */
static bool include_unit_id;

#define NTPD_DEFAULT_HOST "localhost"
#define NTPD_DEFAULT_PORT "123"
static int sock_descr = -1;
static char *ntpd_host;
static char ntpd_port[16];

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * The following definitions were copied from the NTPd distribution  *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
#define MAXFILENAME 128
#define MAXSEQ 127
#define MODE_PRIVATE 7
#define NTP_OLDVERSION ((uint8_t)1) /* oldest credible version */
#define IMPL_XNTPD 3
#define FP_FRAC 65536.0

#define REFCLOCK_ADDR 0x7f7f0000 /* 127.127.0.0 */
#define REFCLOCK_MASK 0xffff0000 /* 255.255.0.0 */

/* This structure is missing the message authentication code, since collectd
 * doesn't use it. */
struct req_pkt {
  uint8_t rm_vn_mode;
  uint8_t auth_seq;
  uint8_t implementation;      /* implementation number */
  uint8_t request;             /* request number */
  uint16_t err_nitems;         /* error code/number of data items */
  uint16_t mbz_itemsize;       /* item size */
  char data[MAXFILENAME + 48]; /* data area [32 prev](176 byte max) */
                               /* struct conf_peer must fit */
};
#define REQ_LEN_NOMAC (sizeof(struct req_pkt))

/*
 * A response packet.  The length here is variable, this is a
 * maximally sized one.  Note that this implementation doesn't
 * authenticate responses.
 */
#define RESP_HEADER_SIZE (8)
#define RESP_DATA_SIZE (500)

struct resp_pkt {
  uint8_t rm_vn_mode;        /* response, more, version, mode */
  uint8_t auth_seq;          /* key, sequence number */
  uint8_t implementation;    /* implementation number */
  uint8_t request;           /* request number */
  uint16_t err_nitems;       /* error code/number of data items */
  uint16_t mbz_itemsize;     /* item size */
  char data[RESP_DATA_SIZE]; /* data area */
};

/*
 * Bit setting macros for multifield items.
 */
#define RESP_BIT 0x80
#define MORE_BIT 0x40

#define ISRESPONSE(rm_vn_mode) (((rm_vn_mode)&RESP_BIT) != 0)
#define ISMORE(rm_vn_mode) (((rm_vn_mode)&MORE_BIT) != 0)
#define INFO_VERSION(rm_vn_mode) ((uint8_t)(((rm_vn_mode) >> 3) & 0x7))
#define INFO_MODE(rm_vn_mode) ((rm_vn_mode)&0x7)

#define RM_VN_MODE(resp, more, version)                                        \
  ((uint8_t)(((resp) ? RESP_BIT : 0) | ((more) ? MORE_BIT : 0) |               \
             ((version ? version : (NTP_OLDVERSION + 1)) << 3) |               \
             (MODE_PRIVATE)))

#define INFO_IS_AUTH(auth_seq) (((auth_seq)&0x80) != 0)
#define INFO_SEQ(auth_seq) ((auth_seq)&0x7f)
#define AUTH_SEQ(auth, seq)                                                    \
  ((uint8_t)((((auth) != 0) ? 0x80 : 0) | ((seq)&0x7f)))

#define INFO_ERR(err_nitems) ((uint16_t)((ntohs(err_nitems) >> 12) & 0xf))
#define INFO_NITEMS(err_nitems) ((uint16_t)(ntohs(err_nitems) & 0xfff))
#define ERR_NITEMS(err, nitems)                                                \
  (htons((uint16_t)((((uint16_t)(err) << 12) & 0xf000) |                       \
                    ((uint16_t)(nitems)&0xfff))))

#define INFO_MBZ(mbz_itemsize) ((ntohs(mbz_itemsize) >> 12) & 0xf)
#define INFO_ITEMSIZE(mbz_itemsize) ((uint16_t)(ntohs(mbz_itemsize) & 0xfff))
#define MBZ_ITEMSIZE(itemsize) (htons((uint16_t)(itemsize)))

/* negate a long float type */
#define M_NEG(v_i, v_f)                                                        \
  do {                                                                         \
    if ((v_f) == 0)                                                            \
      (v_i) = -((uint32_t)(v_i));                                              \
    else {                                                                     \
      (v_f) = -((uint32_t)(v_f));                                              \
      (v_i) = ~(v_i);                                                          \
    }                                                                          \
  } while (0)
/* l_fp to double */
#define M_LFPTOD(r_i, r_uf, d)                                                 \
  do {                                                                         \
    register int32_t ri;                                                       \
    register uint32_t rf;                                                      \
                                                                               \
    ri = (r_i);                                                                \
    rf = (r_uf);                                                               \
    if (ri < 0) {                                                              \
      M_NEG(ri, rf);                                                           \
      (d) = -((double)ri + ((double)rf) / 4294967296.0);                       \
    } else {                                                                   \
      (d) = (double)ri + ((double)rf) / 4294967296.0;                          \
    }                                                                          \
  } while (0)

#define REQ_PEER_LIST_SUM 1
struct info_peer_summary {
  uint32_t dstadr;         /* local address (zero for undetermined) */
  uint32_t srcadr;         /* source address */
  uint16_t srcport;        /* source port */
  uint8_t stratum;         /* stratum of peer */
  int8_t hpoll;            /* host polling interval */
  int8_t ppoll;            /* peer polling interval */
  uint8_t reach;           /* reachability register */
  uint8_t flags;           /* flags, from above */
  uint8_t hmode;           /* peer mode */
  int32_t delay;           /* peer.estdelay; s_fp */
  int32_t offset_int;      /* peer.estoffset; integral part */
  int32_t offset_frc;      /* peer.estoffset; fractional part */
  uint32_t dispersion;     /* peer.estdisp; u_fp */
  uint32_t v6_flag;        /* is this v6 or not */
  uint32_t unused1;        /* (unused) padding for dstadr6 */
  struct in6_addr dstadr6; /* local address (v6) */
  struct in6_addr srcadr6; /* source address (v6) */
};

#define REQ_SYS_INFO 4
struct info_sys {
  uint32_t peer;           /* system peer address (v4) */
  uint8_t peer_mode;       /* mode we are syncing to peer in */
  uint8_t leap;            /* system leap bits */
  uint8_t stratum;         /* our stratum */
  int8_t precision;        /* local clock precision */
  int32_t rootdelay;       /* distance from sync source */
  uint32_t rootdispersion; /* dispersion from sync source */
  uint32_t refid;          /* reference ID of sync source */
  uint64_t reftime;        /* system reference time */
  uint32_t poll;           /* system poll interval */
  uint8_t flags;           /* system flags */
  uint8_t unused1;         /* unused */
  uint8_t unused2;         /* unused */
  uint8_t unused3;         /* unused */
  int32_t bdelay;          /* default broadcast offset */
  int32_t frequency;       /* frequency residual (scaled ppm)  */
  uint64_t authdelay;      /* default authentication delay */
  uint32_t stability;      /* clock stability (scaled ppm) */
  int32_t v6_flag;         /* is this v6 or not */
  int32_t unused4;         /* unused, padding for peer6 */
  struct in6_addr peer6;   /* system peer address (v6) */
};

#define REQ_GET_KERNEL 38
struct info_kernel {
  int32_t offset;
  int32_t freq;
  int32_t maxerror;
  int32_t esterror;
  uint16_t status;
  uint16_t shift;
  int32_t constant;
  int32_t precision;
  int32_t tolerance;
  /* pps stuff */
  int32_t ppsfreq;
  int32_t jitter;
  int32_t stabil;
  int32_t jitcnt;
  int32_t calcnt;
  int32_t errcnt;
  int32_t stbcnt;
};

/* List of reference clock names */
static const char *refclock_names[] = {
    "UNKNOWN",    "LOCAL",        "GPS_TRAK",   "WWV_PST",     /*  0- 3 */
    "SPECTRACOM", "TRUETIME",     "IRIG_AUDIO", "CHU_AUDIO",   /*  4- 7 */
    "GENERIC",    "GPS_MX4200",   "GPS_AS2201", "GPS_ARBITER", /*  8-11 */
    "IRIG_TPRO",  "ATOM_LEITCH",  "MSF_EES",    "GPSTM_TRUE",  /* 12-15 */
    "GPS_BANC",   "GPS_DATUM",    "ACTS_NIST",  "WWV_HEATH",   /* 16-19 */
    "GPS_NMEA",   "GPS_VME",      "PPS",        "ACTS_PTB",    /* 20-23 */
    "ACTS_USNO",  "TRUETIME",     "GPS_HP",     "MSF_ARCRON",  /* 24-27 */
    "SHM",        "GPS_PALISADE", "GPS_ONCORE", "GPS_JUPITER", /* 28-31 */
    "CHRONOLOG",  "DUMBCLOCK",    "ULINK_M320", "PCF",         /* 32-35 */
    "WWV_AUDIO",  "GPS_FG",       "HOPF_S",     "HOPF_P",      /* 36-39 */
    "JJY",        "TT_IRIG",      "GPS_ZYFER",  "GPS_RIPENCC", /* 40-43 */
    "NEOCLK4X",   "PCI_TSYNC",    "GPSD_JSON"                  /* 44-46 */
};
static size_t refclock_names_num = STATIC_ARRAY_SIZE(refclock_names);
/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * End of the copied stuff..                                         *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

static int ntpd_config(const char *key, const char *value) {
  if (strcasecmp(key, "Host") == 0) {
    if (ntpd_host != NULL)
      free(ntpd_host);
    if ((ntpd_host = strdup(value)) == NULL)
      return 1;
  } else if (strcasecmp(key, "Port") == 0) {
    int port = (int)(atof(value));
    if ((port > 0) && (port <= 65535))
      snprintf(ntpd_port, sizeof(ntpd_port), "%i", port);
    else
      sstrncpy(ntpd_port, value, sizeof(ntpd_port));
  } else if (strcasecmp(key, "ReverseLookups") == 0) {
    if (IS_TRUE(value))
      do_reverse_lookups = true;
    else
      do_reverse_lookups = false;
  } else if (strcasecmp(key, "IncludeUnitID") == 0) {
    if (IS_TRUE(value))
      include_unit_id = true;
    else
      include_unit_id = false;
  } else {
    return -1;
  }

  return 0;
}

static void ntpd_submit(const char *type, const char *type_inst,
                        gauge_t value) {
  value_list_t vl = VALUE_LIST_INIT;

  vl.values = &(value_t){.gauge = value};
  vl.values_len = 1;
  sstrncpy(vl.plugin, "ntpd", sizeof(vl.plugin));
  sstrncpy(vl.type, type, sizeof(vl.type));
  sstrncpy(vl.type_instance, type_inst, sizeof(vl.type_instance));

  plugin_dispatch_values(&vl);
}

/* Each time a peer is polled, ntpd shifts the reach register to the left and
 * sets the LSB based on whether the peer was reachable. If the LSB is zero,
 * the values are out of date. */
static void ntpd_submit_reach(const char *type, const char *type_inst,
                              uint8_t reach, gauge_t value) {
  if (!(reach & 1))
    value = NAN;

  ntpd_submit(type, type_inst, value);
}

static int ntpd_connect(void) {
  const char *host;
  const char *port;

  struct addrinfo *ai_list;
  int status;

  if (sock_descr >= 0)
    return sock_descr;

  DEBUG("Opening a new socket");

  host = ntpd_host;
  if (host == NULL)
    host = NTPD_DEFAULT_HOST;

  port = ntpd_port;
  if (strlen(port) == 0)
    port = NTPD_DEFAULT_PORT;

  struct addrinfo ai_hints = {.ai_family = AF_UNSPEC,
                              .ai_flags = AI_ADDRCONFIG,
                              .ai_protocol = IPPROTO_UDP,
                              .ai_socktype = SOCK_DGRAM};

  if ((status = getaddrinfo(host, port, &ai_hints, &ai_list)) != 0) {
    ERROR("ntpd plugin: getaddrinfo (%s, %s): %s", host, port,
          (status == EAI_SYSTEM) ? STRERRNO : gai_strerror(status));
    return -1;
  }

  for (struct addrinfo *ai_ptr = ai_list; ai_ptr != NULL;
       ai_ptr = ai_ptr->ai_next) {
    /* create our socket descriptor */
    if ((sock_descr = socket(ai_ptr->ai_family, ai_ptr->ai_socktype,
                             ai_ptr->ai_protocol)) < 0)
      continue;

    /* connect to the ntpd */
    if (connect(sock_descr, ai_ptr->ai_addr, ai_ptr->ai_addrlen)) {
      close(sock_descr);
      sock_descr = -1;
      continue;
    }

    break;
  }

  freeaddrinfo(ai_list);

  if (sock_descr < 0) {
    ERROR("ntpd plugin: Unable to connect to server.");
  }

  return sock_descr;
}

/* For a description of the arguments see `ntpd_do_query' below. */
static int ntpd_receive_response(int *res_items, int *res_size, char **res_data,
                                 int res_item_size) {
  int sd;
  struct pollfd poll_s;
  struct resp_pkt res;
  int status;
  int done;

  char *items;
  size_t items_num;

  struct timeval time_end;
  struct timeval time_now;
  int timeout;

  int pkt_item_num; /* items in this packet */
  int pkt_item_len; /* size of the items in this packet */
  int pkt_sequence;
  char pkt_recvd[MAXSEQ + 1] = {
      0};              /* sequence numbers that have been received */
  int pkt_recvd_num;   /* number of packets that have been received */
  int pkt_lastseq;     /* the last sequence number */
  ssize_t pkt_padding; /* Padding in this packet */

  if ((sd = ntpd_connect()) < 0)
    return -1;

  items = NULL;
  items_num = 0;

  pkt_recvd_num = 0;
  pkt_lastseq = -1;

  *res_items = 0;
  *res_size = 0;
  *res_data = NULL;

  if (gettimeofday(&time_end, NULL) < 0) {
    ERROR("ntpd plugin: gettimeofday failed: %s", STRERRNO);
    return -1;
  }
  time_end.tv_sec++; /* wait for a most one second */

  done = 0;
  while (done == 0) {
    struct timeval time_left;

    if (gettimeofday(&time_now, NULL) < 0) {
      ERROR("ntpd plugin: gettimeofday failed: %s", STRERRNO);
      return -1;
    }

    if (timeval_cmp(time_end, time_now, &time_left) <= 0)
      timeout = 0;
    else
      timeout = 1000 * time_left.tv_sec + ((time_left.tv_usec + 500) / 1000);

    /* timeout reached */
    if (timeout <= 0)
      break;

    poll_s.fd = sd;
    poll_s.events = POLLIN | POLLPRI;
    poll_s.revents = 0;

    DEBUG("Polling for %ims", timeout);
    status = poll(&poll_s, 1, timeout);

    if ((status < 0) && ((errno == EAGAIN) || (errno == EINTR)))
      continue;

    if (status < 0) {
      ERROR("ntpd plugin: poll failed: %s", STRERRNO);
      return -1;
    }

    if (status == 0) /* timeout */
    {
      DEBUG("timeout reached.");
      break;
    }

    memset(&res, '\0', sizeof(res));
    status = recv(sd, (void *)&res, sizeof(res), 0 /* no flags */);

    if ((status < 0) && ((errno == EAGAIN) || (errno == EINTR)))
      continue;

    if (status < 0) {
      INFO("recv(2) failed: %s", STRERRNO);
      DEBUG("Closing socket #%i", sd);
      close(sd);
      sock_descr = sd = -1;
      return -1;
    }

    DEBUG("recv'd %i bytes", status);

    /*
     * Do some sanity checks first
     */
    if (status < RESP_HEADER_SIZE) {
      WARNING("ntpd plugin: Short (%i bytes) packet received", (int)status);
      continue;
    }
    if (INFO_MODE(res.rm_vn_mode) != MODE_PRIVATE) {
      NOTICE("ntpd plugin: Packet received with mode %i",
             INFO_MODE(res.rm_vn_mode));
      continue;
    }
    if (INFO_IS_AUTH(res.auth_seq)) {
      NOTICE("ntpd plugin: Encrypted packet received");
      continue;
    }
    if (!ISRESPONSE(res.rm_vn_mode)) {
      NOTICE("ntpd plugin: Received request packet, "
             "wanted response");
      continue;
    }
    if (INFO_MBZ(res.mbz_itemsize)) {
      WARNING("ntpd plugin: Received packet with nonzero "
              "MBZ field!");
      continue;
    }
    if (res.implementation != IMPL_XNTPD) {
      WARNING("ntpd plugin: Asked for request of type %i, "
              "got %i",
              (int)IMPL_XNTPD, (int)res.implementation);
      continue;
    }

    /* Check for error code */
    if (INFO_ERR(res.err_nitems) != 0) {
      ERROR("ntpd plugin: Received error code %i",
            (int)INFO_ERR(res.err_nitems));
      return (int)INFO_ERR(res.err_nitems);
    }

    /* extract number of items in this packet and the size of these items */
    pkt_item_num = INFO_NITEMS(res.err_nitems);
    pkt_item_len = INFO_ITEMSIZE(res.mbz_itemsize);
    DEBUG("pkt_item_num = %i; pkt_item_len = %i;", pkt_item_num, pkt_item_len);

    /* Check if the reported items fit in the packet */
    if ((pkt_item_num * pkt_item_len) > (status - RESP_HEADER_SIZE)) {
      ERROR("ntpd plugin: %i items * %i bytes > "
            "%i bytes - %i bytes header",
            (int)pkt_item_num, (int)pkt_item_len, (int)status,
            (int)RESP_HEADER_SIZE);
      continue;
    }

    if (pkt_item_len > res_item_size) {
      ERROR("ntpd plugin: (pkt_item_len = %i) "
            ">= (res_item_size = %i)",
            pkt_item_len, res_item_size);
      continue;
    }

    /* If this is the first packet (time wise, not sequence wise),
     * set `res_size'. If it's not the first packet check if the
     * items have the same size. Discard invalid packets. */
    if (items_num == 0) /* first packet */
    {
      DEBUG("*res_size = %i", pkt_item_len);
      *res_size = pkt_item_len;
    } else if (*res_size != pkt_item_len) {
      DEBUG("Error: *res_size = %i; pkt_item_len = %i;", *res_size,
            pkt_item_len);
      ERROR("Item sizes differ.");
      continue;
    }

    /*
     * Because the items in the packet may be smaller than the
     * items requested, the following holds true:
     */
    assert((*res_size == pkt_item_len) && (pkt_item_len <= res_item_size));

    /* Calculate the padding. No idea why there might be any padding.. */
    pkt_padding = 0;
    if (pkt_item_len < res_item_size)
      pkt_padding = res_item_size - pkt_item_len;
    DEBUG("res_item_size = %i; pkt_padding = %zi;", res_item_size, pkt_padding);

    /* Extract the sequence number */
    pkt_sequence = INFO_SEQ(res.auth_seq);
    if ((pkt_sequence < 0) || (pkt_sequence > MAXSEQ)) {
      ERROR("ntpd plugin: Received packet with sequence %i", pkt_sequence);
      continue;
    }

    /* Check if this sequence has been received before. If so, discard it. */
    if (pkt_recvd[pkt_sequence] != '\0') {
      NOTICE("ntpd plugin: Sequence %i received twice", pkt_sequence);
      continue;
    }

    /* If `pkt_lastseq != -1' another packet without `more bit' has
     * been received. */
    if (!ISMORE(res.rm_vn_mode)) {
      if (pkt_lastseq != -1) {
        ERROR("ntpd plugin: Two packets which both "
              "claim to be the last one in the "
              "sequence have been received.");
        continue;
      }
      pkt_lastseq = pkt_sequence;
      DEBUG("Last sequence = %i;", pkt_lastseq);
    }

    /*
     * Enough with the checks. Copy the data now.
     * We start by allocating some more memory.
     */
    DEBUG("realloc (%p, %" PRIsz ")", (void *)*res_data,
          (items_num + pkt_item_num) * res_item_size);
    items = realloc(*res_data, (items_num + pkt_item_num) * res_item_size);
    if (items == NULL) {
      ERROR("ntpd plugin: realloc failed.");
      continue;
    }
    items_num += pkt_item_num;
    *res_data = items;

    for (int i = 0; i < pkt_item_num; i++) {
      /* dst: There are already `*res_items' items with
       *      res_item_size bytes each in in `*res_data'. Set
       *      dst to the first byte after that. */
      void *dst = (void *)(*res_data + ((*res_items) * res_item_size));
      /* src: We use `pkt_item_len' to calculate the offset
       *      from the beginning of the packet, because the
       *      items in the packet may be smaller than the
       *      items that were requested. We skip `i' such
       *      items. */
      void *src = (void *)(((char *)res.data) + (i * pkt_item_len));

      /* Set the padding to zeros */
      if (pkt_padding != 0)
        memset(dst, '\0', res_item_size);
      memcpy(dst, src, (size_t)pkt_item_len);

      /* Increment `*res_items' by one, so `dst' will end up
       * one further in the next round. */
      (*res_items)++;
    } /* for (pkt_item_num) */

    pkt_recvd[pkt_sequence] = (char)1;
    pkt_recvd_num++;

    if ((pkt_recvd_num - 1) == pkt_lastseq)
      done = 1;
  } /* while (done == 0) */

  return 0;
} /* int ntpd_receive_response */

/* For a description of the arguments see `ntpd_do_query' below. */
static int ntpd_send_request(int req_code, int req_items, int req_size,
                             char *req_data) {
  int sd;
  struct req_pkt req = {0};
  size_t req_data_len;
  int status;

  assert(req_items >= 0);
  assert(req_size >= 0);

  if ((sd = ntpd_connect()) < 0)
    return -1;

  req.rm_vn_mode = RM_VN_MODE(0, 0, 0);
  req.auth_seq = AUTH_SEQ(0, 0);
  req.implementation = IMPL_XNTPD;
  req.request = (unsigned char)req_code;

  req_data_len = (size_t)(req_items * req_size);

  assert(((req_data != NULL) && (req_data_len > 0)) ||
         ((req_data == NULL) && (req_items == 0) && (req_size == 0)));

  req.err_nitems = ERR_NITEMS(0, req_items);
  req.mbz_itemsize = MBZ_ITEMSIZE(req_size);

  if (req_data != NULL)
    memcpy((void *)req.data, (const void *)req_data, req_data_len);

  DEBUG("req_items = %i; req_size = %i; req_data = %p;", req_items, req_size,
        (void *)req_data);

  status = swrite(sd, (const char *)&req, REQ_LEN_NOMAC);
  if (status != 0) {
    DEBUG("`swrite' failed. Closing socket #%i", sd);
    close(sd);
    sock_descr = sd = -1;
    return status;
  }

  return 0;
}

/*
 * ntpd_do_query:
 *
 * req_code:      Type of request packet
 * req_items:     Numver of items in the request
 * req_size:      Size of one item in the request
 * req_data:      Data of the request packet
 * res_items:     Pointer to where the number returned items will be stored.
 * res_size:      Pointer to where the size of one returned item will be stored.
 * res_data:      This is where a pointer to the (allocated) data will be
 * stored.
 * res_item_size: Size of one returned item. (used to calculate padding)
 *
 * returns:       zero upon success, non-zero otherwise.
 */
static int ntpd_do_query(int req_code, int req_items, int req_size,
                         char *req_data, int *res_items, int *res_size,
                         char **res_data, int res_item_size) {
  int status;

  status = ntpd_send_request(req_code, req_items, req_size, req_data);
  if (status != 0)
    return status;

  status = ntpd_receive_response(res_items, res_size, res_data, res_item_size);
  return status;
}

static double ntpd_read_fp(int32_t val_int) {
  double val_double;

  val_int = ntohl(val_int);
  val_double = ((double)val_int) / FP_FRAC;

  return val_double;
}

static uint32_t
ntpd_get_refclock_id(struct info_peer_summary const *peer_info) {
  uint32_t addr = ntohl(peer_info->srcadr);
  uint32_t refclock_id = (addr >> 8) & 0x00FF;

  return refclock_id;
}

static int ntpd_get_name_from_address(char *buffer, size_t buffer_size,
                                      struct info_peer_summary const *peer_info,
                                      bool do_reverse_lookup) {
  struct sockaddr_storage sa = {0};
  socklen_t sa_len;
  int flags = 0;
  int status;

  if (peer_info->v6_flag) {
    struct sockaddr_in6 sa6 = {0};

    assert(sizeof(sa) >= sizeof(sa6));

    sa6.sin6_family = AF_INET6;
    sa6.sin6_port = htons(123);
    memcpy(&sa6.sin6_addr, &peer_info->srcadr6, sizeof(struct in6_addr));
    sa_len = sizeof(sa6);

    memcpy(&sa, &sa6, sizeof(sa6));
  } else {
    struct sockaddr_in sa4 = {0};

    assert(sizeof(sa) >= sizeof(sa4));

    sa4.sin_family = AF_INET;
    sa4.sin_port = htons(123);
    memcpy(&sa4.sin_addr, &peer_info->srcadr, sizeof(struct in_addr));
    sa_len = sizeof(sa4);

    memcpy(&sa, &sa4, sizeof(sa4));
  }

  if (!do_reverse_lookup)
    flags |= NI_NUMERICHOST;

  status = getnameinfo((struct sockaddr const *)&sa, sa_len, buffer,
                       buffer_size, NULL, 0, /* No port name */
                       flags);
  if (status != 0) {
    ERROR("ntpd plugin: getnameinfo failed: %s",
          (status == EAI_SYSTEM) ? STRERRNO : gai_strerror(status));
    return -1;
  }

  return 0;
} /* ntpd_get_name_from_address */

static int ntpd_get_name_refclock(char *buffer, size_t buffer_size,
                                  struct info_peer_summary const *peer_info) {
  uint32_t refclock_id = ntpd_get_refclock_id(peer_info);
  uint32_t unit_id = ntohl(peer_info->srcadr) & 0x00FF;

  if (((size_t)refclock_id) >= refclock_names_num)
    return ntpd_get_name_from_address(buffer, buffer_size, peer_info, 0);

  if (include_unit_id)
    snprintf(buffer, buffer_size, "%s-%" PRIu32, refclock_names[refclock_id],
             unit_id);
  else
    sstrncpy(buffer, refclock_names[refclock_id], buffer_size);

  return 0;
} /* int ntpd_get_name_refclock */

static int ntpd_read(void) {
  struct info_kernel *ik;
  int ik_num;
  int ik_size;

  struct info_peer_summary *ps;
  int ps_num;
  int ps_size;

  gauge_t offset_loop;
  gauge_t freq_loop;
  gauge_t offset_error;

  int status;

  /* On Linux, if the STA_NANO bit is set in ik->status, then ik->offset
   * is is nanoseconds, otherwise it's microseconds. */
  double scale_loop = 1e-6;
  double scale_error = 1e-6;

  ik = NULL;
  ik_num = 0;
  ik_size = 0;

  status = ntpd_do_query(REQ_GET_KERNEL, 0, 0, NULL, /* request data */
                         &ik_num, &ik_size,
                         (char **)((void *)&ik), /* response data */
                         sizeof(struct info_kernel));
  if (status != 0) {
    ERROR("ntpd plugin: ntpd_do_query (REQ_GET_KERNEL) failed with status %i",
          status);
    free(ik);
    return status;
  } else if ((ik == NULL) || (ik_num == 0) || (ik_size == 0)) {
    ERROR("ntpd plugin: ntpd_do_query returned unexpected data. "
          "(ik = %p; ik_num = %i; ik_size = %i)",
          (void *)ik, ik_num, ik_size);
    free(ik);
    return -1;
  }

  if (ntohs(ik->status) & STA_NANO) {
    scale_loop = 1e-9;
    scale_error = 1e-9;
  }

  /* kerninfo -> estimated error */
  offset_loop = (gauge_t)((int32_t)ntohl(ik->offset) * scale_loop);
  freq_loop = ntpd_read_fp(ik->freq);
  offset_error = (gauge_t)((int32_t)ntohl(ik->esterror) * scale_error);

  DEBUG("info_kernel:\n"
        "  pll offset    = %.8g\n"
        "  pll frequency = %.8g\n" /* drift compensation */
        "  est error     = %.8g\n",
        offset_loop, freq_loop, offset_error);

  ntpd_submit("frequency_offset", "loop", freq_loop);
  ntpd_submit("time_offset", "loop", offset_loop);
  ntpd_submit("time_offset", "error", offset_error);

  free(ik);
  ik = NULL;

  status = ntpd_do_query(REQ_PEER_LIST_SUM, 0, 0, NULL, /* request data */
                         &ps_num, &ps_size,
                         (char **)((void *)&ps), /* response data */
                         sizeof(struct info_peer_summary));
  if (status != 0) {
    ERROR(
        "ntpd plugin: ntpd_do_query (REQ_PEER_LIST_SUM) failed with status %i",
        status);
    free(ps);
    return status;
  } else if ((ps == NULL) || (ps_num == 0) || (ps_size == 0)) {
    ERROR("ntpd plugin: ntpd_do_query returned unexpected data. "
          "(ps = %p; ps_num = %i; ps_size = %i)",
          (void *)ps, ps_num, ps_size);
    free(ps);
    return -1;
  }

  for (int i = 0; i < ps_num; i++) {
    struct info_peer_summary *ptr;
    double offset;

    char peername[NI_MAXHOST];
    uint32_t refclock_id;

    ptr = ps + i;

    int is_refclock = !ptr->v6_flag &&
                      ((ntohl(ptr->srcadr) & REFCLOCK_MASK) == REFCLOCK_ADDR);

    if (is_refclock)
      status = ntpd_get_name_refclock(peername, sizeof(peername), ptr);
    else
      status = ntpd_get_name_from_address(peername, sizeof(peername), ptr,
                                          do_reverse_lookups);

    if (status != 0) {
      ERROR("ntpd plugin: Determining name of peer failed.");
      continue;
    }

    // `0.0.0.0` hosts are caused by POOL servers
    // see https://github.com/collectd/collectd/issues/2358
    if (strcmp(peername, "0.0.0.0") == 0) {
      continue;
    }

    refclock_id = ntpd_get_refclock_id(ptr);

    /* Convert the `long floating point' offset value to double */
    M_LFPTOD(ntohl(ptr->offset_int), ntohl(ptr->offset_frc), offset);

    DEBUG("peer %i:\n"
          "  is_refclock= %d\n"
          "  refclock_id= %d\n"
          "  peername   = %s\n"
          "  srcadr     = 0x%08x\n"
          "  reach      = 0%03o\n"
          "  delay      = %f\n"
          "  offset_int = %i\n"
          "  offset_frc = %i\n"
          "  offset     = %f\n"
          "  dispersion = %f\n",
          i, is_refclock, (is_refclock > 0) ? refclock_id : 0, peername,
          ntohl(ptr->srcadr), ptr->reach, ntpd_read_fp(ptr->delay),
          ntohl(ptr->offset_int), ntohl(ptr->offset_frc), offset,
          ntpd_read_fp(ptr->dispersion));

    ntpd_submit_reach("time_dispersion", peername, ptr->reach,
                      ntpd_read_fp(ptr->dispersion));

    /* not the system clock (offset will always be zero) */
    if (!(is_refclock && refclock_id == 1))
      ntpd_submit_reach("time_offset", peername, ptr->reach, offset);

    if (!is_refclock) /* not a reference clock */
      ntpd_submit_reach("delay", peername, ptr->reach,
                        ntpd_read_fp(ptr->delay));
  }

  free(ps);
  ps = NULL;

  return 0;
} /* int ntpd_read */

void module_register(void) {
  plugin_register_config("ntpd", ntpd_config, config_keys, config_keys_num);
  plugin_register_read("ntpd", ntpd_read);
} /* void module_register */