NTPv3/4 queries can request if NTPv5 is supported

Author: beevik

ntp.go

@@ -178,8 +178,12 @@ type QueryOptions struct {
 	RequestReferenceID ReferenceIDRequest
 
 	// RequestSupportedVersions indicates whether to request which versions of
-	// the NTP protocol are supported by the server in its response. Used only
-	// in NTPv5.
+	// the NTP protocol are supported by the server. When used with NTPv5, the
+	// response will list all supported versions. When used with NTPv3 or
+	// NTPv4, the response's supported versions will include the version used
+	// in the query as well as 5 if the server supports it. The response's
+	// ReferenceTime field will be invalid in NTPv3 and NTPv4 if this option
+	// is set.
 	RequestSupportedVersions bool
 
 	// RequestCorrection indicates whether to request delay corrections from
@@ -484,6 +488,7 @@ func (r *Response) Log(w io.Writer) {
 	fmt.Fprintf(w, "  RootDelay: %s\n", r.RootDelay)
 	fmt.Fprintf(w, "   RootDisp: %s\n", r.RootDispersion)
 	fmt.Fprintf(w, "   RootDist: %s\n", r.RootDistance)
+	fmt.Fprintf(w, "  Supported: %v\n", r.SupportedVersions)
 	fmt.Fprintf(w, "   MinError: %s\n", r.MinError)
 	fmt.Fprintf(w, "    RefTime: %s\n", fmtTime(r.ReferenceTime))
 	if r.Version == 5 {
@@ -492,7 +497,6 @@ func (r *Response) Log(w io.Writer) {
 		fmt.Fprintf(w, "    Offsets: %s\n", fmtTimescaleOffsets(r.TimescaleOffsets))
 		fmt.Fprintf(w, " MonoOffset: %s\n", r.MonotonicOffset)
 		fmt.Fprintf(w, "  MonoEpoch: %s\n", fmtEpoch(r.MonotonicEpochID))
-		fmt.Fprintf(w, "  Supported: %v\n", r.SupportedVersions)
 		fmt.Fprintf(w, "  SrvCookie: %s", fmtCookie(r.ServerCookie))
 	} else {
 		fmt.Fprintf(w, "      RefID: %s (0x%08x)\n", r.ReferenceString(), r.ReferenceID)
@@ -559,9 +563,9 @@ func (r *Response) Validate() error {
 		return ErrInvalidStratum
 	}
 
-	// Estimate the "freshness" of the time. If it exceeds the maximum
-	// polling interval (~36 hours), then it cannot be considered "fresh".
-	if r.Version < 5 || (r.Version == 5 && !r.ReferenceTime.IsZero()) {
+	// Estimate the "freshness" of the time. If it exceeds the maximum polling
+	// interval (~36 hours), then it cannot be considered "fresh".
+	if (r.Version < 5 && r.ReferenceTime != ntpEra0) || (r.Version == 5 && !r.ReferenceTime.IsZero()) {
 		if freshness := r.Time.Sub(r.ReferenceTime); freshness > maxPollInterval {
 			return ErrServerClockFreshness
 		}
@@ -1009,8 +1013,8 @@ func fmtFlags(flags ResponseFlags) string {
 }
 
 func fmtTime(value time.Time) string {
-	if value.IsZero() {
-		return "<zero>"
+	if value.Equal(ntpEra0) {
+		return "<invalid>"
 	}
 	return value.Format(timeFormat)
 }

ntp4.go

@@ -21,6 +21,13 @@ type modeV4 uint8
 const (
 	clientMode modeV4 = 3
 	serverMode modeV4 = 4
+
+	// Sentinel value sent in the ReferenceTime field to request whether the
+	// server supports NTPv5. If server responds with the same value, then it
+	// also supports NTPv5. This value is the ASCII representation of
+	// "NTP5DRFT". Once the NTPv5 protocol is finalized, the value will be
+	// changed to "NTP5NTP5".
+	v5sentinel = 0x4e54503544524654
 )
 
 // timeShortV4 is a 32-bit fixed-point (Q16.16) representation of the number
@@ -191,6 +198,18 @@ func queryV4(conn net.Conn, opt *QueryOptions) (*Response, error) {
 		Time:           m.TransmitTime.TimeV4(),
 	}
 
+	// If NTPv5 support was requested, check for it.
+	if opt.RequestSupportedVersions {
+		r.SupportedVersions = []int{opt.Version}
+
+		// If the server responded to the NTPv5 support request in its
+		// ReferenceTime field, add version 5 and clear the ReferenceTime.
+		if m.ReferenceTime == v5sentinel {
+			r.SupportedVersions = append(r.SupportedVersions, 5)
+			r.ReferenceTime = ntpEra0
+		}
+	}
+
 	// Calculate root distance.
 	r.RootDistance = rootDistance(r.RTT, r.RootDelay, r.RootDispersion)
 
@@ -221,6 +240,12 @@ func buildV4Request(opt *QueryOptions) (*bytes.Buffer, error) {
 	m.setMode(clientMode)
 	m.setLeap(LeapNoWarning)
 
+	// To request whether the server supports NTPv5, set the reference time to
+	// "NTP5DRFT". If the server supports it, it will echo this value back.
+	if opt.RequestSupportedVersions {
+		m.ReferenceTime = v5sentinel
+	}
+
 	// Write the message to a buffer.
 	buf := new(bytes.Buffer)
 	binary.Write(buf, binary.BigEndian, m.LiVnMode)

ntp4_test.go

@@ -23,8 +23,12 @@ func TestOnlineBadServerPort(t *testing.T) {
 	assert.NotNil(t, err)
 }
 
-func TestOnlineV3Query(t *testing.T) {
-	r, err := QueryWithOptions(host, QueryOptions{Version: 3})
+func TestOnlineV3QueryWithSupportedVersions(t *testing.T) {
+	opt := QueryOptions{
+		Version:                  3,
+		RequestSupportedVersions: true,
+	}
+	r, err := QueryWithOptions(host, opt)
 	if isError(t, host, err) {
 		return
 	}
@@ -37,7 +41,28 @@ func TestOnlineV3Query(t *testing.T) {
 }
 
 func TestOnlineV4Query(t *testing.T) {
-	r, err := QueryWithOptions(host, QueryOptions{Version: 4})
+	opt := QueryOptions{
+		Version:                  4,
+		RequestSupportedVersions: false,
+	}
+	r, err := QueryWithOptions(host, opt)
+	if isError(t, host, err) {
+		return
+	}
+	assertValid(t, r)
+
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "\n    Address: %s\n", host)
+	r.Log(&buf)
+	t.Log(buf.String())
+}
+
+func TestOnlineV4QueryWithSupportedVersions(t *testing.T) {
+	opt := QueryOptions{
+		Version:                  4,
+		RequestSupportedVersions: true,
+	}
+	r, err := QueryWithOptions(host, opt)
 	if isError(t, host, err) {
 		return
 	}