diff --git a/port.c b/port.c
index e5c079e..43105d1 100644
--- a/port.c
+++ b/port.c
@@ -366,6 +366,21 @@ static void free_foreign_masters(struct port *p)
 	}
 }
 
+static int fup_sync_ok(struct ptp_message *fup, struct ptp_message *sync)
+{
+	int64_t tfup, tsync;
+	tfup = tmv_to_nanoseconds(timespec_to_tmv(fup->hwts.sw));
+	tsync = tmv_to_nanoseconds(timespec_to_tmv(sync->hwts.sw));
+	/*
+	 * NB - If the sk_check_fupsync option is not enabled, then
+	 * both of these time stamps will be zero.
+	 */
+	if (tfup < tsync) {
+		return 0;
+	}
+	return 1;
+}
+
 static int incapable_ignore(struct port *p, struct ptp_message *m)
 {
 	if (port_capable(p)) {
@@ -1808,6 +1823,7 @@ static void process_sync(struct port *p, struct ptp_message *m)
 	}
 
 	if (p->syfu == SF_HAVE_FUP &&
+	    fup_sync_ok(p->last_syncfup, m) &&
 	    p->last_syncfup->header.sequenceId == m->header.sequenceId) {
 		event = SYNC_MATCH;
 	} else {
diff --git a/raw.c b/raw.c
index 326ccd2..d87edbf 100644
--- a/raw.c
+++ b/raw.c
@@ -209,6 +209,9 @@ static int raw_open(struct transport *t, char *name,
 	if (sk_timestamping_init(efd, name, ts_type, TRANS_IEEE_802_3))
 		goto no_timestamping;
 
+	if (sk_general_init(gfd))
+		goto no_timestamping;
+
 	fda->fd[FD_EVENT] = efd;
 	fda->fd[FD_GENERAL] = gfd;
 	return 0;
diff --git a/sk.c b/sk.c
index a13d14d..fe74164 100644
--- a/sk.c
+++ b/sk.c
@@ -36,6 +36,7 @@
 /* globals */
 
 int sk_tx_timeout = 1;
+int sk_check_fupsync;
 
 /* private methods */
 
@@ -91,6 +92,16 @@ int sk_interface_index(int fd, char *name)
 	return ifreq.ifr_ifindex;
 }
 
+int sk_general_init(int fd)
+{
+	int on = sk_check_fupsync ? 1 : 0;
+	if (setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPNS, &on, sizeof(on)) < 0) {
+		pr_err("ioctl SO_TIMESTAMPNS failed: %m");
+		return -1;
+	}
+	return 0;
+}
+
 int sk_get_ts_info(char *name, struct sk_ts_info *sk_info)
 {
 #ifdef ETHTOOL_GET_TS_INFO
@@ -205,7 +216,7 @@ int sk_receive(int fd, void *buf, int buflen,
 	struct cmsghdr *cm;
 	struct iovec iov = { buf, buflen };
 	struct msghdr msg;
-	struct timespec *ts = NULL;
+	struct timespec *sw, *ts = NULL;
 
 	memset(control, 0, sizeof(control));
 	memset(&msg, 0, sizeof(msg));
@@ -241,7 +252,14 @@ int sk_receive(int fd, void *buf, int buflen,
 				return -1;
 			}
 			ts = (struct timespec *) CMSG_DATA(cm);
-			break;
+		}
+		if (SOL_SOCKET == level && SO_TIMESTAMPNS == type) {
+			if (cm->cmsg_len < sizeof(*sw)) {
+				pr_warning("short SO_TIMESTAMPNS message");
+				return -1;
+			}
+			sw = (struct timespec *) CMSG_DATA(cm);
+			hwts->sw = *sw;
 		}
 	}
 
@@ -325,5 +343,10 @@ int sk_timestamping_init(int fd, char *device, enum timestamp_type type,
 		return -1;
 	}
 
+	/* Enable the sk_check_fupsync option, perhaps. */
+	if (sk_general_init(fd)) {
+		return -1;
+	}
+
 	return 0;
 }
diff --git a/sk.h b/sk.h
index 6070537..895840f 100644
--- a/sk.h
+++ b/sk.h
@@ -46,6 +46,13 @@ struct sk_ts_info {
  */
 int sk_interface_index(int fd, char *device);
 
+/**
+ * Prepare a given socket for PTP "general" messages.
+ * @param fd  An open socket.
+ * @return    Zero on success, non-zero otherwise.
+ */
+int sk_general_init(int fd);
+
 /**
  * Obtain supported timestamping information
  * @param name	    The name of the interface
@@ -102,4 +109,11 @@ int sk_timestamping_init(int fd, char *device, enum timestamp_type type,
  */
 extern int sk_tx_timeout;
 
+/**
+ * Enables the SO_TIMESTAMPNS socket option on the both the event and
+ * general sockets in order to test the order of paired sync and
+ * follow up messages using their network stack receipt time stamps.
+ */
+extern int sk_check_fupsync;
+
 #endif
diff --git a/transport.h b/transport.h
index 46af456..34934fe 100644
--- a/transport.h
+++ b/transport.h
@@ -57,6 +57,7 @@ enum timestamp_type {
 struct hw_timestamp {
 	enum timestamp_type type;
 	struct timespec ts;
+	struct timespec sw;
 };
 
 struct transport;
diff --git a/udp.c b/udp.c
index fea0122..be7f2b7 100644
--- a/udp.c
+++ b/udp.c
@@ -179,6 +179,9 @@ static int udp_open(struct transport *t, char *name, struct fdarray *fda,
 	if (sk_timestamping_init(efd, name, ts_type, TRANS_UDP_IPV4))
 		goto no_timestamping;
 
+	if (sk_general_init(gfd))
+		goto no_timestamping;
+
 	fda->fd[FD_EVENT] = efd;
 	fda->fd[FD_GENERAL] = gfd;
 	return 0;
diff --git a/udp6.c b/udp6.c
index 2cb0179..e0d1256 100644
--- a/udp6.c
+++ b/udp6.c
@@ -191,6 +191,9 @@ static int udp6_open(struct transport *t, char *name, struct fdarray *fda,
 	if (sk_timestamping_init(efd, name, ts_type, TRANS_UDP_IPV6))
 		goto no_timestamping;
 
+	if (sk_general_init(gfd))
+		goto no_timestamping;
+
 	fda->fd[FD_EVENT] = efd;
 	fda->fd[FD_GENERAL] = gfd;
 	return 0;