nodemcu-firmware/app/modules/sntp.c

/*
 * Copyright 2015 Dius Computing Pty Ltd. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - 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.
 * - Neither the name of the copyright holders nor the names of
 *   its contributors may be used to endorse or promote products derived
 *   from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "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 COPYRIGHT HOLDER OR CONTRIBUTORS 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.
 *
 * @author Johny Mattsson <jmattsson@dius.com.au>
 */

// Module for Simple Network Time Protocol (SNTP)

#include "module.h"
#include "lauxlib.h"
#include "os_type.h"
#include "osapi.h"
#include "lwip/udp.h"
#include "c_stdlib.h"
#include "user_modules.h"
#include "lwip/dns.h"
#include "user_interface.h"

#ifdef LUA_USE_MODULES_RTCTIME
#include "rtc/rtctime.h"
#endif

#define max(a,b) ((a < b) ? b : a)

#define NTP_PORT 123
#define NTP_ANYCAST_ADDR(dst)  IP4_ADDR(dst, 224, 0, 1, 1)

#define MAX_ATTEMPTS 5

#if 0
# define sntp_dbg(...) dbg_printf(__VA_ARGS__)
#else
# define sntp_dbg(...)
#endif

#define US_TO_FRAC(us)          ((((uint64_t) (us)) << 32) / 1000000)
#define SUS_TO_FRAC(us)         ((((int64_t) (us)) << 32) / 1000000)
#define US_TO_FRAC16(us)        ((((uint64_t) (us)) << 16) / 1000000)
#define FRAC16_TO_US(frac)      ((((uint64_t) (frac)) * 1000000) >> 16)

typedef enum {
  NTP_NO_ERR = 0,
  NTP_DNS_ERR,
  NTP_MEM_ERR,
  NTP_SEND_ERR,
  NTP_TIMEOUT_ERR
} ntp_err_t;

typedef struct
{
  uint32_t sec;
  uint32_t frac;
} ntp_timestamp_t;

typedef struct
{
  uint8_t mode : 3;
  uint8_t ver : 3;
  uint8_t LI : 2;
  uint8_t stratum;
  uint8_t poll;
  uint8_t precision;
  uint32_t root_delay;
  uint32_t root_dispersion;
  uint32_t refid;
  ntp_timestamp_t ref;
  ntp_timestamp_t origin;
  ntp_timestamp_t recv;
  ntp_timestamp_t xmit;
} ntp_frame_t;

typedef struct
{
  struct udp_pcb *pcb;
  ntp_timestamp_t cookie;
  os_timer_t timer;
  int sync_cb_ref;
  int err_cb_ref;
  uint8_t attempts;    // Number of repeats of each entry
  uint8_t server_index;   // index into server table
  uint8_t lookup_pos;
  bool is_on_timeout;
  int server_pos;
  int list_ref;
  struct {
    uint32_t delay_frac;
    uint32_t root_maxerr;
    uint32_t root_delay;
    uint32_t root_dispersion;
    uint8_t LI;
    uint8_t stratum;
    int when;
    int64_t delta;
    ip_addr_t server;
  } best;
} sntp_state_t;

typedef struct {
  int32_t sync_cb_ref;
  int32_t err_cb_ref;
  os_timer_t timer;
} sntp_repeat_t;

static sntp_state_t *state;
static sntp_repeat_t *repeat;
static ip_addr_t *serverp;
static uint8_t server_count;
static uint8_t using_offset;
static uint8_t the_offset;
static uint8_t pending_LI;
static int32_t next_midnight;
static uint64_t pll_increment;

#define PLL_A   (1 << (32 - 11))
#define PLL_B   (1 << (32 - 11 - 2))

static void on_timeout(void *arg);
static void on_long_timeout(void *arg);
static void sntp_dolookups(lua_State *L);

static void cleanup (lua_State *L)
{
  os_timer_disarm (&state->timer);
  udp_remove (state->pcb);
  luaL_unref (L, LUA_REGISTRYINDEX, state->sync_cb_ref);
  luaL_unref (L, LUA_REGISTRYINDEX, state->err_cb_ref);
  luaL_unref (L, LUA_REGISTRYINDEX, state->list_ref);
  os_free (state);
  state = 0;
}

static ip_addr_t* get_free_server() {
  ip_addr_t* temp = (ip_addr_t *) c_malloc((server_count + 1) * sizeof(ip_addr_t));

  if (server_count > 0) {
    memcpy(temp, serverp, server_count * sizeof(ip_addr_t));
  }
  if (serverp) {
    c_free(serverp);
  }
  serverp = temp;

  return serverp + server_count;
}

static void handle_error (lua_State *L, ntp_err_t err, const char *msg)
{
  sntp_dbg("sntp: handle_error\n");
  if (state->err_cb_ref != LUA_NOREF)
  {
    lua_rawgeti (L, LUA_REGISTRYINDEX, state->err_cb_ref);
    lua_pushinteger (L, err);
    lua_pushstring (L, msg);
    cleanup (L);
    lua_call (L, 2, 0);
  }
  else
    cleanup (L);
}

#ifdef LUA_USE_MODULES_RTCTIME
static void get_zero_base_timeofday(struct rtc_timeval *tv) {
  uint32_t now = system_get_time();

  tv->tv_sec = now / 1000000;
  tv->tv_usec = now % 1000000;
}
#endif

static void sntp_handle_result(lua_State *L) {
  const uint32_t MICROSECONDS = 1000000;

  if (state->best.stratum == 0) {
    handle_error(L, NTP_TIMEOUT_ERR, NULL);
    return;
  }

  bool have_cb = (state->sync_cb_ref != LUA_NOREF);

  // if we have rtctime, do higher resolution delta calc, else just use
  // the transmit timestamp
#ifdef LUA_USE_MODULES_RTCTIME
  struct rtc_timeval tv;
  rtctime_gettimeofday (&tv);
  if (tv.tv_sec == 0) {
    get_zero_base_timeofday(&tv);
  }
  tv.tv_sec += (int)(state->best.delta >> 32);
  tv.tv_usec += (int) ((MICROSECONDS * (state->best.delta & 0xffffffff)) >> 32);
  while (tv.tv_usec >= 1000000) {
    tv.tv_usec -= 1000000;
    tv.tv_sec++;
  }
  if (state->is_on_timeout && state->best.delta > SUS_TO_FRAC(-200000) && state->best.delta < SUS_TO_FRAC(200000)) {
    // Adjust rate
    // f is frequency -- f should be 1 << 32 for nominal
    sntp_dbg("delta=%d, increment=%d, ", (int32_t) state->best.delta, (int32_t) pll_increment);
    int64_t f = ((state->best.delta * PLL_A) >> 32) + pll_increment;
    pll_increment += (state->best.delta * PLL_B) >> 32;
    sntp_dbg("f=%d, increment=%d\n", (int32_t) f, (int32_t) pll_increment);
    //rtctime_adjust_rate((int32_t) f);
  } else {
    rtctime_settimeofday (&tv);
  }
#endif

  if (have_cb)
  {
    lua_rawgeti(L, LUA_REGISTRYINDEX, state->sync_cb_ref);
#ifdef LUA_USE_MODULES_RTCTIME
    lua_pushnumber(L, tv.tv_sec);
    lua_pushnumber(L, tv.tv_usec);
    lua_pushstring(L, ipaddr_ntoa (&state->best.server));
    lua_newtable(L);
    int d40 = state->best.delta >> 40;
    if (d40 != 0 && d40 != -1) {
      lua_pushnumber(L, state->best.delta >> 32);
      lua_setfield(L, -2, "offset_s");
    } else {
      lua_pushnumber(L, (state->best.delta * MICROSECONDS) >> 32);
      lua_setfield(L, -2, "offset_us");
    }
#else
    int adjust_us = system_get_time() - state->best.when;
    int tv_sec = state->best.delta >> 32;
    int tv_usec = (int) (((state->best.delta & 0xffffffff) * MICROSECONDS) >> 32) + adjust_us;
    while (tv_usec >= 1000000) {
      tv_usec -= 1000000;
      tv_sec++;
    }
    lua_pushnumber(L, tv_sec);
    lua_pushnumber(L, tv_usec);
    lua_pushstring(L, ipaddr_ntoa (&state->best.server));
    lua_newtable(L);
#endif
    if (state->best.delay_frac > 0) {
      lua_pushnumber(L, FRAC16_TO_US(state->best.delay_frac));
      lua_setfield(L, -2, "delay_us");
    }
    lua_pushnumber(L, FRAC16_TO_US(state->best.root_delay));
    lua_setfield(L, -2, "root_delay_us");
    lua_pushnumber(L, FRAC16_TO_US(state->best.root_dispersion));
    lua_setfield(L, -2, "root_dispersion_us");
    lua_pushnumber(L, FRAC16_TO_US(state->best.root_maxerr + state->best.delay_frac / 2));
    lua_setfield(L, -2, "root_maxerr_us");
    lua_pushnumber(L, state->best.stratum);
    lua_setfield(L, -2, "stratum");
    lua_pushnumber(L, state->best.LI);
    lua_setfield(L, -2, "leap");
    lua_pushnumber(L, pending_LI);
    lua_setfield(L, -2, "pending_leap");
  }

  cleanup (L);

  if (have_cb)
  {
    lua_call (L, 4, 0);
  }
}


static void sntp_dosend (lua_State *L)
{
  if (state->server_pos < 0) {
    os_timer_disarm(&state->timer);
    os_timer_setfn(&state->timer, on_timeout, NULL);
    state->server_pos = 0;
  } else {
    ++state->server_pos;
  }

  if (state->server_pos >= server_count) {
    state->server_pos = 0;
    ++state->attempts;
  }

  if (state->attempts >= MAX_ATTEMPTS || state->attempts * server_count > 10) {
    sntp_handle_result(L);
    return;
  }

  sntp_dbg("sntp: server %s (%d), attempt %d\n", ipaddr_ntoa(serverp + state->server_pos), state->server_pos, state->attempts);

  struct pbuf *p = pbuf_alloc (PBUF_TRANSPORT, sizeof (ntp_frame_t), PBUF_RAM);
  if (!p)
    handle_error (L, NTP_MEM_ERR, NULL);

  ntp_frame_t req;
  os_memset (&req, 0, sizeof (req));
  req.ver = 4;
  req.mode = 3; // client
#ifdef LUA_USE_MODULES_RTCTIME
  const uint32_t NTP_TO_UNIX_EPOCH = 2208988800ul;
  struct rtc_timeval tv;
  rtctime_gettimeofday (&tv);
  if (tv.tv_sec == 0) {
    get_zero_base_timeofday(&tv);
  }
  req.xmit.sec = htonl (tv.tv_sec - the_offset + NTP_TO_UNIX_EPOCH);
  req.xmit.frac = htonl (US_TO_FRAC(tv.tv_usec));
#else
  req.xmit.frac = htonl (system_get_time ());
#endif
  state->cookie = req.xmit;

  os_memcpy (p->payload, &req, sizeof (req));
  int ret = udp_sendto (state->pcb, p, serverp + state->server_pos, NTP_PORT);
  sntp_dbg("sntp: send: %d\n", ret);
  pbuf_free (p);
  if (ret != ERR_OK)
    handle_error (L, NTP_SEND_ERR, NULL);

  os_timer_arm (&state->timer, 1000, 0);
}


static void sntp_dns_found(const char *name, ip_addr_t *ipaddr, void *arg)
{
  (void)arg;

  lua_State *L = lua_getstate ();
  if (ipaddr == NULL)
  {
    sntp_dbg("DNS Fail!\n");
    handle_error(L, NTP_DNS_ERR, name);
  }
  else
  {
    serverp[server_count] = *ipaddr;
    server_count++;
    sntp_dolookups(L);
  }
}


static void on_timeout (void *arg)
{
  (void)arg;
  sntp_dbg("sntp: timer\n");
  lua_State *L = lua_getstate ();
  sntp_dosend (L);
}

static void update_offset()
{
  // This may insert or remove an offset second -- i.e. a leap second
  // This can only happen if it is at midnight UTC.
#ifdef LUA_USE_MODULES_RTCTIME
  struct rtc_timeval tv;

  if (pending_LI && using_offset) {
    rtctime_gettimeofday (&tv);
    if (tv.tv_sec - the_offset >= next_midnight) {
      next_midnight = tv.tv_sec + 86400 - the_offset - (tv.tv_sec - the_offset) % 86400;
      // is this the first day of the month
      int day = (tv.tv_sec - the_offset) / 86400 + 1975 * 365 + 1970 / 4 - 74;

      int century = (4 * day + 3) / 146097;
      day = day - century * 146097 / 4;
      int year = (4 * day + 3) / 1461;
      day = day - year * 1461 / 4;
      int month = (5 * day + 2) / 153;
      day = day - (153 * month + 2) / 5;

      if (day == 0) {
        if (pending_LI == 1) {
          the_offset ++;
        } else {
          the_offset --;
        }
      }
      pending_LI = 0;
    }
  }
#endif
}

static void record_result(ip_addr_t *addr, int64_t delta, int stratum, int LI, uint32_t delay_frac, uint32_t root_maxerr, uint32_t root_dispersion, uint32_t root_delay) {
  sntp_dbg("Recording %s: delta=%08x.%08x, stratum=%d, li=%d, delay=%dus, root_maxerr=%dus",
      ipaddr_ntoa(addr), (uint32_t) (delta >> 32), (uint32_t) (delta & 0xffffffff), stratum, LI, (int32_t) FRAC16_TO_US(delay_frac), (int32_t) FRAC16_TO_US(root_maxerr));
  // I want to favor close by servers as they probably have a more consistent clock,
  if (!state->best.stratum || root_delay * 2 + delay_frac < state->best.root_delay * 2 + state->best.delay_frac) {
    sntp_dbg("   --BEST\n");
    state->best.server = *addr;
    state->best.delay_frac = delay_frac;
    state->best.root_maxerr = root_maxerr;
    state->best.root_dispersion = root_dispersion;
    state->best.root_delay = root_delay;
    state->best.delta = delta;
    state->best.stratum = stratum;
    state->best.LI = LI;
    state->best.when = system_get_time();
  } else {
    sntp_dbg("\n");
  }
}

static void on_recv (void *arg, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, uint16_t port)
{
  (void)port;
#ifdef LUA_USE_MODULES_RTCTIME
  // Ideally this would be done when we receive the packet....
  struct rtc_timeval tv;

  rtctime_gettimeofday (&tv);
  if (tv.tv_sec == 0) {
    get_zero_base_timeofday(&tv);
  }
#endif
  sntp_dbg("sntp: on_recv\n");

  lua_State *L = lua_getstate();

  if (!state || state->pcb != pcb)
  {
    // "impossible", but don't leak if it did happen somehow...
    udp_remove (pcb);
    pbuf_free (p);
    return;
  }

  if (!p)
    return;

  if (p->len < sizeof (ntp_frame_t))
  {
    pbuf_free (p);
    return; // not an ntp frame, ignore
  }

  // make sure we have an aligned copy to work from
  ntp_frame_t ntp;
  os_memcpy (&ntp, p->payload, sizeof (ntp));
  pbuf_free (p);
  sntp_dbg("sntp: transmit timestamp: %u, %u\n", ntp.xmit.sec, ntp.xmit.frac);

  // sanity checks before we touch our clocks
  ip_addr_t anycast;
  NTP_ANYCAST_ADDR(&anycast);
  if (serverp[state->server_pos].addr != anycast.addr && serverp[state->server_pos].addr != addr->addr)
    return; // unknown sender, ignore

  if (ntp.origin.sec  != state->cookie.sec ||
      ntp.origin.frac != state->cookie.frac)
    return; // unsolicited message, ignore

  if (ntp.LI == 3)
    return; // server clock not synchronized (why did it even respond?!)

  os_timer_disarm(&state->timer);

  if (ntp.LI) {
    pending_LI = ntp.LI;
  }

  update_offset();

  ntp.origin.sec  = ntohl (ntp.origin.sec);
  ntp.origin.frac = ntohl (ntp.origin.frac);
  ntp.recv.sec  = ntohl (ntp.recv.sec);
  ntp.recv.frac = ntohl (ntp.recv.frac);
  ntp.xmit.sec  = ntohl (ntp.xmit.sec);
  ntp.xmit.frac = ntohl (ntp.xmit.frac);

  const uint64_t MICROSECONDS = 1000000ull;
  const uint32_t NTP_TO_UNIX_EPOCH = 2208988800ul;

  uint32_t root_maxerr = ntohl(ntp.root_dispersion) + ntohl(ntp.root_delay) / 2;

  // if we have rtctime, do higher resolution delta calc, else just use
  // the transmit timestamp
#ifdef LUA_USE_MODULES_RTCTIME
  ntp_timestamp_t dest;
  dest.sec = tv.tv_sec + NTP_TO_UNIX_EPOCH - the_offset;
  dest.frac = US_TO_FRAC(tv.tv_usec);

  uint64_t ntp_recv = (((uint64_t) ntp.recv.sec) << 32) + (uint64_t) ntp.recv.frac;
  uint64_t ntp_origin = (((uint64_t) ntp.origin.sec) << 32) + (uint64_t) ntp.origin.frac;
  uint64_t ntp_xmit = (((uint64_t) ntp.xmit.sec) << 32) + (uint64_t) ntp.xmit.frac;
  uint64_t ntp_dest = (((uint64_t) dest.sec) << 32) + (uint64_t) dest.frac;

  // Compensation as per RFC2030
  int64_t delta = (int64_t) (ntp_recv - ntp_origin) / 2 + (int64_t) (ntp_xmit - ntp_dest) / 2;

  record_result(addr, delta, ntp.stratum, ntp.LI, ((int64_t)(ntp_dest - ntp_origin - (ntp_xmit - ntp_recv))) >> 16, root_maxerr, ntohl(ntp.root_dispersion), ntohl(ntp.root_delay));

#else
  uint64_t ntp_xmit = (((uint64_t) ntp.xmit.sec - NTP_TO_UNIX_EPOCH) << 32) + (uint64_t) ntp.xmit.frac;
  record_result(addr, ntp_xmit, ntp.stratum, ntp.LI, (((int64_t) (system_get_time() - ntp.origin.frac)) << 16) / MICROSECONDS, root_maxerr, ntohl(ntp.root_dispersion), ntohl(ntp.root_delay));
#endif

  sntp_dosend(L);
}

#ifdef LUA_USE_MODULES_RTCTIME
static int sntp_setoffset(lua_State *L)
{
  the_offset = luaL_checkinteger(L, 1);
  if (!using_offset) {
    struct rtc_timeval tv;
    rtctime_gettimeofday (&tv);
    next_midnight = tv.tv_sec + 86400 - the_offset - (tv.tv_sec - the_offset) % 86400;
  }
  using_offset = 1;

  return 0;
}

static int sntp_getoffset(lua_State *L)
{
  update_offset();
  lua_pushnumber(L, the_offset);

  return 1;
}
#endif

static void sntp_dolookups (lua_State *L) {
  // Step through each element of the table, converting it to an address
  // at the end, start the lookups
  //
  if (state->list_ref == LUA_NOREF) {
    sntp_dosend(L);
  }

  lua_rawgeti(L, LUA_REGISTRYINDEX, state->list_ref);
  while (1) {
    if (lua_objlen(L, -1) <= state->lookup_pos) {
      // We reached the end
      sntp_dosend(L);
      break;
    }

    state->lookup_pos++;

    lua_rawgeti(L, -1, state->lookup_pos);

    int l;

    const char *hostname = luaL_checklstring(L, -1, &l);
    lua_pop(L, 1);

    if (l>128 || hostname == NULL) {
      handle_error(L, NTP_DNS_ERR, hostname);
      break;
    }
    err_t err = dns_gethostbyname(hostname, get_free_server(), sntp_dns_found, state);
    if (err == ERR_INPROGRESS)
      break;  // Callback function sntp_dns_found will handle sntp_dosend for us
    else if (err == ERR_ARG) {
      handle_error(L, NTP_DNS_ERR, hostname);
      break;
    }
    server_count++;
  }
  lua_pop(L, 1);
}

static char *state_init(lua_State *L) {
  state = (sntp_state_t *)c_malloc (sizeof (sntp_state_t));
  if (!state)
    return ("out of memory");

  memset (state, 0, sizeof (sntp_state_t));

  state->sync_cb_ref = LUA_NOREF;
  state->err_cb_ref = LUA_NOREF;
  state->list_ref = LUA_NOREF;

  state->pcb = udp_new ();
  if (!state->pcb)
    return ("out of memory");

  if (udp_bind (state->pcb, IP_ADDR_ANY, 0) != ERR_OK)
    return ("no port available");

  udp_recv (state->pcb, on_recv, L);

  state->server_pos = -1;

  return NULL;
}

static char *set_repeat_mode(lua_State *L, bool enable)
{
  if (enable) {
    set_repeat_mode(L, FALSE);
    repeat = (sntp_repeat_t *) c_malloc(sizeof(sntp_repeat_t));
    if (!repeat) {
      return "no memory";
    }
    memset(repeat, 0, sizeof(repeat));
    lua_rawgeti(L, LUA_REGISTRYINDEX, state->sync_cb_ref);
    repeat->sync_cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
    lua_rawgeti(L, LUA_REGISTRYINDEX, state->err_cb_ref);
    repeat->err_cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
    os_timer_setfn(&repeat->timer, on_long_timeout, NULL);
    os_timer_arm(&repeat->timer, 1000 * 1000, 1);
  } else {
    if (repeat) {
      os_timer_disarm (&repeat->timer);
      luaL_unref (L, LUA_REGISTRYINDEX, repeat->sync_cb_ref);
      luaL_unref (L, LUA_REGISTRYINDEX, repeat->err_cb_ref);
      c_free(repeat);
      repeat = NULL;
    }
  }
  return NULL;
}

static void on_long_timeout (void *arg)
{
  (void)arg;
  sntp_dbg("sntp: long timer\n");
  lua_State *L = lua_getstate ();
  if (!state) {
    if (!state_init(L)) {
      // Good.
      lua_rawgeti(L, LUA_REGISTRYINDEX, repeat->sync_cb_ref);
      state->sync_cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
      lua_rawgeti(L, LUA_REGISTRYINDEX, repeat->err_cb_ref);
      state->err_cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
      state->is_on_timeout = 1;
      sntp_dosend (L);
    }
  }
}

// sntp.sync (server or nil, syncfn or nil, errfn or nil)
static int sntp_sync (lua_State *L)
{
  // default to anycast address, then allow last server to stick
  if (server_count == 0) {
    NTP_ANYCAST_ADDR(get_free_server());
    server_count++;
  }

  set_repeat_mode(L, 0);

  const char *errmsg = 0;
#define sync_err(x) do { errmsg = x; goto error; } while (0)

  if (state)
    return luaL_error (L, "sync in progress");

  char *state_err;
  state_err = state_init(L);
  if (state_err) {
    sync_err(state_err);
  }

  if (!lua_isnoneornil (L, 2))
  {
    lua_pushvalue (L, 2);
    state->sync_cb_ref = luaL_ref (L, LUA_REGISTRYINDEX);
  }

  if (!lua_isnoneornil (L, 3))
  {
    lua_pushvalue (L, 3);
    state->err_cb_ref = luaL_ref (L, LUA_REGISTRYINDEX);
  }

  // use last server, unless new one specified
  if (!lua_isnoneornil (L, 1))
  {
    server_count = 0;
    if (lua_istable(L, 1)) {
      // Save a reference to the table
      lua_pushvalue(L, 1);
      state->list_ref = luaL_ref(L, LUA_REGISTRYINDEX);
      sntp_dolookups(L);
      goto good_ret;
    } else {
      size_t l;
      const char *hostname = luaL_checklstring(L, 1, &l);
      if (l>128 || hostname == NULL)
        sync_err("need <128 hostname");
      err_t err = dns_gethostbyname(hostname, get_free_server(), sntp_dns_found, state);
      if (err == ERR_INPROGRESS) {
        goto good_ret;
      } else if (err == ERR_ARG)
        sync_err("bad hostname");

      server_count++;
    }
  }

  sntp_dosend (L);

good_ret:
  if (!lua_isnoneornil(L, 4)) {
    set_repeat_mode(L, 1);
  }

  return 0;

error:
  if (state)
  {
    if (state->pcb)
      udp_remove (state->pcb);
    c_free (state);
    state = 0;
  }
  return luaL_error (L, errmsg);
}


// Module function map
static const LUA_REG_TYPE sntp_map[] = {
  { LSTRKEY("sync"),  LFUNCVAL(sntp_sync)  },
#ifdef LUA_USE_MODULES_RTCTIME
  { LSTRKEY("setoffset"),  LFUNCVAL(sntp_setoffset)  },
  { LSTRKEY("getoffset"),  LFUNCVAL(sntp_getoffset)  },
#endif
  { LNILKEY, LNILVAL }
};

NODEMCU_MODULE(SNTP, "sntp", sntp_map, NULL);