source: MondoRescue/branches/2.2.9/mindi-busybox/networking/ntpd_simple.c@ 3320

/*
 * NTP client/server, based on OpenNTPD 3.9p1
 *
 * Author: Adam Tkac <vonsch@gmail.com>
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
#include "libbb.h"
#include <netinet/ip.h> /* For IPTOS_LOWDELAY definition */
#ifndef IPTOS_LOWDELAY
# define IPTOS_LOWDELAY 0x10
#endif
#ifndef IP_PKTINFO
# error "Sorry, your kernel has to support IP_PKTINFO"
#endif


/* Sync to peers every N secs */
#define INTERVAL_QUERY_NORMAL    30
#define INTERVAL_QUERY_PATHETIC  60
#define INTERVAL_QUERY_AGRESSIVE  5

/* Bad if *less than* TRUSTLEVEL_BADPEER */
#define TRUSTLEVEL_BADPEER        6
#define TRUSTLEVEL_PATHETIC       2
#define TRUSTLEVEL_AGRESSIVE      8
#define TRUSTLEVEL_MAX           10

#define QSCALE_OFF_MIN         0.05
#define QSCALE_OFF_MAX         0.50

/* Single query might take N secs max */
#define QUERYTIME_MAX            15
/* Min offset for settime at start. "man ntpd" says it's 128 ms */
#define STEPTIME_MIN_OFFSET   0.128

typedef struct {
    uint32_t int_partl;
    uint32_t fractionl;
} l_fixedpt_t;

typedef struct {
    uint16_t int_parts;
    uint16_t fractions;
} s_fixedpt_t;

enum {
    NTP_DIGESTSIZE     = 16,
    NTP_MSGSIZE_NOAUTH = 48,
    NTP_MSGSIZE        = (NTP_MSGSIZE_NOAUTH + 4 + NTP_DIGESTSIZE),
};

typedef struct {
    uint8_t     m_status;     /* status of local clock and leap info */
    uint8_t     m_stratum;    /* stratum level */
    uint8_t     m_ppoll;      /* poll value */
    int8_t      m_precision_exp;
    s_fixedpt_t m_rootdelay;
    s_fixedpt_t m_dispersion;
    uint32_t    m_refid;
    l_fixedpt_t m_reftime;
    l_fixedpt_t m_orgtime;
    l_fixedpt_t m_rectime;
    l_fixedpt_t m_xmttime;
    uint32_t    m_keyid;
    uint8_t     m_digest[NTP_DIGESTSIZE];
} msg_t;

enum {
    NTP_VERSION     = 4,
    NTP_MAXSTRATUM  = 15,

    /* Status Masks */
    MODE_MASK       = (7 << 0),
    VERSION_MASK    = (7 << 3),
    VERSION_SHIFT   = 3,
    LI_MASK         = (3 << 6),

    /* Leap Second Codes (high order two bits of m_status) */
    LI_NOWARNING    = (0 << 6),    /* no warning */
    LI_PLUSSEC      = (1 << 6),    /* add a second (61 seconds) */
    LI_MINUSSEC     = (2 << 6),    /* minus a second (59 seconds) */
    LI_ALARM        = (3 << 6),    /* alarm condition */

    /* Mode values */
    MODE_RES0       = 0,    /* reserved */
    MODE_SYM_ACT    = 1,    /* symmetric active */
    MODE_SYM_PAS    = 2,    /* symmetric passive */
    MODE_CLIENT     = 3,    /* client */
    MODE_SERVER     = 4,    /* server */
    MODE_BROADCAST  = 5,    /* broadcast */
    MODE_RES1       = 6,    /* reserved for NTP control message */
    MODE_RES2       = 7,    /* reserved for private use */
};

#define OFFSET_1900_1970 2208988800UL  /* 1970 - 1900 in seconds */

typedef struct {
    double   d_offset;
    double   d_delay;
    //UNUSED: double d_error;
    time_t   d_rcv_time;
    uint32_t d_refid4;
    uint8_t  d_leap;
    uint8_t  d_stratum;
    uint8_t  d_good;
} datapoint_t;

#define NUM_DATAPOINTS  8
typedef struct {
    len_and_sockaddr *p_lsa;
    char             *p_dotted;
    /* When to send new query (if p_fd == -1)
     * or when receive times out (if p_fd >= 0): */
    time_t           next_action_time;
    int              p_fd;
    uint8_t          p_datapoint_idx;
    uint8_t          p_trustlevel;
    double           p_xmttime;
    datapoint_t      update;
    datapoint_t      p_datapoint[NUM_DATAPOINTS];
    msg_t            p_xmt_msg;
} peer_t;

enum {
    OPT_n = (1 << 0),
    OPT_q = (1 << 1),
    OPT_N = (1 << 2),
    OPT_x = (1 << 3),
    /* Insert new options above this line. */
    /* Non-compat options: */
    OPT_p = (1 << 4),
    OPT_l = (1 << 5) * ENABLE_FEATURE_NTPD_SERVER,
};


struct globals {
    /* total round trip delay to currently selected reference clock */
    double   rootdelay;
    /* reference timestamp: time when the system clock was last set or corrected */
    double   reftime;
    llist_t  *ntp_peers;
#if ENABLE_FEATURE_NTPD_SERVER
    int      listen_fd;
#endif
    unsigned verbose;
    unsigned peer_cnt;
    unsigned scale;
    uint32_t refid;
    uint32_t refid4;
    uint8_t  synced;
    uint8_t  leap;
#define G_precision_exp -6
//  int8_t   precision_exp;
    uint8_t  stratum;
    uint8_t  time_was_stepped;
    uint8_t  first_adj_done;
};
#define G (*ptr_to_globals)

static const int const_IPTOS_LOWDELAY = IPTOS_LOWDELAY;


static void
set_next(peer_t *p, unsigned t)
{
    p->next_action_time = time(NULL) + t;
}

static void
add_peers(char *s)
{
    peer_t *p;

    p = xzalloc(sizeof(*p));
    p->p_lsa = xhost2sockaddr(s, 123);
    p->p_dotted = xmalloc_sockaddr2dotted_noport(&p->p_lsa->u.sa);
    p->p_fd = -1;
    p->p_xmt_msg.m_status = MODE_CLIENT | (NTP_VERSION << 3);
    p->p_trustlevel = TRUSTLEVEL_PATHETIC;
    p->next_action_time = time(NULL); /* = set_next(p, 0); */

    llist_add_to(&G.ntp_peers, p);
    G.peer_cnt++;
}

static double
gettime1900d(void)
{
    struct timeval tv;
    gettimeofday(&tv, NULL); /* never fails */
    return (tv.tv_sec + 1.0e-6 * tv.tv_usec + OFFSET_1900_1970);
}

static void
d_to_tv(double d, struct timeval *tv)
{
    tv->tv_sec = (long)d;
    tv->tv_usec = (d - tv->tv_sec) * 1000000;
}

static double
lfp_to_d(l_fixedpt_t lfp)
{
    double ret;
    lfp.int_partl = ntohl(lfp.int_partl);
    lfp.fractionl = ntohl(lfp.fractionl);
    ret = (double)lfp.int_partl + ((double)lfp.fractionl / UINT_MAX);
    return ret;
}

#if 0 //UNUSED
static double
sfp_to_d(s_fixedpt_t sfp)
{
    double ret;
    sfp.int_parts = ntohs(sfp.int_parts);
    sfp.fractions = ntohs(sfp.fractions);
    ret = (double)sfp.int_parts + ((double)sfp.fractions / USHRT_MAX);
    return ret;
}
#endif

#if ENABLE_FEATURE_NTPD_SERVER
static l_fixedpt_t
d_to_lfp(double d)
{
    l_fixedpt_t lfp;
    lfp.int_partl = (uint32_t)d;
    lfp.fractionl = (uint32_t)((d - lfp.int_partl) * UINT_MAX);
    lfp.int_partl = htonl(lfp.int_partl);
    lfp.fractionl = htonl(lfp.fractionl);
    return lfp;
}

static s_fixedpt_t
d_to_sfp(double d)
{
    s_fixedpt_t sfp;
    sfp.int_parts = (uint16_t)d;
    sfp.fractions = (uint16_t)((d - sfp.int_parts) * USHRT_MAX);
    sfp.int_parts = htons(sfp.int_parts);
    sfp.fractions = htons(sfp.fractions);
    return sfp;
}
#endif

static unsigned
error_interval(void)
{
    unsigned interval, r;
    interval = INTERVAL_QUERY_PATHETIC * QSCALE_OFF_MAX / QSCALE_OFF_MIN;
    r = (unsigned)random() % (unsigned)(interval / 10);
    return (interval + r);
}

static int
do_sendto(int fd,
        const struct sockaddr *from, const struct sockaddr *to, socklen_t addrlen,
        msg_t *msg, ssize_t len)
{
    ssize_t ret;

    errno = 0;
    if (!from) {
        ret = sendto(fd, msg, len, MSG_DONTWAIT, to, addrlen);
    } else {
        ret = send_to_from(fd, msg, len, MSG_DONTWAIT, to, from, addrlen);
    }
    if (ret != len) {
        bb_perror_msg("send failed");
        return -1;
    }
    return 0;
}

static int
send_query_to_peer(peer_t *p)
{
    // Why do we need to bind()?
    // See what happens when we don't bind:
    //
    // socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
    // setsockopt(3, SOL_IP, IP_TOS, [16], 4) = 0
    // gettimeofday({1259071266, 327885}, NULL) = 0
    // sendto(3, "xxx", 48, MSG_DONTWAIT, {sa_family=AF_INET, sin_port=htons(123), sin_addr=inet_addr("10.34.32.125")}, 16) = 48
    // ^^^ we sent it from some source port picked by kernel.
    // time(NULL)              = 1259071266
    // write(2, "ntpd: entering poll 15 secs\n", 28) = 28
    // poll([{fd=3, events=POLLIN}], 1, 15000) = 1 ([{fd=3, revents=POLLIN}])
    // recv(3, "yyy", 68, MSG_DONTWAIT) = 48
    // ^^^ this recv will receive packets to any local port!
    //
    // Uncomment this and use strace to see it in action:
#define PROBE_LOCAL_ADDR // { len_and_sockaddr lsa; lsa.len = LSA_SIZEOF_SA; getsockname(p->query.fd, &lsa.u.sa, &lsa.len); }

    if (p->p_fd == -1) {
        int fd, family;
        len_and_sockaddr *local_lsa;

        family = p->p_lsa->u.sa.sa_family;
        p->p_fd = fd = xsocket_type(&local_lsa, family, SOCK_DGRAM);
        /* local_lsa has "null" address and port 0 now.
         * bind() ensures we have a *particular port* selected by kernel
         * and remembered in p->p_fd, thus later recv(p->p_fd)
         * receives only packets sent to this port.
         */
        PROBE_LOCAL_ADDR
        xbind(fd, &local_lsa->u.sa, local_lsa->len);
        PROBE_LOCAL_ADDR
#if ENABLE_FEATURE_IPV6
        if (family == AF_INET)
#endif
            setsockopt(fd, IPPROTO_IP, IP_TOS, &const_IPTOS_LOWDELAY, sizeof(const_IPTOS_LOWDELAY));
        free(local_lsa);
    }

    /*
     * Send out a random 64-bit number as our transmit time.  The NTP
     * server will copy said number into the originate field on the
     * response that it sends us.  This is totally legal per the SNTP spec.
     *
     * The impact of this is two fold: we no longer send out the current
     * system time for the world to see (which may aid an attacker), and
     * it gives us a (not very secure) way of knowing that we're not
     * getting spoofed by an attacker that can't capture our traffic
     * but can spoof packets from the NTP server we're communicating with.
     *
     * Save the real transmit timestamp locally.
     */
    p->p_xmt_msg.m_xmttime.int_partl = random();
    p->p_xmt_msg.m_xmttime.fractionl = random();
    p->p_xmttime = gettime1900d();

    if (do_sendto(p->p_fd, /*from:*/ NULL, /*to:*/ &p->p_lsa->u.sa, /*addrlen:*/ p->p_lsa->len,
            &p->p_xmt_msg, NTP_MSGSIZE_NOAUTH) == -1
    ) {
        close(p->p_fd);
        p->p_fd = -1;
        set_next(p, INTERVAL_QUERY_PATHETIC);
        return -1;
    }

    if (G.verbose)
        bb_error_msg("sent query to %s", p->p_dotted);
    set_next(p, QUERYTIME_MAX);

    return 0;
}


/* Time is stepped only once, when the first packet from a peer is received.
 */
static void
step_time_once(double offset)
{
    double dtime;
    llist_t *item;
    struct timeval tv;
    char buf[80];
    time_t tval;

    if (G.time_was_stepped)
        goto bail;
    G.time_was_stepped = 1;

    /* if the offset is small, don't step, slew (later) */
    if (offset < STEPTIME_MIN_OFFSET && offset > -STEPTIME_MIN_OFFSET)
        goto bail;

    gettimeofday(&tv, NULL); /* never fails */
    dtime = offset + tv.tv_sec;
    dtime += 1.0e-6 * tv.tv_usec;
    d_to_tv(dtime, &tv);

    if (settimeofday(&tv, NULL) == -1)
        bb_perror_msg_and_die("settimeofday");

    tval = tv.tv_sec;
    strftime(buf, sizeof(buf), "%a %b %e %H:%M:%S %Z %Y", localtime(&tval));

    bb_error_msg("setting clock to %s (offset %fs)", buf, offset);

    for (item = G.ntp_peers; item != NULL; item = item->link) {
        peer_t *p = (peer_t *) item->data;
        p->next_action_time -= (time_t)offset;
    }

 bail:
    if (option_mask32 & OPT_q)
        exit(0);
}


/* Time is periodically slewed when we collect enough
 * good data points.
 */
static int
compare_offsets(const void *aa, const void *bb)
{
    const peer_t *const *a = aa;
    const peer_t *const *b = bb;
    if ((*a)->update.d_offset < (*b)->update.d_offset)
        return -1;
    return ((*a)->update.d_offset > (*b)->update.d_offset);
}
static unsigned
updated_scale(double offset)
{
    if (offset < 0)
        offset = -offset;
    if (offset > QSCALE_OFF_MAX)
        return 1;
    if (offset < QSCALE_OFF_MIN)
        return QSCALE_OFF_MAX / QSCALE_OFF_MIN;
    return QSCALE_OFF_MAX / offset;
}
static void
slew_time(void)
{
    llist_t *item;
    double offset_median;
    struct timeval tv;

    {
        peer_t **peers = xzalloc(sizeof(peers[0]) * G.peer_cnt);
        unsigned goodpeer_cnt = 0;
        unsigned middle;

        for (item = G.ntp_peers; item != NULL; item = item->link) {
            peer_t *p = (peer_t *) item->data;
            if (p->p_trustlevel < TRUSTLEVEL_BADPEER)
                continue;
            if (!p->update.d_good) {
                free(peers);
                return;
            }
            peers[goodpeer_cnt++] = p;
        }

        if (goodpeer_cnt == 0) {
            free(peers);
            goto clear_good;
        }

        qsort(peers, goodpeer_cnt, sizeof(peers[0]), compare_offsets);

        middle = goodpeer_cnt / 2;
        if (middle != 0 && (goodpeer_cnt & 1) == 0) {
            offset_median = (peers[middle-1]->update.d_offset + peers[middle]->update.d_offset) / 2;
            G.rootdelay = (peers[middle-1]->update.d_delay + peers[middle]->update.d_delay) / 2;
            G.stratum = 1 + MAX(peers[middle-1]->update.d_stratum, peers[middle]->update.d_stratum);
        } else {
            offset_median = peers[middle]->update.d_offset;
            G.rootdelay = peers[middle]->update.d_delay;
            G.stratum = 1 + peers[middle]->update.d_stratum;
        }
        G.leap = peers[middle]->update.d_leap;
        G.refid4 = peers[middle]->update.d_refid4;
        G.refid =
#if ENABLE_FEATURE_IPV6
            peers[middle]->p_lsa->u.sa.sa_family != AF_INET ?
                G.refid4 :
#endif
                peers[middle]->p_lsa->u.sin.sin_addr.s_addr;
        free(peers);
    }
//TODO: if (offset_median > BIG) step_time(offset_median)?

    G.scale = updated_scale(offset_median);

    bb_error_msg("adjusting clock by %fs, our stratum is %u, time scale %u",
            offset_median, G.stratum, G.scale);

    errno = 0;
    d_to_tv(offset_median, &tv);
    if (adjtime(&tv, &tv) == -1)
        bb_perror_msg_and_die("adjtime failed");
    if (G.verbose >= 2)
        bb_error_msg("old adjust: %d.%06u", (int)tv.tv_sec, (unsigned)tv.tv_usec);

    if (G.first_adj_done) {
        uint8_t synced = (tv.tv_sec == 0 && tv.tv_usec == 0);
        if (synced != G.synced) {
            G.synced = synced;
            bb_error_msg("clock is %ssynced", synced ? "" : "un");
        }
    }
    G.first_adj_done = 1;

    G.reftime = gettime1900d();

 clear_good:
    for (item = G.ntp_peers; item != NULL; item = item->link) {
        peer_t *p = (peer_t *) item->data;
        p->update.d_good = 0;
    }
}

static void
update_peer_data(peer_t *p)
{
    /* Clock filter.
     * Find the datapoint with the lowest delay.
     * Use that as the peer update.
     * Invalidate it and all older ones.
     */
    int i;
    int best = -1;
    int good = 0;

    for (i = 0; i < NUM_DATAPOINTS; i++) {
        if (p->p_datapoint[i].d_good) {
            good++;
            if (best < 0 || p->p_datapoint[i].d_delay < p->p_datapoint[best].d_delay)
                best = i;
        }
    }

    if (good < 8) //FIXME: was it meant to be NUM_DATAPOINTS, not 8?
        return;

    p->update = p->p_datapoint[best]; /* struct copy */
    slew_time();

    for (i = 0; i < NUM_DATAPOINTS; i++)
        if (p->p_datapoint[i].d_rcv_time <= p->p_datapoint[best].d_rcv_time)
            p->p_datapoint[i].d_good = 0;
}

static unsigned
scale_interval(unsigned requested)
{
    unsigned interval, r;
    interval = requested * G.scale;
    r = (unsigned)random() % (unsigned)(MAX(5, interval / 10));
    return (interval + r);
}
static void
recv_and_process_peer_pkt(peer_t *p)
{
    ssize_t     size;
    msg_t       msg;
    double      T1, T2, T3, T4;
    unsigned    interval;
    datapoint_t *datapoint;

    /* We can recvfrom here and check from.IP, but some multihomed
     * ntp servers reply from their *other IP*.
     * TODO: maybe we should check at least what we can: from.port == 123?
     */
    size = recv(p->p_fd, &msg, sizeof(msg), MSG_DONTWAIT);
    if (size == -1) {
        bb_perror_msg("recv(%s) error", p->p_dotted);
        if (errno == EHOSTUNREACH || errno == EHOSTDOWN
         || errno == ENETUNREACH || errno == ENETDOWN
         || errno == ECONNREFUSED || errno == EADDRNOTAVAIL
         || errno == EAGAIN
        ) {
//TODO: always do this?
            set_next(p, error_interval());
            goto close_sock;
        }
        xfunc_die();
    }

    if (size != NTP_MSGSIZE_NOAUTH && size != NTP_MSGSIZE) {
        bb_error_msg("malformed packet received from %s", p->p_dotted);
        goto bail;
    }

    if (msg.m_orgtime.int_partl != p->p_xmt_msg.m_xmttime.int_partl
     || msg.m_orgtime.fractionl != p->p_xmt_msg.m_xmttime.fractionl
    ) {
        goto bail;
    }

    if ((msg.m_status & LI_ALARM) == LI_ALARM
     || msg.m_stratum == 0
     || msg.m_stratum > NTP_MAXSTRATUM
    ) {
// TODO: stratum 0 responses may have commands in 32-bit m_refid field:
// "DENY", "RSTR" - peer does not like us at all
// "RATE" - peer is overloaded, reduce polling freq
        interval = error_interval();
        bb_error_msg("reply from %s: not synced, next query in %us", p->p_dotted, interval);
        goto close_sock;
    }

    /*
     * From RFC 2030 (with a correction to the delay math):
     *
     * Timestamp Name          ID   When Generated
     * ------------------------------------------------------------
     * Originate Timestamp     T1   time request sent by client
     * Receive Timestamp       T2   time request received by server
     * Transmit Timestamp      T3   time reply sent by server
     * Destination Timestamp   T4   time reply received by client
     *
     * The roundtrip delay and local clock offset are defined as
     *
     * delay = (T4 - T1) - (T3 - T2); offset = ((T2 - T1) + (T3 - T4)) / 2
     */
    T1 = p->p_xmttime;
    T2 = lfp_to_d(msg.m_rectime);
    T3 = lfp_to_d(msg.m_xmttime);
    T4 = gettime1900d();

    datapoint = &p->p_datapoint[p->p_datapoint_idx];

    datapoint->d_offset = ((T2 - T1) + (T3 - T4)) / 2;
    datapoint->d_delay = (T4 - T1) - (T3 - T2);
    if (datapoint->d_delay < 0) {
        bb_error_msg("reply from %s: negative delay %f", p->p_dotted, datapoint->d_delay);
        interval = error_interval();
        set_next(p, interval);
        goto close_sock;
    }
    //UNUSED: datapoint->d_error = (T2 - T1) - (T3 - T4);
    datapoint->d_rcv_time = (time_t)(T4 - OFFSET_1900_1970); /* = time(NULL); */
    datapoint->d_good = 1;

    datapoint->d_leap = (msg.m_status & LI_MASK);
    //UNUSED: datapoint->o_precision = msg.m_precision_exp;
    //UNUSED: datapoint->o_rootdelay = sfp_to_d(msg.m_rootdelay);
    //UNUSED: datapoint->o_rootdispersion = sfp_to_d(msg.m_dispersion);
    //UNUSED: datapoint->d_refid = ntohl(msg.m_refid);
    datapoint->d_refid4 = msg.m_xmttime.fractionl;
    //UNUSED: datapoint->o_reftime = lfp_to_d(msg.m_reftime);
    //UNUSED: datapoint->o_poll = msg.m_ppoll;
    datapoint->d_stratum = msg.m_stratum;

    if (p->p_trustlevel < TRUSTLEVEL_PATHETIC)
        interval = scale_interval(INTERVAL_QUERY_PATHETIC);
    else if (p->p_trustlevel < TRUSTLEVEL_AGRESSIVE)
        interval = scale_interval(INTERVAL_QUERY_AGRESSIVE);
    else
        interval = scale_interval(INTERVAL_QUERY_NORMAL);

    set_next(p, interval);

    /* Every received reply which we do not discard increases trust */
    if (p->p_trustlevel < TRUSTLEVEL_MAX) {
        p->p_trustlevel++;
        if (p->p_trustlevel == TRUSTLEVEL_BADPEER)
            bb_error_msg("peer %s now valid", p->p_dotted);
    }

    if (G.verbose)
        bb_error_msg("reply from %s: offset %f delay %f, next query in %us", p->p_dotted,
            datapoint->d_offset, datapoint->d_delay, interval);

    update_peer_data(p);
//TODO: do it after all peers had a chance to return at least one reply?
    step_time_once(datapoint->d_offset);

    p->p_datapoint_idx++;
    if (p->p_datapoint_idx >= NUM_DATAPOINTS)
        p->p_datapoint_idx = 0;

 close_sock:
    /* We do not expect any more packets from this peer for now.
     * Closing the socket informs kernel about it.
     * We open a new socket when we send a new query.
     */
    close(p->p_fd);
    p->p_fd = -1;
 bail:
    return;
}

#if ENABLE_FEATURE_NTPD_SERVER
static void
recv_and_process_client_pkt(void /*int fd*/)
{
    ssize_t          size;
    uint8_t          version;
    double           rectime;
    len_and_sockaddr *to;
    struct sockaddr  *from;
    msg_t            msg;
    uint8_t          query_status;
    uint8_t          query_ppoll;
    l_fixedpt_t      query_xmttime;

    to = get_sock_lsa(G.listen_fd);
    from = xzalloc(to->len);

    size = recv_from_to(G.listen_fd, &msg, sizeof(msg), MSG_DONTWAIT, from, &to->u.sa, to->len);
    if (size != NTP_MSGSIZE_NOAUTH && size != NTP_MSGSIZE) {
        char *addr;
        if (size < 0) {
            if (errno == EAGAIN)
                goto bail;
            bb_perror_msg_and_die("recv");
        }
        addr = xmalloc_sockaddr2dotted_noport(from);
        bb_error_msg("malformed packet received from %s: size %u", addr, (int)size);
        free(addr);
        goto bail;
    }

    query_status = msg.m_status;
    query_ppoll = msg.m_ppoll;
    query_xmttime = msg.m_xmttime;

    /* Build a reply packet */
    memset(&msg, 0, sizeof(msg));
    msg.m_status = G.synced ? G.leap : LI_ALARM;
    msg.m_status |= (query_status & VERSION_MASK);
    msg.m_status |= ((query_status & MODE_MASK) == MODE_CLIENT) ?
             MODE_SERVER : MODE_SYM_PAS;
    msg.m_stratum = G.stratum;
    msg.m_ppoll = query_ppoll;
    msg.m_precision_exp = G_precision_exp;
    rectime = gettime1900d();
    msg.m_xmttime = msg.m_rectime = d_to_lfp(rectime);
    msg.m_reftime = d_to_lfp(G.reftime);
    //msg.m_xmttime = d_to_lfp(gettime1900d()); // = msg.m_rectime
    msg.m_orgtime = query_xmttime;
    msg.m_rootdelay = d_to_sfp(G.rootdelay);
    version = (query_status & VERSION_MASK); /* ... >> VERSION_SHIFT - done below instead */
    msg.m_refid = (version > (3 << VERSION_SHIFT)) ? G.refid4 : G.refid;

    /* We reply from the local address packet was sent to,
     * this makes to/from look swapped here: */
    do_sendto(G.listen_fd,
        /*from:*/ &to->u.sa, /*to:*/ from, /*addrlen:*/ to->len,
        &msg, size);

 bail:
    free(to);
    free(from);
}
#endif

/* Upstream ntpd's options:
 *
 * -4   Force DNS resolution of host names to the IPv4 namespace.
 * -6   Force DNS resolution of host names to the IPv6 namespace.
 * -a   Require cryptographic authentication for broadcast client,
 *      multicast client and symmetric passive associations.
 *      This is the default.
 * -A   Do not require cryptographic authentication for broadcast client,
 *      multicast client and symmetric passive associations.
 *      This is almost never a good idea.
 * -b   Enable the client to synchronize to broadcast servers.
 * -c conffile
 *      Specify the name and path of the configuration file,
 *      default /etc/ntp.conf
 * -d   Specify debugging mode. This option may occur more than once,
 *      with each occurrence indicating greater detail of display.
 * -D level
 *      Specify debugging level directly.
 * -f driftfile
 *      Specify the name and path of the frequency file.
 *      This is the same operation as the "driftfile FILE"
 *      configuration command.
 * -g   Normally, ntpd exits with a message to the system log
 *      if the offset exceeds the panic threshold, which is 1000 s
 *      by default. This option allows the time to be set to any value
 *      without restriction; however, this can happen only once.
 *      If the threshold is exceeded after that, ntpd will exit
 *      with a message to the system log. This option can be used
 *      with the -q and -x options. See the tinker command for other options.
 * -i jaildir
 *      Chroot the server to the directory jaildir. This option also implies
 *      that the server attempts to drop root privileges at startup
 *      (otherwise, chroot gives very little additional security).
 *      You may need to also specify a -u option.
 * -k keyfile
 *      Specify the name and path of the symmetric key file,
 *      default /etc/ntp/keys. This is the same operation
 *      as the "keys FILE" configuration command.
 * -l logfile
 *      Specify the name and path of the log file. The default
 *      is the system log file. This is the same operation as
 *      the "logfile FILE" configuration command.
 * -L   Do not listen to virtual IPs. The default is to listen.
 * -n   Don't fork.
 * -N   To the extent permitted by the operating system,
 *      run the ntpd at the highest priority.
 * -p pidfile
 *      Specify the name and path of the file used to record the ntpd
 *      process ID. This is the same operation as the "pidfile FILE"
 *      configuration command.
 * -P priority
 *      To the extent permitted by the operating system,
 *      run the ntpd at the specified priority.
 * -q   Exit the ntpd just after the first time the clock is set.
 *      This behavior mimics that of the ntpdate program, which is
 *      to be retired. The -g and -x options can be used with this option.
 *      Note: The kernel time discipline is disabled with this option.
 * -r broadcastdelay
 *      Specify the default propagation delay from the broadcast/multicast
 *      server to this client. This is necessary only if the delay
 *      cannot be computed automatically by the protocol.
 * -s statsdir
 *      Specify the directory path for files created by the statistics
 *      facility. This is the same operation as the "statsdir DIR"
 *      configuration command.
 * -t key
 *      Add a key number to the trusted key list. This option can occur
 *      more than once.
 * -u user[:group]
 *      Specify a user, and optionally a group, to switch to.
 * -v variable
 * -V variable
 *      Add a system variable listed by default.
 * -x   Normally, the time is slewed if the offset is less than the step
 *      threshold, which is 128 ms by default, and stepped if above
 *      the threshold. This option sets the threshold to 600 s, which is
 *      well within the accuracy window to set the clock manually.
 *      Note: since the slew rate of typical Unix kernels is limited
 *      to 0.5 ms/s, each second of adjustment requires an amortization
 *      interval of 2000 s. Thus, an adjustment as much as 600 s
 *      will take almost 14 days to complete. This option can be used
 *      with the -g and -q options. See the tinker command for other options.
 *      Note: The kernel time discipline is disabled with this option.
 */

/* By doing init in a separate function we decrease stack usage
 * in main loop.
 */
static NOINLINE void ntp_init(char **argv)
{
    unsigned opts;
    llist_t *peers;

    srandom(getpid());

    if (getuid())
        bb_error_msg_and_die(bb_msg_you_must_be_root);

    peers = NULL;
    opt_complementary = "dd:p::"; /* d: counter, p: list */
    opts = getopt32(argv,
            "nqNx" /* compat */
            "p:"IF_FEATURE_NTPD_SERVER("l") /* NOT compat */
            "d" /* compat */
            "46aAbgL", /* compat, ignored */
            &peers, &G.verbose);
    if (!(opts & (OPT_p|OPT_l)))
        bb_show_usage();
    if (opts & OPT_x) /* disable stepping, only slew is allowed */
        G.time_was_stepped = 1;
    while (peers)
        add_peers(llist_pop(&peers));
    if (!(opts & OPT_n)) {
        bb_daemonize_or_rexec(DAEMON_DEVNULL_STDIO, argv);
        logmode = LOGMODE_NONE;
    }
#if ENABLE_FEATURE_NTPD_SERVER
    G.listen_fd = -1;
    if (opts & OPT_l) {
        G.listen_fd = create_and_bind_dgram_or_die(NULL, 123);
        socket_want_pktinfo(G.listen_fd);
        setsockopt(G.listen_fd, IPPROTO_IP, IP_TOS, &const_IPTOS_LOWDELAY, sizeof(const_IPTOS_LOWDELAY));
    }
#endif
    /* I hesitate to set -20 prio. -15 should be high enough for timekeeping */
    if (opts & OPT_N)
        setpriority(PRIO_PROCESS, 0, -15);

    /* Set some globals */
#if 0
    /* With constant b = 100, G.precision_exp is also constant -6.
     * Uncomment this and you'll see */
    {
        int prec = 0;
        int b;
# if 0
        struct timespec tp;
        /* We can use sys_clock_getres but assuming 10ms tick should be fine */
        clock_getres(CLOCK_REALTIME, &tp);
        tp.tv_sec = 0;
        tp.tv_nsec = 10000000;
        b = 1000000000 / tp.tv_nsec;  /* convert to Hz */
# else
        b = 100; /* b = 1000000000/10000000 = 100 */
# endif
        while (b > 1)
            prec--, b >>= 1;
        //G.precision_exp = prec;
        bb_error_msg("G.precision_exp:%d", prec); /* -6 */
    }
#endif
    G.scale = 1;

    bb_signals((1 << SIGTERM) | (1 << SIGINT), record_signo);
    bb_signals((1 << SIGPIPE) | (1 << SIGHUP), SIG_IGN);
}

int ntpd_main(int argc UNUSED_PARAM, char **argv) MAIN_EXTERNALLY_VISIBLE;
int ntpd_main(int argc UNUSED_PARAM, char **argv)
{
    struct globals g;
    struct pollfd *pfd;
    peer_t **idx2peer;

    memset(&g, 0, sizeof(g));
    SET_PTR_TO_GLOBALS(&g);

    ntp_init(argv);

    {
        /* if ENABLE_FEATURE_NTPD_SERVER, + 1 for listen_fd: */
        unsigned cnt = g.peer_cnt + ENABLE_FEATURE_NTPD_SERVER;
        idx2peer = xzalloc(sizeof(idx2peer[0]) * cnt);
        pfd = xzalloc(sizeof(pfd[0]) * cnt);
    }

    while (!bb_got_signal) {
        llist_t *item;
        unsigned i, j;
        unsigned sent_cnt, trial_cnt;
        int nfds, timeout;
        time_t cur_time, nextaction;

        /* Nothing between here and poll() blocks for any significant time */

        cur_time = time(NULL);
        nextaction = cur_time + 3600;

        i = 0;
#if ENABLE_FEATURE_NTPD_SERVER
        if (g.listen_fd != -1) {
            pfd[0].fd = g.listen_fd;
            pfd[0].events = POLLIN;
            i++;
        }
#endif
        /* Pass over peer list, send requests, time out on receives */
        sent_cnt = trial_cnt = 0;
        for (item = g.ntp_peers; item != NULL; item = item->link) {
            peer_t *p = (peer_t *) item->data;

            /* Overflow-safe "if (p->next_action_time <= cur_time) ..." */
            if ((int)(cur_time - p->next_action_time) >= 0) {
                if (p->p_fd == -1) {
                    /* Time to send new req */
                    trial_cnt++;
                    if (send_query_to_peer(p) == 0)
                        sent_cnt++;
                } else {
                    /* Timed out waiting for reply */
                    close(p->p_fd);
                    p->p_fd = -1;
                    timeout = error_interval();
                    bb_error_msg("timed out waiting for %s, "
                            "next query in %us", p->p_dotted, timeout);
                    if (p->p_trustlevel >= TRUSTLEVEL_BADPEER) {
                        p->p_trustlevel /= 2;
                        if (p->p_trustlevel < TRUSTLEVEL_BADPEER)
                            bb_error_msg("peer %s now invalid", p->p_dotted);
                    }
                    set_next(p, timeout);
                }
            }

            if (p->next_action_time < nextaction)
                nextaction = p->next_action_time;

            if (p->p_fd >= 0) {
                /* Wait for reply from this peer */
                pfd[i].fd = p->p_fd;
                pfd[i].events = POLLIN;
                idx2peer[i] = p;
                i++;
            }
        }

        if ((trial_cnt > 0 && sent_cnt == 0) || g.peer_cnt == 0)
            step_time_once(0); /* no good peers, don't wait */

        timeout = nextaction - cur_time;
        if (timeout < 1)
            timeout = 1;

        /* Here we may block */
        if (g.verbose >= 2)
            bb_error_msg("poll %us, sockets:%u", timeout, i);
        nfds = poll(pfd, i, timeout * 1000);
        if (nfds <= 0)
            continue;

        /* Process any received packets */
        j = 0;
#if ENABLE_FEATURE_NTPD_SERVER
        if (g.listen_fd != -1) {
            if (pfd[0].revents /* & (POLLIN|POLLERR)*/) {
                nfds--;
                recv_and_process_client_pkt(/*g.listen_fd*/);
            }
            j = 1;
        }
#endif
        for (; nfds != 0 && j < i; j++) {
            if (pfd[j].revents /* & (POLLIN|POLLERR)*/) {
                nfds--;
                recv_and_process_peer_pkt(idx2peer[j]);
            }
        }
    } /* while (!bb_got_signal) */

    kill_myself_with_sig(bb_got_signal);
}