ntpd: new applet by Adam Tkac. +5k.

Signed-off-by: Adam Tkac <vonsch@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

5 files changed, 1026 insertions, 0 deletions

diff --git a/include/applets.h b/include/applets.h
index 15ccf9cea..a2d1e95a5 100644
--- a/include/applets.h
+++ b/include/applets.h
@@ -285,6 +285,7 @@ IF_NICE(APPLET(nice, _BB_DIR_BIN, _BB_SUID_DROP))
 IF_NMETER(APPLET(nmeter, _BB_DIR_USR_BIN, _BB_SUID_DROP))
 IF_NOHUP(APPLET(nohup, _BB_DIR_USR_BIN, _BB_SUID_DROP))
 IF_NSLOOKUP(APPLET(nslookup, _BB_DIR_USR_BIN, _BB_SUID_DROP))
+IF_NTPD(APPLET(ntpd, _BB_DIR_USR_SBIN, _BB_SUID_DROP))
 IF_OD(APPLET(od, _BB_DIR_USR_BIN, _BB_SUID_DROP))
 IF_OPENVT(APPLET(openvt, _BB_DIR_USR_BIN, _BB_SUID_DROP))
 //IF_PARSE(APPLET(parse, _BB_DIR_USR_BIN, _BB_SUID_DROP))
diff --git a/include/usage.h b/include/usage.h
index eae9650ca..3ccf160bd 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -3210,6 +3210,17 @@
        "Name:       debian\n" \
        "Address:    127.0.0.1\n"
 
+#define ntpd_trivial_usage \
+	"[-dngl] [-p PEER]..."
+#define ntpd_full_usage "\n\n" \
+       "NTP client/server\n" \
+     "\nOptions:" \
+     "\n	-d	Verbose" \
+     "\n	-n	Do not daemonize" \
+     "\n	-g	Set system time even if offset is > 1000 sec" \
+     "\n	-l	Run as server on port 123" \
+     "\n	-p PEER	Obtain time from PEER (may be repeated)" \
+
 #define od_trivial_usage \
        "[-aBbcDdeFfHhIiLlOovXx] " IF_DESKTOP("[-t TYPE] ") "[FILE]"
 #define od_full_usage "\n\n" \
diff --git a/networking/Config.in b/networking/Config.in
index 5f1b6f636..3d29622da 100644
--- a/networking/Config.in
+++ b/networking/Config.in
@@ -661,6 +661,20 @@ config NSLOOKUP
 	help
 	  nslookup is a tool to query Internet name servers.
 
+config NTPD
+	bool "ntpd"
+	default n
+	help
+	  The NTP client/server daemon.
+
+config FEATURE_NTPD_SERVER
+        bool "Make ntpd usable as a NTP server"
+        default y
+        depends on NTPD
+        help
+	  Make ntpd usable as a NTP server. If you disable this option 
+	  ntpd will be usable only as a NTP client.
+
 config PING
 	bool "ping"
 	default n
diff --git a/networking/Kbuild b/networking/Kbuild
index 866d42f6b..b0765bcf6 100644
--- a/networking/Kbuild
+++ b/networking/Kbuild
@@ -27,6 +27,7 @@ lib-$(CONFIG_NAMEIF)       += nameif.o
 lib-$(CONFIG_NC)           += nc.o
 lib-$(CONFIG_NETSTAT)      += netstat.o
 lib-$(CONFIG_NSLOOKUP)     += nslookup.o
+lib-$(CONFIG_NTPD)         += ntpd.o
 lib-$(CONFIG_PING)         += ping.o
 lib-$(CONFIG_PING6)        += ping.o
 lib-$(CONFIG_PSCAN)        += pscan.o
diff --git a/networking/ntpd.c b/networking/ntpd.c
new file mode 100644
index 000000000..62a00a5bf
--- /dev/null
+++ b/networking/ntpd.c
@@ -0,0 +1,999 @@
+/*
+ * NTP client/server, based on OpenNTPD 3.9p1
+ *
+ * Author: Adam Tkac <vonsch@gmail.com>
+ *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
+ */
+
+#include "libbb.h"
+#include <netinet/ip.h> /* For IPTOS_LOWDELAY definition */
+
+#ifndef IP_PKTINFO
+# error "Sorry, your kernel has to support IP_PKTINFO"
+#endif
+
+#define	INTERVAL_QUERY_NORMAL		30	/* sync to peers every n secs */
+#define	INTERVAL_QUERY_PATHETIC		60
+#define	INTERVAL_QUERY_AGRESSIVE	5
+
+#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
+
+#define	QUERYTIME_MAX		15	/* single query might take n secs max */
+#define	OFFSET_ARRAY_SIZE	8
+#define	SETTIME_MIN_OFFSET	180	/* min offset for settime at start */
+#define	SETTIME_TIMEOUT		15	/* max seconds to wait with -s */
+
+/* Style borrowed from NTP ref/tcpdump and updated for SNTPv4 (RFC2030). */
+
+/*
+ * RFC Section 3
+ *
+ *    0                   1                   2                   3
+ *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *   |                         Integer Part                          |
+ *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *   |                         Fraction Part                         |
+ *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ *    0                   1                   2                   3
+ *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *   |            Integer Part       |     Fraction Part             |
+ *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+*/
+typedef struct {
+	uint32_t int_partl;
+	uint32_t fractionl;
+} l_fixedpt_t;
+
+typedef struct {
+	uint16_t int_parts;
+	uint16_t fractions;
+} s_fixedpt_t;
+
+#define	NTP_DIGESTSIZE		16
+#define	NTP_MSGSIZE_NOAUTH	48
+#define	NTP_MSGSIZE		(NTP_MSGSIZE_NOAUTH + 4 + NTP_DIGESTSIZE)
+
+typedef struct {
+	uint8_t status;	/* status of local clock and leap info */
+	uint8_t stratum;	/* Stratum level */
+	uint8_t ppoll;		/* poll value */
+	int8_t precision;
+	s_fixedpt_t rootdelay;
+	s_fixedpt_t dispersion;
+	uint32_t refid;
+	l_fixedpt_t reftime;
+	l_fixedpt_t orgtime;
+	l_fixedpt_t rectime;
+	l_fixedpt_t xmttime;
+	uint32_t keyid;
+	uint8_t digest[NTP_DIGESTSIZE];
+} ntp_msg_t;
+
+typedef struct {
+	int			fd;
+	ntp_msg_t		msg;
+	double			xmttime;
+} ntp_query_t;
+
+enum {
+	NTP_VERSION	= 4,
+	NTP_MAXSTRATUM	= 15,
+	/* Leap Second Codes (high order two bits) */
+	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 */
+
+	/* Status Masks */
+	MODE_MASK	= (7 << 0),
+	VERSION_MASK	= (7 << 3),
+	LI_MASK		= (3 << 6),
+
+	/* 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	JAN_1970	2208988800UL	/* 1970 - 1900 in seconds */
+
+enum client_state {
+	STATE_NONE,
+	STATE_QUERY_SENT,
+	STATE_REPLY_RECEIVED
+};
+
+typedef struct {
+	double		rootdelay;
+	double		rootdispersion;
+	double		reftime;
+	uint32_t	refid;
+	uint32_t	refid4;
+	uint8_t		synced;
+	uint8_t		leap;
+	int8_t		precision;
+	uint8_t		poll;
+	uint8_t		stratum;
+} ntp_status_t;
+
+typedef struct {
+	ntp_status_t		status;
+	double			offset;
+	double			delay;
+	double			error;
+	time_t			rcvd;
+	uint8_t			good;
+} ntp_offset_t;
+
+typedef struct {
+	len_and_sockaddr	*lsa;
+	ntp_query_t		 query;
+	ntp_offset_t		 reply[OFFSET_ARRAY_SIZE];
+	ntp_offset_t		 update;
+	enum client_state	 state;
+	time_t			 next;
+	time_t			 deadline;
+	uint8_t			 shift;
+	uint8_t			 trustlevel;
+} ntp_peer_t;
+
+
+struct globals {
+	unsigned	verbose;
+#if ENABLE_FEATURE_NTPD_SERVER
+	int		listen_fd;
+#endif
+	llist_t		*ntp_peers;
+	ntp_status_t	status;
+	uint32_t	scale;
+	uint8_t		settime;
+	uint8_t		firstadj;
+	smallint	peer_cnt;
+
+};
+#define G (*ptr_to_globals)
+
+
+static const int const_IPTOS_LOWDELAY = IPTOS_LOWDELAY;
+
+
+static void
+set_next(ntp_peer_t *p, time_t t)
+{
+	p->next = time(NULL) + t;
+	p->deadline = 0;
+}
+
+static void
+add_peers(const char *s)
+{
+	ntp_peer_t *p;
+
+	p = xzalloc(sizeof(*p));
+//TODO: big ntpd uses all IPs, not just 1st, do we need to mimic that?
+	p->lsa = xhost2sockaddr(s, 123);
+	p->query.fd = -1;
+	p->query.msg.status = MODE_CLIENT | (NTP_VERSION << 3);
+	if (STATE_NONE != 0)
+		p->state = STATE_NONE;
+	p->trustlevel = TRUSTLEVEL_PATHETIC;
+	p->query.fd = -1;
+	set_next(p, 0);
+
+	llist_add_to(&G.ntp_peers, p);
+	G.peer_cnt++;
+}
+
+static double
+gettime(void)
+{
+	struct timeval	tv;
+
+	gettimeofday(&tv, NULL); /* never fails */
+	return (tv.tv_sec + JAN_1970 + 1.0e-6 * tv.tv_usec);
+}
+
+
+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 ENABLE_FEATURE_NTPD_SERVER
+static l_fixedpt_t
+d_to_lfp(double d)
+{
+	l_fixedpt_t	lfp;
+
+	lfp.int_partl = htonl((uint32_t)d);
+	lfp.fractionl = htonl((uint32_t)((d - (u_int32_t)d) * UINT_MAX));
+	return lfp;
+}
+#endif
+
+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;
+}
+
+#if ENABLE_FEATURE_NTPD_SERVER
+static s_fixedpt_t
+d_to_sfp(double d)
+{
+	s_fixedpt_t	sfp;
+
+	sfp.int_parts = htons((uint16_t)d);
+	sfp.fractions = htons((uint16_t)((d - (u_int16_t)d) * USHRT_MAX));
+	return sfp;
+}
+#endif
+
+static void
+set_deadline(ntp_peer_t *p, time_t t)
+{
+	p->deadline = time(NULL) + t;
+	p->next = 0;
+}
+
+static time_t
+error_interval(void)
+{
+	time_t interval, r;
+
+	interval = INTERVAL_QUERY_PATHETIC * QSCALE_OFF_MAX / QSCALE_OFF_MIN;
+	r = random() % (interval / 10);
+	return (interval + r);
+}
+
+static int
+sendmsg_wrap(int fd,
+		const struct sockaddr *from, const struct sockaddr *to, socklen_t addrlen,
+		ntp_msg_t *msg, ssize_t len)
+{
+	ssize_t ret;
+
+	errno = 0;
+	if (!from) {
+		ret = sendto(fd, msg, len, 0, to, addrlen);
+	} else {
+		ret = send_to_from(fd, msg, len, 0, to, from, addrlen);
+	}
+	if (ret != len) {
+		bb_perror_msg("send failed");
+		return -1;
+	}
+	return 0;
+}
+
+static int
+client_query(ntp_peer_t *p)
+{
+	if (p->query.fd == -1) {
+		p->query.fd = xsocket(p->lsa->u.sa.sa_family, SOCK_DGRAM, 0);
+#if ENABLE_FEATURE_IPV6
+		if (p->lsa->u.sa.sa_family == AF_INET)
+#endif
+			setsockopt(p->query.fd, IPPROTO_IP, IP_TOS, &const_IPTOS_LOWDELAY, sizeof(const_IPTOS_LOWDELAY));
+	}
+
+	/*
+	 * 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->query.msg.xmttime.int_partl = random();
+	p->query.msg.xmttime.fractionl = random();
+	p->query.xmttime = gettime();
+
+	if (sendmsg_wrap(p->query.fd, /*from:*/ NULL, /*to:*/ &p->lsa->u.sa, /*addrlen:*/ p->lsa->len,
+			&p->query.msg, NTP_MSGSIZE_NOAUTH) == -1) {
+		set_next(p, INTERVAL_QUERY_PATHETIC);
+		return -1;
+	}
+
+	p->state = STATE_QUERY_SENT;
+	set_deadline(p, QUERYTIME_MAX);
+
+	return 0;
+}
+
+static int
+offset_compare(const void *aa, const void *bb)
+{
+	const ntp_peer_t * const *a;
+	const ntp_peer_t * const *b;
+
+	a = aa;
+	b = bb;
+
+	if ((*a)->update.offset < (*b)->update.offset)
+		return -1;
+	return ((*a)->update.offset > (*b)->update.offset);
+}
+
+static uint32_t
+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
+adjtime_wrap(void)
+{
+	ntp_peer_t	 *p;
+	unsigned	  offset_cnt;
+	int		  i = 0;
+	ntp_peer_t	**peers;
+	double		  offset_median;
+	llist_t		 *item;
+	len_and_sockaddr *lsa;
+	struct timeval	  tv, olddelta;
+
+	offset_cnt = 0;
+	for (item = G.ntp_peers; item != NULL; item = item->link) {
+		p = (ntp_peer_t *) item->data;
+		if (p->trustlevel < TRUSTLEVEL_BADPEER)
+			continue;
+		if (!p->update.good)
+			return;
+		offset_cnt++;
+	}
+
+	peers = xzalloc(sizeof(ntp_peer_t *) * offset_cnt);
+	for (item = G.ntp_peers; item != NULL; item = item->link) {
+		p = (ntp_peer_t *) item->data;
+		if (p->trustlevel < TRUSTLEVEL_BADPEER)
+			continue;
+		peers[i++] = p;
+	}
+
+	qsort(peers, offset_cnt, sizeof(ntp_peer_t *), offset_compare);
+
+	if (offset_cnt != 0) {
+		if ((offset_cnt & 1) == 0) {
+//TODO: try offset_cnt /= 2...
+			offset_median =
+			    (peers[offset_cnt / 2 - 1]->update.offset +
+			    peers[offset_cnt / 2]->update.offset) / 2;
+			G.status.rootdelay =
+			    (peers[offset_cnt / 2 - 1]->update.delay +
+			    peers[offset_cnt / 2]->update.delay) / 2;
+			G.status.stratum = MAX(
+			    peers[offset_cnt / 2 - 1]->update.status.stratum,
+			    peers[offset_cnt / 2]->update.status.stratum);
+		} else {
+			offset_median = peers[offset_cnt / 2]->update.offset;
+			G.status.rootdelay = peers[offset_cnt / 2]->update.delay;
+			G.status.stratum = peers[offset_cnt / 2]->update.status.stratum;
+		}
+		G.status.leap = peers[offset_cnt / 2]->update.status.leap;
+
+		bb_info_msg("adjusting local clock by %fs", offset_median);
+
+		d_to_tv(offset_median, &tv);
+		if (adjtime(&tv, &olddelta) == -1)
+			bb_error_msg("adjtime failed");
+		else if (!G.firstadj
+		 && olddelta.tv_sec == 0
+		 && olddelta.tv_usec == 0
+		 && !G.status.synced
+		) {
+			bb_info_msg("clock synced");
+			G.status.synced = 1;
+		} else if (G.status.synced) {
+			bb_info_msg("clock unsynced");
+			G.status.synced = 0;
+		}
+
+		G.firstadj = 0;
+		G.status.reftime = gettime();
+		G.status.stratum++;	/* one more than selected peer */
+		G.scale = updated_scale(offset_median);
+
+		G.status.refid4 = peers[offset_cnt / 2]->update.status.refid4;
+
+		lsa = peers[offset_cnt / 2]->lsa;
+		G.status.refid =
+#if ENABLE_FEATURE_IPV6
+			lsa->u.sa.sa_family != AF_INET ?
+				G.status.refid4 :
+#endif
+				lsa->u.sin.sin_addr.s_addr;
+	}
+
+	free(peers);
+
+	for (item = G.ntp_peers; item != NULL; item = item->link) {
+		p = (ntp_peer_t *) item->data;
+		p->update.good = 0;
+	}
+}
+
+static void
+settime(double offset)
+{
+	ntp_peer_t *p;
+	llist_t		*item;
+	struct timeval	tv, curtime;
+	char		buf[80];
+	time_t		tval;
+
+#if 0
+	if (!G.settime)
+		return;
+#endif
+
+	/* if the offset is small, don't call settimeofday */
+	if (offset < SETTIME_MIN_OFFSET && offset > -SETTIME_MIN_OFFSET)
+		return;
+
+	gettimeofday(&curtime, NULL); /* never fails */
+
+	d_to_tv(offset, &tv);
+	curtime.tv_usec += tv.tv_usec + 1000000;
+	curtime.tv_sec += tv.tv_sec - 1 + (curtime.tv_usec / 1000000);
+	curtime.tv_usec %= 1000000;
+
+	if (settimeofday(&curtime, NULL) == -1) {
+		bb_error_msg("settimeofday");
+		return;
+	}
+
+	G.settime = 0;
+
+	tval = curtime.tv_sec;
+	strftime(buf, sizeof(buf), "%a %b %e %H:%M:%S %Z %Y", localtime(&tval));
+
+	/* Do we want to print message below to system log when daemonized? */
+	bb_info_msg("set local clock to %s (offset %fs)", buf, offset);
+
+	for (item = G.ntp_peers; item != NULL; item = item->link) {
+		p = (ntp_peer_t *) item->data;
+		if (p->next)
+			p->next -= offset;
+		if (p->deadline)
+			p->deadline -= offset;
+	}
+}
+
+static void
+client_update(ntp_peer_t *p)
+{
+	int i, best = 0, good = 0;
+
+	/*
+	 * clock filter
+	 * find the offset which arrived with the lowest delay
+	 * use that as the peer update
+	 * invalidate it and all older ones
+	 */
+
+	for (i = 0; good == 0 && i < OFFSET_ARRAY_SIZE; i++) {
+		if (p->reply[i].good) {
+			good++;
+			best = i;
+		}
+	}
+
+	for (; i < OFFSET_ARRAY_SIZE; i++) {
+		if (p->reply[i].good) {
+			good++;
+			if (p->reply[i].delay < p->reply[best].delay)
+				best = i;
+		}
+	}
+
+	if (good < 8)
+		return;
+
+	memcpy(&p->update, &p->reply[best], sizeof(p->update));
+	adjtime_wrap();
+
+	for (i = 0; i < OFFSET_ARRAY_SIZE; i++)
+		if (p->reply[i].rcvd <= p->reply[best].rcvd)
+			p->reply[i].good = 0;
+}
+
+static time_t
+scale_interval(time_t requested)
+{
+	time_t interval, r;
+
+	interval = requested * G.scale;
+	r = random() % MAX(5, interval / 10);
+	return (interval + r);
+}
+
+static void
+client_dispatch(ntp_peer_t *p)
+{
+	char			 *addr;
+	ssize_t			 size;
+	ntp_msg_t		 msg;
+	double			 T1, T2, T3, T4;
+	time_t			 interval;
+	ntp_offset_t		*offset;
+
+	addr = xmalloc_sockaddr2dotted_noport(&p->lsa->u.sa);
+
+	size = recvfrom(p->query.fd, &msg, sizeof(msg), 0, NULL, NULL);
+	if (size == -1) {
+		bb_perror_msg("recvfrom(%s) error", addr);
+		if (errno == EHOSTUNREACH || errno == EHOSTDOWN
+		 || errno == ENETUNREACH || errno == ENETDOWN
+		 || errno == ECONNREFUSED || errno == EADDRNOTAVAIL
+		) {
+//TODO: always do this?
+			set_next(p, error_interval());
+			goto bail;
+		}
+		xfunc_die();
+	}
+
+	T4 = gettime();
+
+	if (size != NTP_MSGSIZE_NOAUTH && size != NTP_MSGSIZE) {
+		bb_error_msg("malformed packet received from %s", addr);
+		goto bail;
+	}
+
+	if (msg.orgtime.int_partl != p->query.msg.xmttime.int_partl
+	 || msg.orgtime.fractionl != p->query.msg.xmttime.fractionl
+	) {
+		goto bail;
+	}
+
+	if ((msg.status & LI_ALARM) == LI_ALARM
+	 || msg.stratum == 0
+	 || msg.stratum > NTP_MAXSTRATUM
+	) {
+		interval = error_interval();
+		bb_info_msg("reply from %s: not synced, next query %ds", addr, (int) interval);
+		goto bail;
+	}
+
+	/*
+	 * 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 d and local clock offset t are defined as
+	 *
+	 *    d = (T4 - T1) - (T3 - T2)     t = ((T2 - T1) + (T3 - T4)) / 2.
+	 */
+
+	T1 = p->query.xmttime;
+	T2 = lfp_to_d(msg.rectime);
+	T3 = lfp_to_d(msg.xmttime);
+
+	offset = &p->reply[p->shift];
+
+	offset->offset = ((T2 - T1) + (T3 - T4)) / 2;
+	offset->delay = (T4 - T1) - (T3 - T2);
+	if (offset->delay < 0) {
+		interval = error_interval();
+		set_next(p, interval);
+		bb_info_msg("reply from %s: negative delay %f", addr, p->reply[p->shift].delay);
+		goto bail;
+	}
+	offset->error = (T2 - T1) - (T3 - T4);
+	offset->rcvd = time(NULL);
+	offset->good = 1;
+
+	offset->status.leap = (msg.status & LI_MASK);
+	offset->status.precision = msg.precision;
+	offset->status.rootdelay = sfp_to_d(msg.rootdelay);
+	offset->status.rootdispersion = sfp_to_d(msg.dispersion);
+	offset->status.refid = ntohl(msg.refid);
+	offset->status.refid4 = msg.xmttime.fractionl;
+	offset->status.reftime = lfp_to_d(msg.reftime);
+	offset->status.poll = msg.ppoll;
+	offset->status.stratum = msg.stratum;
+
+	if (p->trustlevel < TRUSTLEVEL_PATHETIC)
+		interval = scale_interval(INTERVAL_QUERY_PATHETIC);
+	else if (p->trustlevel < TRUSTLEVEL_AGRESSIVE)
+		interval = scale_interval(INTERVAL_QUERY_AGRESSIVE);
+	else
+		interval = scale_interval(INTERVAL_QUERY_NORMAL);
+
+	set_next(p, interval);
+	p->state = STATE_REPLY_RECEIVED;
+
+	/* every received reply which we do not discard increases trust */
+	if (p->trustlevel < TRUSTLEVEL_MAX) {
+		if (p->trustlevel < TRUSTLEVEL_BADPEER
+		 && p->trustlevel + 1 >= TRUSTLEVEL_BADPEER
+		) {
+			bb_info_msg("peer %s now valid", addr);
+		}
+		p->trustlevel++;
+	}
+
+	bb_info_msg("reply from %s: offset %f delay %f, next query %ds", addr,
+			offset->offset, offset->delay, (int) interval);
+
+	client_update(p);
+	if (!G.settime)
+		settime(offset->offset);
+
+	if (++p->shift >= OFFSET_ARRAY_SIZE)
+		p->shift = 0;
+
+ bail:
+	free(addr);
+}
+
+#if ENABLE_FEATURE_NTPD_SERVER
+static void
+server_dispatch(int fd)
+{
+	ssize_t			size;
+	uint8_t			version;
+	double			rectime;
+	len_and_sockaddr	*to;
+	struct sockaddr		*from;
+	ntp_msg_t		query, reply;
+
+	to = get_sock_lsa(G.listen_fd);
+	from = xzalloc(to->len);
+
+	size = recv_from_to(fd, &query, sizeof(query), 0, from, &to->u.sa, to->len);
+	if (size == -1)
+		bb_error_msg_and_die("recv_from_to");
+	if (size != NTP_MSGSIZE_NOAUTH && size != NTP_MSGSIZE) {
+		char *addr = xmalloc_sockaddr2dotted_noport(from);
+		bb_error_msg("malformed packet received from %s", addr);
+		free(addr);
+		goto bail;
+	}
+
+	rectime = gettime();
+	version = (query.status & VERSION_MASK) >> 3;
+
+	memset(&reply, 0, sizeof(reply));
+	reply.status = G.status.synced ? G.status.leap : LI_ALARM;
+	reply.status |= (query.status & VERSION_MASK);
+	reply.status |= ((query.status & MODE_MASK) == MODE_CLIENT) ?
+			 MODE_SERVER : MODE_SYM_PAS;
+	reply.stratum =	G.status.stratum;
+	reply.ppoll = query.ppoll;
+	reply.precision = G.status.precision;
+	reply.rectime = d_to_lfp(rectime);
+	reply.reftime = d_to_lfp(G.status.reftime);
+	reply.xmttime = d_to_lfp(gettime());
+	reply.orgtime = query.xmttime;
+	reply.rootdelay = d_to_sfp(G.status.rootdelay);
+	reply.refid = (version > 3) ? G.status.refid4 : G.status.refid;
+
+	/* We reply from the address packet was sent to,
+	 * this makes to/from look swapped here: */
+	sendmsg_wrap(fd, /*from:*/ &to->u.sa, /*to:*/ from, /*addrlen:*/ to->len,
+			&reply, 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.
+ */
+
+enum {
+	OPT_n = (1 << 0),
+	OPT_g = (1 << 1),
+	OPT_p = (1 << 2),
+	OPT_l = (1 << 3),
+};
+
+/* 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;
+
+	tzset();
+
+	if (getuid())
+		bb_error_msg_and_die("need root privileges");
+
+	peers = NULL;
+	opt_complementary = "dd:p::"; /* d: counter, p: list */
+	opts = getopt32(argv,
+			"ng" /* compat */
+			"p:"IF_FEATURE_NTPD_SERVER("l") /* NOT compat */
+			"d" /* compat */
+			"46aAbLNx", /* compat, ignored */
+			&peers, &G.verbose);
+#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
+	if (opts & OPT_g)
+		G.settime = 1;
+	while (peers)
+		add_peers(llist_pop(&peers));
+	if (!(opts & OPT_n)) {
+		logmode = LOGMODE_NONE;
+		bb_daemonize(DAEMON_DEVNULL_STDIO);
+	}
+
+	/* Set some globals */
+	{
+		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.status.precision = prec;
+	}
+	G.scale = 1;
+	G.firstadj = 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;
+	unsigned new_cnt;
+	struct pollfd *pfd;
+	ntp_peer_t **idx2peer;
+
+	memset(&g, 0, sizeof(g));
+	SET_PTR_TO_GLOBALS(&g);
+
+	ntp_init(argv);
+
+	new_cnt = g.peer_cnt;
+	idx2peer = xzalloc(sizeof(void *) * new_cnt);
+#if ENABLE_FEATURE_NTPD_SERVER
+	if (g.listen_fd != -1)
+		new_cnt++;
+#endif
+	pfd = xzalloc(sizeof(pfd[0]) * new_cnt);
+
+	while (!bb_got_signal) {
+		llist_t *item;
+		unsigned i, j, idx_peers;
+		unsigned sent_cnt, trial_cnt;
+		int nfds, timeout;
+		time_t nextaction;
+
+		nextaction = time(NULL) + 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
+		idx_peers = i;
+		sent_cnt = trial_cnt = 0;
+		for (item = g.ntp_peers; item != NULL; item = item->link) {
+			ntp_peer_t *p = (ntp_peer_t *) item->data;
+
+			if (p->next > 0 && p->next <= time(NULL)) {
+				trial_cnt++;
+				if (client_query(p) == 0)
+					sent_cnt++;
+			}
+			if (p->next > 0 && p->next < nextaction)
+				nextaction = p->next;
+			if (p->deadline > 0 && p->deadline < nextaction)
+				nextaction = p->deadline;
+
+			if (p->deadline > 0 && p->deadline <= time(NULL)) {
+				char *addr = xmalloc_sockaddr2dotted_noport(&p->lsa->u.sa);
+
+				timeout = error_interval();
+				bb_info_msg("no reply from %s received in time, "
+						"next query %ds", addr, timeout);
+				if (p->trustlevel >= TRUSTLEVEL_BADPEER) {
+					p->trustlevel /= 2;
+					if (p->trustlevel < TRUSTLEVEL_BADPEER)
+						bb_info_msg("peer %s now invalid", addr);
+				}
+				free(addr);
+
+				set_next(p, timeout);
+			}
+
+			if (p->state == STATE_QUERY_SENT) {
+				pfd[i].fd = p->query.fd;
+				pfd[i].events = POLLIN;
+				idx2peer[i - idx_peers] = p;
+				i++;
+			}
+		}
+
+		if (g.settime
+		 && ((trial_cnt > 0 && sent_cnt == 0) || g.peer_cnt == 0)
+		) {
+			settime(0);	/* no good peers, don't wait */
+		}
+
+		timeout = nextaction - time(NULL);
+		if (timeout < 0)
+			timeout = 0;
+
+		if (g.verbose)
+			bb_error_msg("entering poll %u secs", timeout);
+		nfds = poll(pfd, i, timeout * 1000);
+
+		j = 0;
+#if ENABLE_FEATURE_NTPD_SERVER
+		for (; nfds > 0 && j < idx_peers; j++) {
+			if (pfd[j].revents & (POLLIN|POLLERR)) {
+				nfds--;
+				server_dispatch(pfd[j].fd);
+			}
+		}
+#endif
+		for (; nfds > 0 && j < i; j++) {
+			if (pfd[j].revents & (POLLIN|POLLERR)) {
+				nfds--;
+				client_dispatch(idx2peer[j - idx_peers]);
+			}
+		}
+	} /* while (!bb_got_signal) */
+
+	kill_myself_with_sig(bb_got_signal);
+}