check local clock is accurate

main.go

@@ -13,6 +13,7 @@ import (
 	"time"
 	_ "time/tzdata"
 
+	"github.com/beevik/ntp"
 	"github.com/vmware/govmomi"
 	"github.com/vmware/govmomi/find"
 	"github.com/vmware/govmomi/view"
@@ -39,6 +40,9 @@ var (
 	hostTimeErrors []HostTimeErrors
 )
 
+// maxAllowedDrift is the maximum allowed time difference (1 second).
+const maxAllowedDrift = 2 * time.Second
+
 /*
 type dateInfo struct {
 	types.HostDateTimeInfo
@@ -47,6 +51,37 @@ type dateInfo struct {
 }
 */
 
+func checkClockDrift(ntpServers []string) bool {
+	localTime := time.Now()
+	inSync := false
+
+	for _, server := range ntpServers {
+		ntpTime, err := ntp.Time(server)
+		if err != nil {
+			log.Printf("Failed to get time from %s: %v\n", server, err)
+			continue
+		}
+
+		diff := localTime.Sub(ntpTime)
+		if diff < 0 {
+			diff = -diff
+		}
+
+		log.Printf("NTP Server: %s\n", server)
+		log.Printf("Local Time: %s\n", localTime)
+		log.Printf("NTP Time  : %s\n", ntpTime)
+		log.Printf("Time Difference: %v\n", diff)
+
+		if diff <= maxAllowedDrift {
+			inSync = true
+		}
+
+		log.Println("----------")
+	}
+
+	return inSync
+}
+
 func prettyPrint(args ...interface{}) {
 	var caller string
 
@@ -82,6 +117,7 @@ func main() {
 	vTZ := flag.String("tz", "Australia/Sydney", "The timezone to use when converting vCenter UTC times")
 	vInsecure := flag.Bool("insecure", true, "Allow insecure connections to vCenter")
 	vAllowedDiff := flag.Int("diff", 300, "Permitted time difference in seconds")
+	vNtpServers := flag.String("ntp-servers", "pool.ntp.org", "A comma separated list of NTP sources to validate the local system clock")
 
 	flag.Parse()
 
@@ -99,7 +135,7 @@ func main() {
 	log.Printf("Setting timezone to '%s'\n", *vTZ)
 	location, err = time.LoadLocation(*vTZ)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Error setting timezone to %s : %s\n", *vTZ, err)
+		log.Printf("Error setting timezone to %s : %s\n", *vTZ, err)
 		os.Exit(1)
 	}
 
@@ -114,6 +150,15 @@ func main() {
 		}
 	}
 
+	// Validate system clock
+	ntpList := strings.FieldsFunc(*vNtpServers, func(r rune) bool {
+		return r == ','
+	})
+	if !checkClockDrift(ntpList) {
+		log.Fatalf("Local system clock is not in sync, unable to proceed")
+		os.Exit(1)
+	}
+
 	log.Printf("Connecting to vCenter %s\n", u)
 	u.User = url.UserPassword(*vUser, *vPass)