Add support for "constraints": when configured, ntpd(8) will query the

time from HTTPS servers, by parsing the Date: header, and use the
median constraint time as a boundary to verify NTP responses.  This
adds some level of authentication and protection against MITM attacks
while preserving the accuracy of the NTP protocol; without relying on
authentication options for NTP that are basically unavailable at
present.  This is an initial implementation and the semantics will be
improved once it is in the tree.

Discussed with deraadt@ and henning@
OK henning@

12 files changed, 977 insertions, 26 deletions

diff --git a/usr.sbin/ntpd/Makefile b/usr.sbin/ntpd/Makefile
index e77935f8b2c..0025aae413b 100644
--- a/usr.sbin/ntpd/Makefile
+++ b/usr.sbin/ntpd/Makefile
@@ -1,17 +1,17 @@
-#	$OpenBSD: Makefile,v 1.12 2013/10/04 14:28:15 phessler Exp $
+#	$OpenBSD: Makefile,v 1.13 2015/02/10 06:40:08 reyk Exp $
 
 PROG=	ntpd
 SRCS=	ntpd.c log.c ntp.c ntp_msg.c parse.y config.c \
 	server.c client.c sensors.c util.c ntp_dns.c \
-	control.c
+	control.c constraint.c
 CFLAGS+= -Wall -I${.CURDIR}
 CFLAGS+= -Wstrict-prototypes -Wmissing-prototypes
 CFLAGS+= -Wmissing-declarations
 CFLAGS+= -Wshadow -Wpointer-arith -Wcast-qual
 CFLAGS+= -Wsign-compare
 YFLAGS=
-LDADD+=	-lutil
-DPADD+= ${LIBUTIL}
+LDADD+=	-lutil -lcrypto -lssl -ltls
+DPADD+= ${LIBUTIL} ${LIBCRYPTO} ${LIBSSL} ${LIBTLS}
 LINKS=	${BINDIR}/ntpd ${BINDIR}/ntpctl
 MAN=	ntpd.8 ntpd.conf.5 ntpctl.8
 
diff --git a/usr.sbin/ntpd/client.c b/usr.sbin/ntpd/client.c
index 06c41fda259..06c5e92457b 100644
--- a/usr.sbin/ntpd/client.c
+++ b/usr.sbin/ntpd/client.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: client.c,v 1.98 2015/01/19 20:47:03 bcook Exp $ */
+/*	$OpenBSD: client.c,v 1.99 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -324,6 +324,15 @@ client_dispatch(struct ntp_peer *p, u_int8_t settime)
 		return (0);
 	}
 
+	/* Detect liars */
+	if (conf->constraint_median != 0 &&
+	    (constraint_check(T2) != 0 || constraint_check(T3) != 0)) {
+		log_info("reply from %s: constraint check failed",
+		    log_sockaddr((struct sockaddr *)&p->addr->ss));
+		set_next(p, error_interval());
+		return (0);
+	}
+
 	p->reply[p->shift].offset = ((T2 - T1) + (T3 - T4)) / 2;
 	p->reply[p->shift].delay = (T4 - T1) - (T3 - T2);
 	p->reply[p->shift].status.stratum = msg.stratum;
diff --git a/usr.sbin/ntpd/config.c b/usr.sbin/ntpd/config.c
index e2443e65445..35d7b31f742 100644
--- a/usr.sbin/ntpd/config.c
+++ b/usr.sbin/ntpd/config.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: config.c,v 1.25 2015/02/08 04:54:15 reyk Exp $ */
+/*	$OpenBSD: config.c,v 1.26 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -33,6 +33,7 @@ struct ntp_addr	*host_v4(const char *);
 struct ntp_addr	*host_v6(const char *);
 
 static u_int32_t		 maxid = 0;
+static u_int32_t		 constraint_maxid = 0;
 
 void
 host(const char *s, struct ntp_addr **hn)
@@ -193,3 +194,16 @@ new_sensor(char *device)
 
 	return (s);
 }
+
+struct constraint *
+new_constraint(void)
+{
+	struct constraint	*p;
+
+	if ((p = calloc(1, sizeof(struct constraint))) == NULL)
+		fatal("new_constraint calloc");
+	p->id = ++constraint_maxid;
+
+	return (p);
+}
+
diff --git a/usr.sbin/ntpd/constraint.c b/usr.sbin/ntpd/constraint.c
new file mode 100644
index 00000000000..4a6265cf0f7
--- /dev/null
+++ b/usr.sbin/ntpd/constraint.c
@@ -0,0 +1,659 @@
+/*	$OpenBSD: constraint.c,v 1.1 2015/02/10 06:40:08 reyk Exp $	*/
+
+/*
+ * Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/queue.h>
+#include <sys/socket.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/uio.h>
+
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <imsg.h>
+#include <netdb.h>
+#include <poll.h>
+#include <signal.h>
+#include <string.h>
+#include <unistd.h>
+#include <time.h>
+#include <tls.h>
+
+#include "log.h"
+#include "ntpd.h"
+
+int	 constraint_addr_init(struct constraint *);
+struct constraint *
+	 constraint_byfd(int);
+struct constraint *
+	 constraint_bypid(pid_t);
+int	 constraint_close(int);
+void	 constraint_update(void);
+void	 constraint_reset(void);
+int	 constraint_cmp(const void *, const void *);
+
+struct httpsdate *
+	 httpsdate_init(const char *, const char *, const char *,
+	    const char *, const u_int8_t *, size_t);
+void	 httpsdate_free(void *);
+int	 httpsdate_request(struct httpsdate *, struct timeval *);
+void	*httpsdate_query(const char *, const char *, const char *,
+	    const char *, const u_int8_t *, size_t,
+	    struct timeval *, struct timeval *);
+
+char	*tls_readline(struct tls *, size_t *, size_t *, struct timeval *);
+
+struct httpsdate {
+	char			*tls_host;
+	char			*tls_port;
+	char			*tls_name;
+	char			*tls_path;
+	char			*tls_request;
+	struct tls_config	*tls_config;
+	struct tls		*tls_ctx;
+	struct tm		 tls_tm;
+};
+
+int
+constraint_init(struct constraint *cstr)
+{
+	cstr->state = STATE_NONE;
+	cstr->fd = -1;
+	cstr->last = getmonotime();
+	cstr->constraint = 0;
+
+	return (constraint_addr_init(cstr));
+}
+
+int
+constraint_addr_init(struct constraint *cstr)
+{
+	struct sockaddr_in	*sa_in;
+	struct sockaddr_in6	*sa_in6;
+	struct ntp_addr		*h;
+	int			 cnt = 0;
+
+	for (h = cstr->addr; h != NULL; h = h->next) {
+		switch (h->ss.ss_family) {
+		case AF_INET:
+			sa_in = (struct sockaddr_in *)&h->ss;
+			if (ntohs(sa_in->sin_port) == 0)
+				sa_in->sin_port = htons(443);
+			cstr->state = STATE_DNS_DONE;
+			break;
+		case AF_INET6:
+			sa_in6 = (struct sockaddr_in6 *)&h->ss;
+			if (ntohs(sa_in6->sin6_port) == 0)
+				sa_in6->sin6_port = htons(443);
+			cstr->state = STATE_DNS_DONE;
+			break;
+		default:
+			/* XXX king bula sez it? */
+			fatalx("wrong AF in constraint_addr_init");
+			/* NOTREACHED */
+		}
+		cnt++;
+	}
+
+	return (cnt);
+}
+
+int
+constraint_query(struct constraint *cstr)
+{
+	int		 pipes[2];
+	struct timeval	 rectv, xmttv;
+	void		*ctx;
+	static char	 hname[NI_MAXHOST];
+	time_t		 now;
+	struct iovec	 iov[2];
+
+	now = getmonotime();
+	if (cstr->state >= STATE_REPLY_RECEIVED) {
+		if (cstr->last + CONSTRAINT_SCAN_INTERVAL > now) {
+			/* Nothing to do */
+			return (-1);
+		}
+
+		/* Reset */
+		cstr->senderrors = 0;
+		constraint_close(cstr->fd);
+	} else if (cstr->state == STATE_QUERY_SENT) {
+		if (cstr->last + CONSTRAINT_SCAN_TIMEOUT > now) {
+			/* The caller should expect a reply */
+			return (0);
+		}
+
+		/* Timeout, just kill the process to reset it */
+		kill(cstr->pid, SIGTERM);
+		return (-1);
+	}
+
+	cstr->last = now;
+	if (getnameinfo((struct sockaddr *)&cstr->addr->ss,
+	    SA_LEN((struct sockaddr *)&cstr->addr->ss),
+	    hname, sizeof(hname), NULL, 0,
+	    NI_NUMERICHOST) != 0)
+		fatalx("%s getnameinfo %s", __func__, cstr->addr_head.name);
+
+	log_debug("constraint request to %s", hname);
+
+	if (socketpair(AF_UNIX, SOCK_DGRAM, AF_UNSPEC, pipes) == -1)
+		fatal("%s pipes", __func__);
+
+	/* Fork child handlers */
+	switch (cstr->pid = fork()) {
+	case -1:
+		cstr->senderrors++;
+		close(pipes[0]);
+		close(pipes[1]);
+		return (-1);
+	case 0:
+		tzset();
+		log_init(conf->debug);
+
+		setproctitle("constraint from %s", hname);
+
+		/* Child process */
+		if (dup2(pipes[1], CONSTRAINT_PASSFD) == -1)
+			fatal("%s dup2 CONSTRAINT_PASSFD", __func__);
+		if (pipes[0] != CONSTRAINT_PASSFD)
+			close(pipes[0]);
+		if (pipes[1] != CONSTRAINT_PASSFD)
+			close(pipes[1]);
+		(void)closefrom(CONSTRAINT_PASSFD + 1);
+
+		if (fcntl(CONSTRAINT_PASSFD, F_SETFD, FD_CLOEXEC) == -1)
+			fatal("%s fcntl F_SETFD", __func__);
+
+		cstr->fd = CONSTRAINT_PASSFD;
+		imsg_init(&cstr->ibuf, cstr->fd);
+
+		if ((ctx = httpsdate_query(hname,
+		    CONSTRAINT_PORT, cstr->addr_head.name, cstr->addr_head.path,
+		    conf->ca, conf->ca_len, &rectv, &xmttv)) == NULL) {
+			/* Abort with failure but without warning */
+			exit(1);
+		}
+
+		iov[0].iov_base = &rectv;
+		iov[0].iov_len = sizeof(rectv);
+		iov[1].iov_base = &xmttv;
+		iov[1].iov_len = sizeof(xmttv);
+		imsg_composev(&cstr->ibuf, IMSG_CONSTRAINT, 0, 0, -1, iov, 2);
+		imsg_flush(&cstr->ibuf);
+
+		/* Tear down the TLS connection after sending the result */
+		httpsdate_free(ctx);
+
+		_exit(0);
+		/* NOTREACHED */
+	default:
+		/* Parent */
+		close(pipes[1]);
+		cstr->fd = pipes[0];
+		cstr->state = STATE_QUERY_SENT;
+
+		imsg_init(&cstr->ibuf, cstr->fd);
+		break;
+	}
+
+
+	return (0);
+}
+
+void
+constraint_check_child(void)
+{
+	struct constraint	*cstr;
+	int			 status;
+	int			 fail;
+	char			*cause;
+	pid_t			 pid;
+
+	do {
+		pid = waitpid(WAIT_ANY, &status, WNOHANG);
+		if (pid <= 0)
+			continue;
+
+		fail = 0;
+		if (WIFSIGNALED(status)) {
+			fail = 1;
+			asprintf(&cause, "terminated; signal %d",
+			    WTERMSIG(status));
+		} else if (WIFEXITED(status)) {
+			if (WEXITSTATUS(status) != 0) {
+				fail = 1;
+				asprintf(&cause, "exited abnormally");
+			} else
+				asprintf(&cause, "exited okay");
+		} else
+			fatalx("unexpected cause of SIGCHLD");
+
+		if ((cstr = constraint_bypid(pid)) != NULL) {
+			if (fail) {
+				log_info("no constraint reply from %s"
+				    " received in time, next query %ds",
+				    log_sockaddr((struct sockaddr *)
+				    &cstr->addr->ss), CONSTRAINT_SCAN_INTERVAL);
+			}
+
+			if (fail || cstr->state < STATE_REPLY_RECEIVED) {
+				cstr->senderrors++;
+				constraint_close(cstr->fd);
+			}
+		}
+		free(cause);
+	} while (pid > 0 || (pid == -1 && errno == EINTR));
+}
+
+struct constraint *
+constraint_byfd(int fd)
+{
+	struct constraint	*cstr;
+
+	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+		if (cstr->fd == fd)
+			return (cstr);
+	}
+
+	return (NULL);
+}
+
+struct constraint *
+constraint_bypid(pid_t pid)
+{
+	struct constraint	*cstr;
+
+	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+		if (cstr->pid == pid)
+			return (cstr);
+	}
+
+	return (NULL);
+}
+
+int
+constraint_close(int fd)
+{
+	struct constraint	*cstr;
+
+	if ((cstr = constraint_byfd(fd)) == NULL) {
+		log_warn("%s: fd %d: not found", __func__, fd);
+		return (0);
+	}
+
+	msgbuf_clear(&cstr->ibuf.w);
+	close(cstr->fd);
+	cstr->fd = -1;
+	if (cstr->senderrors)
+		cstr->state = STATE_INVALID;
+	else if (cstr->state >= STATE_QUERY_SENT)
+		cstr->state = STATE_DNS_DONE;
+
+	cstr->last = getmonotime();
+
+	return (1);
+}
+
+int
+constraint_dispatch_msg(struct pollfd *pfd)
+{
+	struct imsg		 imsg;
+	struct constraint	*cstr;
+	ssize_t			 n;
+	struct timeval		 tv[2];
+	double			 offset;
+
+	if ((cstr = constraint_byfd(pfd->fd)) == NULL)
+		return (0);
+
+	if (!(pfd->revents & POLLIN))
+		return (0);
+
+	if ((n = imsg_read(&cstr->ibuf)) == -1 || n == 0) {
+		constraint_close(pfd->fd);
+		return (1);
+	}
+
+	for (;;) {
+		if ((n = imsg_get(&cstr->ibuf, &imsg)) == -1) {
+			constraint_close(pfd->fd);
+			return (1);
+		}
+		if (n == 0)
+			break;
+
+		switch (imsg.hdr.type) {
+		case IMSG_CONSTRAINT:
+			 if (imsg.hdr.len != IMSG_HEADER_SIZE + sizeof(tv))
+				fatalx("invalid IMSG_CONSTRAINT received");
+
+			memcpy(tv, imsg.data, sizeof(tv));
+
+			offset = gettime_from_timeval(&tv[0]) -
+			    gettime_from_timeval(&tv[1]);
+
+			log_info("constraint reply from %s: offset %f",
+			    log_sockaddr((struct sockaddr *)&cstr->addr->ss),
+			    offset);
+
+			cstr->state = STATE_REPLY_RECEIVED;
+			cstr->last = getmonotime();
+			cstr->constraint = tv[0].tv_sec;
+
+			constraint_update();
+			break;
+		default:
+			break;
+		}
+		imsg_free(&imsg);
+	}
+
+	return (0);
+}
+
+int
+constraint_cmp(const void *a, const void *b)
+{
+	return (*(const time_t *)a - *(const time_t *)b);
+}
+
+void
+constraint_update(void)
+{
+	struct constraint *cstr;
+	int	 cnt, i;
+	time_t	*sum;
+	time_t	 now;
+
+	now = getmonotime();
+
+	cnt = 0;
+	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+		if (cstr->state != STATE_REPLY_RECEIVED)
+			continue;
+		cnt++;
+	}
+
+	if ((sum = calloc(cnt, sizeof(time_t))) == NULL)
+		fatal("calloc");
+
+	i = 0;
+	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+		if (cstr->state != STATE_REPLY_RECEIVED)
+			continue;
+		sum[i++] = cstr->constraint + (now - cstr->last);
+	}
+
+	qsort(sum, cnt, sizeof(time_t), constraint_cmp);
+
+	/* calculate median */
+	i = cnt / 2;
+	if (cnt % 2 == 0)
+		if (sum[i - 1] < sum[i])
+			i -= 1;
+
+	conf->constraint_last = now;
+	conf->constraint_median = sum[i];
+
+	free(sum);
+}
+
+void
+constraint_reset(void)
+{
+	struct constraint *cstr;
+
+	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+		if (cstr->state == STATE_QUERY_SENT)
+			continue;
+		constraint_close(cstr->fd);
+	}
+	conf->constraint_errors = 0;
+}
+
+int
+constraint_check(double val)
+{
+	struct timeval	tv;
+	double		constraint;
+	time_t		now;
+
+	if (conf->constraint_median == 0)
+		return (0);
+
+	/* Calculate the constraint with the current offset */
+	now = getmonotime();
+	tv.tv_sec = conf->constraint_median + (now - conf->constraint_last);
+	tv.tv_usec = 0;
+	constraint = gettime_from_timeval(&tv);
+
+	if (((val - constraint) > CONSTRAINT_MARGIN) ||
+	    ((constraint - val) > CONSTRAINT_MARGIN)) {
+		/* XXX get new constraint if too many errors happened */
+		if (conf->constraint_errors++ > CONSTRAINT_ERROR_MARGIN) {
+			constraint_reset();
+		}
+
+		return (-1);
+	}
+
+	return (0);
+}
+
+struct httpsdate *
+httpsdate_init(const char *hname, const char *port, const char *name,
+    const char *path, const u_int8_t *ca, size_t ca_len)
+{
+	struct httpsdate	*httpsdate = NULL;
+
+	if (tls_init() == -1)
+		return (NULL);
+
+	if ((httpsdate = calloc(1, sizeof(*httpsdate))) == NULL)
+		goto fail;
+
+	if (name == NULL)
+		name = hname;
+
+	if ((httpsdate->tls_host = strdup(hname)) == NULL ||
+	    (httpsdate->tls_port = strdup(port)) == NULL ||
+	    (httpsdate->tls_name = strdup(name)) == NULL ||
+	    (httpsdate->tls_path = strdup(path)) == NULL)
+		goto fail;
+
+	if (asprintf(&httpsdate->tls_request,
+	    "HEAD %s HTTP/1.1\r\nHost: %s\r\nConnection: close\r\n\r\n",
+	    httpsdate->tls_path, httpsdate->tls_name) == -1)
+		goto fail;
+
+	if ((httpsdate->tls_config = tls_config_new()) == NULL)
+		goto fail;
+
+	/* XXX we have to pre-resolve, so name and host are not equal */
+	tls_config_insecure_noverifyhost(httpsdate->tls_config);
+
+	if (ca == NULL || ca_len == 0)
+		tls_config_insecure_noverifycert(httpsdate->tls_config);
+	else
+		tls_config_set_ca_mem(httpsdate->tls_config, ca, ca_len);
+
+	return (httpsdate);
+
+ fail:
+	httpsdate_free(httpsdate);
+	return (NULL);
+}
+
+void
+httpsdate_free(void *arg)
+{
+	struct httpsdate *httpsdate = arg;
+	if (httpsdate == NULL)
+		return;
+	if (httpsdate->tls_ctx)
+		tls_close(httpsdate->tls_ctx);
+	tls_free(httpsdate->tls_ctx);
+	tls_config_free(httpsdate->tls_config);
+	free(httpsdate->tls_host);
+	free(httpsdate->tls_port);
+	free(httpsdate->tls_name);
+	free(httpsdate->tls_path);
+	free(httpsdate->tls_request);
+	free(httpsdate);
+}
+
+int
+httpsdate_request(struct httpsdate *httpsdate, struct timeval *when)
+{
+	size_t	 outlen = 0, maxlength = CONSTRAINT_MAXHEADERLENGTH;
+	char	*line, *p;
+
+	if ((httpsdate->tls_ctx = tls_client()) == NULL)
+		goto fail;
+
+	if (tls_configure(httpsdate->tls_ctx, httpsdate->tls_config) == -1)
+		goto fail;
+
+	if (tls_connect(httpsdate->tls_ctx,
+	    httpsdate->tls_host, httpsdate->tls_port) == -1) {
+		log_warnx("tls failed: %s: %s", httpsdate->tls_host,
+		    tls_error(httpsdate->tls_ctx));
+		goto fail;
+	}
+
+	if (tls_write(httpsdate->tls_ctx,
+	    httpsdate->tls_request, strlen(httpsdate->tls_request),
+	    &outlen) == -1)
+		goto fail;
+
+	while ((line = tls_readline(httpsdate->tls_ctx, &outlen,
+	    &maxlength, when)) != NULL) {
+		line[strcspn(line, "\r\n")] = '\0';
+
+		if ((p = strchr(line, ' ')) == NULL || *p == '\0')
+			goto next;
+		*p++ = '\0';
+		if (strcasecmp("Date:", line) != 0)
+			goto next;
+
+		/*
+		 * Expect the date/time format as IMF-fixdate which is
+		 * mandated by HTTP/1.1 in the new RFC 7231 and was
+		 * preferred by RFC 2616.  Other formats would be RFC 850
+		 * or ANSI C's asctime() - the latter doesn't include
+		 * the timezone which is required here.
+		 */
+		if (strptime(p, "%a, %d %h %Y %T %Z",
+		    &httpsdate->tls_tm) == NULL) {
+			log_warnx("unsupported date format");
+			free(line);
+			return (-1);
+		}
+
+		free(line);
+		break;
+ next:
+		free(line);
+	}
+
+
+	return (0);
+ fail:
+	httpsdate_free(httpsdate);
+	return (-1);
+}
+
+void *
+httpsdate_query(const char *hname, const char *port, const char *name,
+    const char *path, const u_int8_t *ca, size_t ca_len,
+    struct timeval *rectv, struct timeval *xmttv)
+{
+	struct httpsdate	*httpsdate;
+	struct timeval		 when;
+	time_t			 t;
+
+	if ((httpsdate = httpsdate_init(hname, port, name, path,
+	    ca, ca_len)) == NULL)
+		return (NULL);
+
+	if (httpsdate_request(httpsdate, &when) == -1)
+		return (NULL);
+
+	/* Return parsed date as local time */
+	t = timegm(&httpsdate->tls_tm);
+
+	/* Report parsed Date: as "received time" */
+	rectv->tv_sec = t;
+	rectv->tv_usec = 0;
+
+	/* And add delay as "transmit time" */
+	xmttv->tv_sec = when.tv_sec;
+	xmttv->tv_usec = when.tv_usec;
+
+	return (httpsdate);
+}
+
+/* Based on SSL_readline in ftp/fetch.c */
+char *
+tls_readline(struct tls *tls, size_t *lenp, size_t *maxlength,
+    struct timeval *when)
+{
+	size_t i, len, nr;
+	char *buf, *q, c;
+	int ret;
+
+	len = 128;
+	if ((buf = malloc(len)) == NULL)
+		fatal("Can't allocate memory for transfer buffer");
+	for (i = 0; ; i++) {
+		if (i >= len - 1) {
+			if ((q = reallocarray(buf, len, 2)) == NULL)
+				fatal("Can't expand transfer buffer");
+			buf = q;
+			len *= 2;
+		}
+ again:
+		ret = tls_read(tls, &c, 1, &nr);
+		if (ret == TLS_READ_AGAIN)
+			goto again;
+		if (ret != 0) {
+			/* SSL read error, ignore */
+			return (NULL);
+		}
+
+		if (maxlength != NULL && (*maxlength)-- == 0) {
+			log_warnx("maximum length exceeded");
+			return (NULL);
+		}
+
+		buf[i] = c;
+		if (c == '\n')
+			break;
+	}
+	*lenp = i;
+	if (gettimeofday(when, NULL) == -1)
+		fatal("gettimeofday");
+	return (buf);
+}
diff --git a/usr.sbin/ntpd/control.c b/usr.sbin/ntpd/control.c
index e6d91bf3e17..867f91122fa 100644
--- a/usr.sbin/ntpd/control.c
+++ b/usr.sbin/ntpd/control.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: control.c,v 1.4 2015/01/09 07:35:37 deraadt Exp $ */
+/*	$OpenBSD: control.c,v 1.5 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -309,6 +309,9 @@ build_show_status(struct ctl_show_status *cs)
 	cs->synced = conf->status.synced;
 	cs->stratum = conf->status.stratum;
 	cs->clock_offset = getoffset() * 1000.0;
+	cs->constraint_median = conf->constraint_median;
+	cs->constraint_last = conf->constraint_last;
+	cs->constraint_errors = conf->constraint_errors;
 }
 
 void
diff --git a/usr.sbin/ntpd/ntp.c b/usr.sbin/ntpd/ntp.c
index 5a4e336aeec..76629af5ef3 100644
--- a/usr.sbin/ntpd/ntp.c
+++ b/usr.sbin/ntpd/ntp.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntp.c,v 1.127 2015/02/10 06:03:43 bcook Exp $ */
+/*	$OpenBSD: ntp.c,v 1.128 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -30,6 +30,7 @@
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
+#include <tls.h>
 
 #include "ntpd.h"
 
@@ -41,6 +42,7 @@
 
 volatile sig_atomic_t	 ntp_quit = 0;
 volatile sig_atomic_t	 ntp_report = 0;
+volatile sig_atomic_t	 ntp_sigchld = 0;
 struct imsgbuf		*ibuf_main;
 struct imsgbuf		*ibuf_dns;
 struct ntpd_conf	*conf;
@@ -67,6 +69,9 @@ ntp_sighdlr(int sig)
 	case SIGINFO:
 		ntp_report = 1;
 		break;
+	case SIGCHLD:
+		ntp_sigchld = 1;
+		break;
 	}
 }
 
@@ -76,9 +81,10 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 {
 	int			 a, b, nfds, i, j, idx_peers, timeout;
 	int			 hotplugfd, nullfd, pipe_dns[2], idx_clients;
+	int			 ctls, boundaries;
 	u_int			 pfd_elms = 0, idx2peer_elms = 0;
 	u_int			 listener_cnt, new_cnt, sent_cnt, trial_cnt;
-	u_int			 ctl_cnt;
+	u_int			 ctl_cnt, constraint_cnt;
 	pid_t			 pid;
 	struct pollfd		*pfd = NULL;
 	struct servent		*se;
@@ -86,10 +92,11 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 	struct ntp_peer		*p;
 	struct ntp_peer		**idx2peer = NULL;
 	struct ntp_sensor	*s, *next_s;
+	struct constraint	*cstr;
 	struct timespec		 tp;
 	struct stat		 stb;
 	struct ctl_conn		*cc;
-	time_t			 nextaction, last_sensor_scan = 0;
+	time_t			 nextaction, last_sensor_scan = 0, now;
 	void			*newp;
 
 	switch (pid = fork()) {
@@ -102,6 +109,13 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 		return (pid);
 	}
 
+	tls_init();
+
+	/* Verification will be turned off if CA is not found */
+	if ((conf->ca = tls_load_file(CONSTRAINT_CA,
+	    &conf->ca_len, NULL)) == NULL)
+		log_warnx("constraint certificate verification turned off");
+
 	/* in this case the parent didn't init logging and didn't daemonize */
 	if (nconf->settime && !nconf->debug) {
 		log_init(nconf->debug);
@@ -157,7 +171,7 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 	signal(SIGINFO, ntp_sighdlr);
 	signal(SIGPIPE, SIG_IGN);
 	signal(SIGHUP, SIG_IGN);
-	signal(SIGCHLD, SIG_DFL);
+	signal(SIGCHLD, ntp_sighdlr);
 
 	if ((ibuf_main = malloc(sizeof(struct imsgbuf))) == NULL)
 		fatal(NULL);
@@ -166,6 +180,12 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 		fatal(NULL);
 	imsg_init(ibuf_dns, pipe_dns[0]);
 
+	constraint_cnt = 0;
+	conf->constraint_median = 0;
+	conf->constraint_last = getmonotime();
+	TAILQ_FOREACH(cstr, &conf->constraints, entry)
+		constraint_cnt += constraint_init(cstr);
+
 	TAILQ_FOREACH(p, &conf->ntp_peers, entry)
 		client_peer_init(p);
 
@@ -213,7 +233,8 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 			idx2peer_elms = peer_cnt;
 		}
 
-		new_cnt = PFD_MAX + peer_cnt + listener_cnt + ctl_cnt;
+		new_cnt = PFD_MAX +
+		    peer_cnt + listener_cnt + ctl_cnt + constraint_cnt;
 		if (new_cnt > pfd_elms) {
 			if ((newp = reallocarray(pfd, new_cnt,
 			    sizeof(*pfd))) == NULL) {
@@ -248,6 +269,9 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 		idx_peers = i;
 		sent_cnt = trial_cnt = 0;
 		TAILQ_FOREACH(p, &conf->ntp_peers, entry) {
+			if (constraint_cnt && conf->constraint_median == 0)
+				continue;
+
 			if (p->next > 0 && p->next <= getmonotime()) {
 				if (p->state > STATE_DNS_INPROGRESS)
 					trial_cnt++;
@@ -326,8 +350,23 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 				pfd[i].events |= POLLOUT;
 			i++;
 		}
+		ctls = i;
 
-		timeout = nextaction - getmonotime();
+		boundaries = 0;
+		TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+			if (constraint_query(cstr) == -1)
+				continue;
+			pfd[i].fd = cstr->fd;
+			pfd[i].events = POLLIN;
+			i++;
+		}
+		boundaries = i;
+
+		now = getmonotime();
+		if (constraint_cnt)
+			nextaction = now + 1;
+
+		timeout = nextaction - now;
 		if (timeout < 0)
 			timeout = 0;
 
@@ -389,8 +428,13 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 			}
 		}
 
-		for (; nfds > 0 && j < i; j++)
+		for (; nfds > 0 && j < ctls; j++) {
 			nfds -= control_dispatch_msg(&pfd[j], &ctl_cnt);
+		}
+
+		for (; nfds > 0 && j < i; j++) {
+			nfds -= constraint_dispatch_msg(&pfd[j]);
+		}
 
 		for (s = TAILQ_FIRST(&conf->ntp_sensors); s != NULL;
 		    s = next_s) {
@@ -400,6 +444,11 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 		}
 		report_peers(ntp_report);
 		ntp_report = 0;
+
+		if (ntp_sigchld) {
+			constraint_check_child();
+			ntp_sigchld = 0;
+		}
 	}
 
 	msgbuf_write(&ibuf_main->w);
diff --git a/usr.sbin/ntpd/ntpctl.8 b/usr.sbin/ntpd/ntpctl.8
index dcf0088f00e..b09c3310eb8 100644
--- a/usr.sbin/ntpd/ntpctl.8
+++ b/usr.sbin/ntpd/ntpctl.8
@@ -1,4 +1,4 @@
-.\" $OpenBSD: ntpctl.8,v 1.6 2014/01/22 02:55:15 benno Exp $
+.\" $OpenBSD: ntpctl.8,v 1.7 2015/02/10 06:40:08 reyk Exp $
 .\"
 .\" Copyright (c) 2012 Mike Miller <mmiller@mgm51.com>
 .\"
@@ -14,7 +14,7 @@
 .\" AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 .\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: January 22 2014 $
+.Dd $Mdocdate: February 10 2015 $
 .Dt NTPCTL 8
 .Os
 .Sh NAME
@@ -53,6 +53,7 @@ When the system clock is not synced, the offset of the system clock,
 as reported by the
 .Xr adjtime 2
 system call, is displayed.
+When the median constraint is set, the offset to the local time is displayed.
 .El
 .Sh FILES
 .Bl -tag -width "/var/run/ntpd.sockXXX" -compact
diff --git a/usr.sbin/ntpd/ntpd.c b/usr.sbin/ntpd/ntpd.c
index c22e5b73272..622fd223dbc 100644
--- a/usr.sbin/ntpd/ntpd.c
+++ b/usr.sbin/ntpd/ntpd.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.c,v 1.88 2015/01/21 03:14:10 bcook Exp $ */
+/*	$OpenBSD: ntpd.c,v 1.89 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -709,6 +709,7 @@ show_status_msg(struct imsg *imsg)
 {
 	struct ctl_show_status	*cstatus;
 	double			 clock_offset;
+	struct timeval		 tv;
 
 	if (imsg->hdr.len != IMSG_HEADER_SIZE + sizeof(struct ctl_show_status))
 		fatalx("invalid IMSG_CTL_SHOW_STATUS received");
@@ -723,6 +724,18 @@ show_status_msg(struct imsg *imsg)
 		printf("%d/%d sensors valid, ",
 		    cstatus->valid_sensors, cstatus->sensorcnt);
 
+	if (cstatus->constraint_median) {
+		tv.tv_sec = cstatus->constraint_median +
+                    (getmonotime() - cstatus->constraint_last);
+		tv.tv_usec = 0;
+		printf("constraint offset %f",
+		    gettime_from_timeval(&tv) - gettime());
+		if (cstatus->constraint_errors)
+			printf(" (%d errors)",
+			    cstatus->constraint_errors);
+		printf(", ");
+	}
+
 	if (cstatus->peercnt + cstatus->sensorcnt == 0)
 		printf("no peers and no sensors configured\n");
 
diff --git a/usr.sbin/ntpd/ntpd.conf.5 b/usr.sbin/ntpd/ntpd.conf.5
index 3e8020ba41b..8466076fd1b 100644
--- a/usr.sbin/ntpd/ntpd.conf.5
+++ b/usr.sbin/ntpd/ntpd.conf.5
@@ -1,4 +1,4 @@
-.\" $OpenBSD: ntpd.conf.5,v 1.24 2012/09/20 12:43:16 patrick Exp $
+.\" $OpenBSD: ntpd.conf.5,v 1.25 2015/02/10 06:40:08 reyk Exp $
 .\"
 .\" Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
 .\"
@@ -14,7 +14,7 @@
 .\" AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 .\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: September 20 2012 $
+.Dd $Mdocdate: February 10 2015 $
 .Dt NTPD.CONF 5
 .Os
 .Sh NAME
@@ -35,6 +35,40 @@ character are ignored.
 Keywords may be specified multiple times within the configuration file.
 They are as follows:
 .Bl -tag -width Ds
+.It Ic constraint from Ar url
+Specify the URL, IP address or the hostname of a HTTPS server to
+provide a constraint.
+.Xr ntpd 8
+will connect to the server and retrieve the remote time from the
+.Eq Date
+header.
+This time will be used as a constraint on time synchronization;
+received NTP packets with time information that is more than a few
+minutes off will be discarded and the NTP
+.Ic server
+will be marked as invalid.
+If multiple
+.Ic constraint
+keywords are used,
+.Xr ntpd 8
+will calculate a median constraint from all the servers specified.
+.Bd -literal -offset indent
+server ntp.example.org
+constraint www.example.com
+.Ed
+.It Ic constraints from Ar url
+As with
+.Ic constraint ,
+specify the URL, IP address or the hostname of a HTTPS server to
+provide a constraint.
+Should the hostname resolve to multiple IP addresses,
+.Xr ntpd 8
+will calculate a median constraint from all of them.
+For example:
+.Bd -literal -offset indent
+servers pool.ntp.org
+constraints from "https://www.google.com/search?q=openntpd"
+.Ed
 .It Xo Ic listen on Ar address
 .Op Ic rtable Ar table-id
 .Xc
diff --git a/usr.sbin/ntpd/ntpd.h b/usr.sbin/ntpd/ntpd.h
index 44f91780f7a..9c65ee2d4bc 100644
--- a/usr.sbin/ntpd/ntpd.h
+++ b/usr.sbin/ntpd/ntpd.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.h,v 1.117 2015/01/13 02:28:56 bcook Exp $ */
+/*	$OpenBSD: ntpd.h,v 1.118 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -72,16 +72,26 @@
 #define	SENSOR_DATA_MAXAGE		(15*60)
 #define	SENSOR_QUERY_INTERVAL		15
 #define	SENSOR_QUERY_INTERVAL_SETTIME	(SETTIME_TIMEOUT/3)
-#define	SENSOR_SCAN_INTERVAL		(5*60)
+#define	SENSOR_SCAN_INTERVAL		(1*60)
 #define	SENSOR_DEFAULT_REFID		"HARD"
 
+#define CONSTRAINT_ERROR_MARGIN		(4)
+#define CONSTRAINT_SCAN_INTERVAL	(15*60)
+#define CONSTRAINT_SCAN_TIMEOUT		(10)
+#define CONSTRAINT_MARGIN		(2.0*60)
+#define CONSTRAINT_PORT			"443"	/* HTTPS port */
+#define	CONSTRAINT_MAXHEADERLENGTH	8192
+#define CONSTRAINT_PASSFD		(STDERR_FILENO + 1)
+#define CONSTRAINT_CA			"/etc/ssl/cert.pem"
+
 enum client_state {
 	STATE_NONE,
 	STATE_DNS_INPROGRESS,
 	STATE_DNS_TEMPFAIL,
 	STATE_DNS_DONE,
 	STATE_QUERY_SENT,
-	STATE_REPLY_RECEIVED
+	STATE_REPLY_RECEIVED,
+	STATE_INVALID
 };
 
 struct listen_addr {
@@ -99,6 +109,7 @@ struct ntp_addr {
 
 struct ntp_addr_wrap {
 	char			*name;
+	char			*path;
 	struct ntp_addr		*a;
 	u_int8_t		 pool;
 };
@@ -160,6 +171,20 @@ struct ntp_sensor {
 	u_int8_t			 shift;
 };
 
+struct constraint {
+	TAILQ_ENTRY(constraint)		 entry;
+	struct ntp_addr_wrap		 addr_head;
+	struct ntp_addr			*addr;
+	int				 senderrors;
+	enum client_state		 state;
+	u_int32_t			 id;
+	int				 fd;
+	pid_t				 pid;
+	struct imsgbuf			 ibuf;
+	time_t				 last;
+	time_t				 constraint;
+};
+
 struct ntp_conf_sensor {
 	TAILQ_ENTRY(ntp_conf_sensor)		 entry;
 	char					*device;
@@ -182,6 +207,7 @@ struct ntpd_conf {
 	TAILQ_HEAD(ntp_peers, ntp_peer)			ntp_peers;
 	TAILQ_HEAD(ntp_sensors, ntp_sensor)		ntp_sensors;
 	TAILQ_HEAD(ntp_conf_sensors, ntp_conf_sensor)	ntp_conf_sensors;
+	TAILQ_HEAD(constraints, constraint)		constraints;
 	struct ntp_status				status;
 	struct ntp_freq					freq;
 	u_int32_t					scale;
@@ -190,6 +216,11 @@ struct ntpd_conf {
 	u_int8_t					debug;
 	u_int8_t					noaction;
 	u_int8_t					filters;
+	time_t						constraint_last;
+	time_t						constraint_median;
+	u_int						constraint_errors;
+	u_int8_t					*ca;
+	size_t						ca_len;
 };
 
 struct ctl_show_status {
@@ -200,6 +231,9 @@ struct ctl_show_status {
 	u_int8_t	 synced;
 	u_int8_t	 stratum;
 	double		 clock_offset;
+	time_t		 constraint_median;
+	time_t		 constraint_last;
+	u_int		 constraint_errors;
 };
 
 struct ctl_show_peer {
@@ -245,6 +279,7 @@ enum imsg_type {
 	IMSG_ADJFREQ,
 	IMSG_SETTIME,
 	IMSG_HOST_DNS,
+	IMSG_CONSTRAINT,
 	IMSG_CTL_SHOW_STATUS,
 	IMSG_CTL_SHOW_PEERS,
 	IMSG_CTL_SHOW_PEERS_END,
@@ -284,6 +319,7 @@ int			 host_dns(const char *, struct ntp_addr **);
 void			 host_dns_free(struct ntp_addr *);
 struct ntp_peer		*new_peer(void);
 struct ntp_conf_sensor	*new_sensor(char *);
+struct constraint	*new_constraint(void);
 
 /* ntp_msg.c */
 int	ntp_getmsg(struct sockaddr *, char *, ssize_t, struct ntp_msg *);
@@ -303,8 +339,16 @@ int	client_dispatch(struct ntp_peer *, u_int8_t);
 void	client_log_error(struct ntp_peer *, const char *, int);
 void	set_next(struct ntp_peer *, time_t);
 
+/* constraint.c */
+int	 constraint_init(struct constraint *);
+int	 constraint_query(struct constraint *);
+int	 constraint_dispatch_msg(struct pollfd *);
+void	 constraint_check_child(void);
+int	 constraint_check(double);
+
 /* util.c */
 double			 gettime_corrected(void);
+double			 gettime_from_timeval(struct timeval *);
 double			 getoffset(void);
 double			 gettime(void);
 time_t			 getmonotime(void);
diff --git a/usr.sbin/ntpd/parse.y b/usr.sbin/ntpd/parse.y
index 1d4595800df..49432fc33fc 100644
--- a/usr.sbin/ntpd/parse.y
+++ b/usr.sbin/ntpd/parse.y
@@ -1,4 +1,4 @@
-/*	$OpenBSD: parse.y,v 1.57 2015/01/10 13:47:05 tedu Exp $ */
+/*	$OpenBSD: parse.y,v 1.58 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2002, 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -80,12 +80,12 @@ typedef struct {
 
 %}
 
-%token	LISTEN ON
+%token	LISTEN ON CONSTRAINT CONSTRAINTS FROM
 %token	SERVER SERVERS SENSOR CORRECTION RTABLE REFID STRATUM WEIGHT
 %token	ERROR
 %token	<v.string>		STRING
 %token	<v.number>		NUMBER
-%type	<v.addr>		address
+%type	<v.addr>		address url
 %type	<v.opts>		listen_opts listen_opts_l listen_opt
 %type	<v.opts>		server_opts server_opts_l server_opt
 %type	<v.opts>		sensor_opts sensor_opts_l sensor_opt
@@ -208,6 +208,82 @@ main		: LISTEN ON address listen_opts	{
 			free($2->name);
 			free($2);
 		}
+		| CONSTRAINTS FROM url		{
+			struct constraint	*p;
+			struct ntp_addr		*h, *next;
+
+			h = $3->a;
+			do {
+				if (h != NULL) {
+					next = h->next;
+					if (h->ss.ss_family != AF_INET &&
+					    h->ss.ss_family != AF_INET6) {
+						yyerror("IPv4 or IPv6 address "
+						    "or hostname expected");
+						free(h);
+						free($3->name);
+						free($3->path);
+						free($3);
+						YYERROR;
+					}
+					h->next = NULL;
+				} else
+					next = NULL;
+
+				p = new_constraint();
+				p->addr = h;
+				p->addr_head.a = h;
+				p->addr_head.pool = 1;
+				p->addr_head.name = strdup($3->name);
+				p->addr_head.path = strdup($3->path);
+				if (p->addr_head.name == NULL ||
+				    p->addr_head.path == NULL)
+					fatal(NULL);
+				if (p->addr != NULL)
+					p->state = STATE_DNS_DONE;
+				TAILQ_INSERT_TAIL(&conf->constraints,
+				    p, entry);
+				h = next;
+			} while (h != NULL);
+
+			free($3->name);
+			free($3);
+		}
+		| CONSTRAINT FROM url		{
+			struct constraint	*p;
+			struct ntp_addr		*h, *next;
+
+			p = new_constraint();
+			for (h = $3->a; h != NULL; h = next) {
+				next = h->next;
+				if (h->ss.ss_family != AF_INET &&
+				    h->ss.ss_family != AF_INET6) {
+					yyerror("IPv4 or IPv6 address "
+					    "or hostname expected");
+					free(h);
+					free(p);
+					free($3->name);
+					free($3->path);
+					free($3);
+					YYERROR;
+				}
+				h->next = p->addr;
+				p->addr = h;
+			}
+
+			p->addr_head.a = p->addr;
+			p->addr_head.pool = 0;
+			p->addr_head.name = strdup($3->name);
+			p->addr_head.path = strdup($3->path);
+			if (p->addr_head.name == NULL ||
+			    p->addr_head.path == NULL)
+				fatal(NULL);
+			if (p->addr != NULL)
+				p->state = STATE_DNS_DONE;
+			TAILQ_INSERT_TAIL(&conf->constraints, p, entry);
+			free($3->name);
+			free($3);
+		}
 		| SENSOR STRING	sensor_opts {
 			struct ntp_conf_sensor	*s;
 
@@ -230,6 +306,45 @@ address		: STRING		{
 		}
 		;
 
+url		: STRING		{
+			char	*hname, *path;
+
+			if (($$ = calloc(1, sizeof(struct ntp_addr_wrap))) ==
+			    NULL)
+				fatal("calloc");
+
+			if (strncmp("https://", $1,
+			    strlen("https://")) != 0) {
+				host($1, &$$->a);
+				$$->name = $1;
+				if (($$->path = strdup("/")) == NULL)
+					fatal("strdup");
+			} else {
+				hname = $1 + strlen("https://");
+
+				path = hname + strcspn(hname, "/\\");
+				if (*path == '\0')
+					path = "/";
+				if (($$->path = strdup(path)) == NULL)
+					fatal("strdup");
+				*path = '\0';
+				host(hname, &$$->a);
+				if (($$->name = strdup(hname)) == NULL)
+					fatal("strdup");
+			}
+
+			if ($$->a == NULL &&
+			    (host_dns($$->name, &$$->a) == -1 ||
+			    $$->a == NULL)) {
+				yyerror("could not resolve \"%s\"", $$->name);
+				free($$->name);
+				free($$->path);
+				free($$);
+				YYERROR;
+			}
+		}
+		;
+
 listen_opts	:	{ opts_default(); }
 		  listen_opts_l
 			{ $$ = opts; }
@@ -358,7 +473,10 @@ lookup(char *s)
 {
 	/* this has to be sorted always */
 	static const struct keywords keywords[] = {
+		{ "constraint",		CONSTRAINT},
+		{ "constraints",	CONSTRAINTS},
 		{ "correction",		CORRECTION},
+		{ "from",		FROM},
 		{ "listen",		LISTEN},
 		{ "on",			ON},
 		{ "refid",		REFID},
@@ -646,6 +764,7 @@ parse_config(const char *filename, struct ntpd_conf *xconf)
 	TAILQ_INIT(&conf->listen_addrs);
 	TAILQ_INIT(&conf->ntp_peers);
 	TAILQ_INIT(&conf->ntp_conf_sensors);
+	TAILQ_INIT(&conf->constraints);
 
 	if ((file = pushfile(filename)) == NULL) {
 		return (-1);
diff --git a/usr.sbin/ntpd/util.c b/usr.sbin/ntpd/util.c
index 88f9a517a59..79b1bb250d9 100644
--- a/usr.sbin/ntpd/util.c
+++ b/usr.sbin/ntpd/util.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: util.c,v 1.16 2015/01/04 01:11:24 bcook Exp $ */
+/*	$OpenBSD: util.c,v 1.17 2015/02/10 06:40:08 reyk Exp $ */
 
 /*
  * Copyright (c) 2004 Alexander Guy <alexander.guy@andern.org>
@@ -48,6 +48,12 @@ gettime(void)
 	return (tv.tv_sec + JAN_1970 + 1.0e-6 * tv.tv_usec);
 }
 
+double
+gettime_from_timeval(struct timeval *tv)
+{
+	return (tv->tv_sec + JAN_1970 + 1.0e-6 * tv->tv_usec);
+}
+
 time_t
 getmonotime(void)
 {