Reference: https://src.fedoraproject.org/rpms/dhcp/blob/rawhide/f/0024-Detect-system-time-changes.patch
From a2fb8759ab48c88e3f8df94ae6e156c357d932a2 Mon Sep 17 00:00:00 2001
From: Pavel Zhukov <pzhukov@redhat.com>
Date: Tue, 22 Oct 2019 16:23:01 +0200
Subject: [PATCH 24/28] Detect system time changes

---
 client/dhclient.c |  6 ++++++
 common/dispatch.c | 11 ++++++++++-
 includes/dhcpd.h  |  3 ++-
 server/dhcpd.c    |  6 ++++++
 4 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/client/dhclient.c b/client/dhclient.c
index 60836b4..fd18813 100644
--- a/client/dhclient.c
+++ b/client/dhclient.c
@@ -5665,6 +5665,12 @@ isc_result_t dhcp_set_control_state (control_object_state_t oldstate,
 		  case server_awaken:
 		    state_reboot (client);
 		    break;
+
+                  case server_time_changed:
+                    if (client->active){
+                      state_reboot (client);
+                    }
+                    break;
 		}
 	    }
 	}
diff --git a/common/dispatch.c b/common/dispatch.c
index 9741ff5..11c1787 100644
--- a/common/dispatch.c
+++ b/common/dispatch.c
@@ -118,7 +118,6 @@ dispatch(void)
 		 * signal. It will return ISC_R_RELOAD in that
 		 * case. That is a normal behavior.
 		 */
-
 		if (status == ISC_R_RELOAD) {
 			/*
 			 * dhcp_set_control_state() will do the job.
@@ -129,6 +128,16 @@ dispatch(void)
 			if (status == ISC_R_SUCCESS)
 				status = ISC_R_RELOAD;
 		}
+
+                
+                if (status == ISC_R_TIMESHIFTED){
+                  status = dhcp_set_control_state(server_time_changed,
+                                                  server_time_changed);
+                  status = ISC_R_RELOAD;
+                  log_info ("System time has been changed. Unable to use existing leases. Restarting");
+                  // do nothing, restart context
+                };
+
 	} while (status == ISC_R_RELOAD);
 
 	log_fatal ("Dispatch routine failed: %s -- exiting",
diff --git a/includes/dhcpd.h b/includes/dhcpd.h
index fabad01..9663508 100644
--- a/includes/dhcpd.h
+++ b/includes/dhcpd.h
@@ -524,7 +524,8 @@ typedef enum {
 	server_running = 1,
 	server_shutdown = 2,
 	server_hibernate = 3,
-	server_awaken = 4
+	server_awaken = 4,
+        server_time_changed = 5
 } control_object_state_t;
 
 typedef struct {
diff --git a/server/dhcpd.c b/server/dhcpd.c
index 845d0cc..3b3bd3b 100644
--- a/server/dhcpd.c
+++ b/server/dhcpd.c
@@ -1767,6 +1767,12 @@ isc_result_t dhcp_set_control_state (control_object_state_t oldstate,
 {
 	struct timeval tv;
 
+        if (newstate == server_time_changed){
+          log_error ("System time has been changed. Leases information unreliable!");
+          return ISC_R_SUCCESS;
+        }
+
+                
 	if (newstate != server_shutdown)
 		return DHCP_R_INVALIDARG;
 	/* Re-entry. */
-- 
2.35.1