Use monotonic clock.

configure.ac

@@ -114,6 +114,10 @@ AC_CHECK_LIB([cap], [cap_set_proc], [],
   AS_IF([test "$host_os" = linux-gnu],
     AC_MSG_WARN([Capabilities support is strongly recommended for increased security.  See SECURITY for more information.])))
 
+AC_CHECK_FUNC([clock_gettime], [
+  AC_DEFINE([HAVE_CLOCK_GETTIME], [1], [Define if your system has clock_gettime])
+])
+
 # Enable ipinfo
 AC_ARG_WITH([ipinfo],
   [AS_HELP_STRING([--without-ipinfo], [Do not try to use ipinfo lookup at all])],

packet/probe_unix.c

@@ -566,8 +566,8 @@ void send_probe(
         return;
     }
 
-    if (gettimeofday(&probe->platform.departure_time, NULL)) {
-        error(EXIT_FAILURE, errno, "gettimeofday failure");
+    if (getmonotime(&probe->platform.departure_time)) {
+        error(EXIT_FAILURE, errno, "getmonotime failure");
     }
 
     // there might be an off-by-one in the number of tries here. 
@@ -667,8 +667,8 @@ void receive_probe(
     struct timeval now;
 
     if (timestamp == NULL) {
-        if (gettimeofday(&now, NULL)) {
-            error(EXIT_FAILURE, errno, "gettimeofday failure");
+        if (getmonotime(&now)) {
+            error(EXIT_FAILURE, errno, "getmonotime failure");
         }
 
         timestamp = &now;
@@ -726,8 +726,8 @@ void receive_replies_from_recv_socket(
            Get the time immediately after reading the packet to
            keep the timing as precise as we can.
          */
-        if (gettimeofday(&timestamp, NULL)) {
-            error(EXIT_FAILURE, errno, "gettimeofday failure");
+        if (getmonotime(&timestamp)) {
+            error(EXIT_FAILURE, errno, "getmonotime failure");
         }
 
         if (packet_length == -1) {
@@ -975,8 +975,8 @@ void check_probe_timeouts(
     struct probe_t *probe;
     struct probe_t *probe_safe_iter;
 
-    if (gettimeofday(&now, NULL)) {
-        error(EXIT_FAILURE, errno, "gettimeofday failure");
+    if (getmonotime(&now)) {
+        error(EXIT_FAILURE, errno, "getmonotime failure");
     }
 
     LIST_FOREACH_SAFE(probe, &net_state->outstanding_probes,
@@ -1008,8 +1008,8 @@ bool get_next_probe_timeout(
     struct timeval now;
     struct timeval probe_timeout;
 
-    if (gettimeofday(&now, NULL)) {
-        error(EXIT_FAILURE, errno, "gettimeofday failure");
+    if (getmonotime(&now)) {
+        error(EXIT_FAILURE, errno, "getmonotime failure");
     }
 
     have_timeout = false;

packet/timeval.c

@@ -16,8 +16,30 @@
     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
+#ifdef HAVE_CLOCK_GETTIME
+#include <time.h>
+#endif
+
 #include "timeval.h"
 
+/*
+    Return timeval using monotonic clock.
+*/
+int getmonotime(
+    struct timeval *tv)
+{
+#ifdef HAVE_CLOCK_GETTIME
+    struct timespec ts;
+    if (clock_gettime(CLOCK_MONOTONIC, &ts) != 0) {
+        return -1;
+    }
+    TIMESPEC_TO_TIMEVAL(tv, &ts);
+    return 0;
+#else
+    return gettimeofday(tv, NULL);
+#endif
+}
+
 /*
     Ensure that a timevalue has a microsecond value in the range
     [0.0, 1.0e6) microseconds by converting microseconds to full seconds.

packet/timeval.h

@@ -21,6 +21,9 @@
 
 #include <sys/time.h>
 
+int getmonotime(
+    struct timeval *timeval);
+
 void normalize_timeval(
     struct timeval *timeval);
 

ui/select.c

@@ -62,9 +62,15 @@ void select_loop(
     struct timeval intervaltime;
     static double dnsinterval = 0;
 
-    memset(&startgrace, 0, sizeof(startgrace));
-
+#ifdef HAVE_CLOCK_GETTIME
+    struct timespec ts;
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    TIMESPEC_TO_TIMEVAL(&lasttime, &ts);
+#else
     gettimeofday(&lasttime, NULL);
+#endif
+
+    memset(&startgrace, 0, sizeof(startgrace));
 
     while (1) {
         dt = calc_deltatime(ctl->WaitTime);
@@ -123,8 +129,12 @@ void select_loop(
                 if (ctl->Interactive)
                     display_redraw(ctl);
 
+#ifdef HAVE_CLOCK_GETTIME
+                clock_gettime(CLOCK_MONOTONIC, &ts);
+                TIMESPEC_TO_TIMEVAL(&thistime, &ts);
+#else
                 gettimeofday(&thistime, NULL);
-
+#endif
                 if (thistime.tv_sec > lasttime.tv_sec + intervaltime.tv_sec
                     || (thistime.tv_sec ==
                         lasttime.tv_sec + intervaltime.tv_sec