diff -up chrony-3.1/configure.timestamping chrony-3.1/configure

--- chrony-3.1/configure.timestamping	2017-01-31 11:22:11.000000000 +0100

+++ chrony-3.1/configure	2017-02-03 12:09:22.911633573 +0100

@@ -651,8 +651,8 @@ if [ $feat_timestamping = "1" ] && [ $tr

   test_code 'SW/HW timestamping' 'sys/types.h sys/socket.h linux/net_tstamp.h

                                   linux/errqueue.h linux/ptp_clock.h' '' '' '

     int val = SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_RX_SOFTWARE |

-              SOF_TIMESTAMPING_RAW_HARDWARE | SOF_TIMESTAMPING_OPT_CMSG;

-    return sizeof (struct scm_timestamping) + SCM_TSTAMP_SND + PTP_SYS_OFFSET +

+              SOF_TIMESTAMPING_RAW_HARDWARE | 1;

+    return 3 * sizeof (struct timespec) + 0 + PTP_SYS_OFFSET +

            setsockopt(0, SOL_SOCKET, SO_SELECT_ERR_QUEUE + SO_TIMESTAMPING,

                       &val, sizeof (val));'

 then

diff -up chrony-3.1/doc/chrony.conf.man.in.timestamping chrony-3.1/doc/chrony.conf.man.in

--- chrony-3.1/doc/chrony.conf.man.in.timestamping	2017-01-31 11:34:16.000000000 +0100

+++ chrony-3.1/doc/chrony.conf.man.in	2017-02-03 12:09:22.911633573 +0100

@@ -3065,7 +3065,7 @@ timestamping. If the server or peer supp

 be enabled by the \fBxleave\fP option in the \fBserver\fP or the

 \fBpeer\fP directive.

 .sp

-This directive is supported on Linux 3.19 and newer. The NIC must support HW

+This directive is supported on Linux. The NIC must support HW

 timestamping, which can be verified with the \fBethtool \-T\fP command. The list of

 capabilities should include \fISOF_TIMESTAMPING_RAW_HARDWARE\fP,

 \fISOF_TIMESTAMPING_TX_HARDWARE\fP, \fISOF_TIMESTAMPING_RX_HARDWARE\fP, and the filter

diff -up chrony-3.1/ntp_io_linux.c.timestamping chrony-3.1/ntp_io_linux.c

--- chrony-3.1/ntp_io_linux.c.timestamping	2017-01-31 11:22:11.000000000 +0100

+++ chrony-3.1/ntp_io_linux.c	2017-02-03 12:10:10.720767667 +0100

@@ -36,6 +36,10 @@

 #include <linux/sockios.h>

 #include <net/if.h>

+/* Missing in older kernel headers */

+#define SOF_TIMESTAMPING_OPT_CMSG (1<<10)

+#define SCM_TSTAMP_SND 0

+

 #include "array.h"

 #include "conf.h"

 #include "hwclock.h"

@@ -95,6 +99,10 @@ static int ts_tx_flags;

 /* Flag indicating the socket options can't be changed in control messages */

 static int permanent_ts_options;

+/* Index of a HW-timestamping interface, but only if the machine has not more

+   than one */

+static int single_hwts_if_index;

+

 /* ================================================== */

 static int

@@ -253,6 +261,84 @@ update_interface_speed(struct Interface

 /* ================================================== */

+static int

+check_sof_opt_cmsg()

+{

+  int sock_fd, flags;

+

+  sock_fd = socket(AF_INET, SOCK_DGRAM, 0);

+  if (sock_fd < 0)

+    return 0;

+

+  flags = SOF_TIMESTAMPING_OPT_CMSG;

+

+  if (setsockopt(sock_fd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof (flags)) < 0) {

+    DEBUG_LOG(LOGF_NtpIOLinux, "SOF_TIMESTAMPING_OPT_CMSG not supported");

+    close(sock_fd);

+    return 0;

+  }

+

+  close(sock_fd);

+  return 1;

+}

+

+/* ================================================== */

+

+static int

+get_single_hwts_index()

+{

+  struct ifaddrs *ifaddr, *ifa;

+  struct ethtool_ts_info ts_info;

+  struct ifreq req;

+  int sock_fd, if_index, hwts_if_index = INVALID_IF_INDEX;

+

+  sock_fd = socket(AF_INET, SOCK_DGRAM, 0);

+  if (sock_fd < 0)

+    return INVALID_IF_INDEX;

+

+  if (getifaddrs(&ifaddr)) {

+    DEBUG_LOG(LOGF_NtpIOLinux, "getifaddrs() failed : %s", strerror(errno));

+    close(sock_fd);

+    return INVALID_IF_INDEX;

+  }

+

+  for (ifa = ifaddr; ifa; ifa = ifa->ifa_next) {

+    memset(&req, 0, sizeof (req));

+    memset(&ts_info, 0, sizeof (ts_info));

+

+    if (snprintf(req.ifr_name, sizeof (req.ifr_name), "%s", ifa->ifa_name) >=

+        sizeof (req.ifr_name))

+      break;

+

+    if (ioctl(sock_fd, SIOCGIFINDEX, &req))

+      break;

+

+    if_index = req.ifr_ifindex;

+    ts_info.cmd = ETHTOOL_GET_TS_INFO;

+    req.ifr_data = (char *)&ts_info;

+

+    if (ioctl(sock_fd, SIOCETHTOOL, &req))

+      break;

+

+    if (ts_info.phc_index < 0)

+      continue;

+

+    if (hwts_if_index != INVALID_IF_INDEX && hwts_if_index != if_index)

+      break;

+

+    hwts_if_index = if_index;

+  }

+

+  close(sock_fd);

+  freeifaddrs(ifaddr);

+

+  if (ifa)

+    return INVALID_IF_INDEX;

+

+  return hwts_if_index;

+}

+

+/* ================================================== */

 void

 NIO_Linux_Initialise(void)

 {

@@ -289,8 +375,20 @@ NIO_Linux_Initialise(void)

     ts_tx_flags = SOF_TIMESTAMPING_TX_SOFTWARE;

   }

-  /* Enable IP_PKTINFO in messages looped back to the error queue */

-  ts_flags |= SOF_TIMESTAMPING_OPT_CMSG;

+  single_hwts_if_index = INVALID_IF_INDEX;

+

+  /* Enable IP_PKTINFO in messages looped back to the error queue if possible.

+     If not, HW timestamping of IPv4 packets can be supported only with one

+     interface capable of HW timestamping. */

+  if (check_sof_opt_cmsg()) {

+    ts_flags |= SOF_TIMESTAMPING_OPT_CMSG;

+  } else if (ARR_GetSize(interfaces) > 0) {

+    single_hwts_if_index = get_single_hwts_index();

+    if (single_hwts_if_index == INVALID_IF_INDEX)

+      LOG(LOGS_WARN, LOGF_NtpIOLinux, "Missing SOF_TIMESTAMPING_OPT_CMSG option for HW timestamping with multiple HW-timestamping interfaces");

+    else

+      LOG(LOGS_INFO, LOGF_NtpIOLinux, "Enabled single-interface HW-timestamping mode");

+  }

   /* Kernels before 4.7 ignore timestamping flags set in control messages */

   permanent_ts_options = !SYS_Linux_CheckKernelVersion(4, 7);

@@ -499,7 +597,9 @@ NIO_Linux_ProcessMessage(NTP_Remote_Addr

   for (cmsg = CMSG_FIRSTHDR(hdr); cmsg; cmsg = CMSG_NXTHDR(hdr, cmsg)) {

     if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_TIMESTAMPING) {

-      struct scm_timestamping ts3;

+      struct {

+        struct timespec ts[3];

+      } ts3;

       memcpy(&ts3, CMSG_DATA(cmsg), sizeof (ts3));

@@ -507,6 +607,10 @@ NIO_Linux_ProcessMessage(NTP_Remote_Addr

         LCL_CookTime(&ts3.ts[0], &local_ts->ts, &local_ts->err);

         local_ts->source = NTP_TS_KERNEL;

       } else if (!UTI_IsZeroTimespec(&ts3.ts[2])) {

+        if (local_addr->if_index == INVALID_IF_INDEX &&

+            single_hwts_if_index != INVALID_IF_INDEX)

+          local_addr->if_index = single_hwts_if_index;

+

         iface = get_interface(local_addr->if_index);

         if (iface) {

           process_hw_timestamp(iface, &ts3.ts[2], local_ts, !is_tx ? length : 0,