1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
|
.\"
.\" $FreeBSD$
.\"
.Dd December 21, 2006
.Dt NTP.CONF 5
.Os
.Sh NAME
.Nm ntp.conf
.Nd Network Time Protocol (NTP) daemon configuration file
.Sh SYNOPSIS
.Nm /etc/ntp.conf
.Sh DESCRIPTION
The
.Nm
configuration file is read at initial startup by the
.Xr ntpd 8
daemon in order to specify the synchronization sources,
modes and other related information.
Usually, it is installed in the
.Pa /etc
directory,
but could be installed elsewhere
(see the daemon's
.Fl c
command line option).
.Pp
The
.Pa /etc/rc.d/ntpdate
script reads this file to get a list of NTP servers to use if the
variable
.Dq Li ntpdate_hosts
was not declared.
Refer to the
.Xr rc.conf 5
man page for further info about this.
.Pp
The file format is similar to other
.Ux
configuration files.
Comments begin with a
.Ql #
character and extend to the end of the line;
blank lines are ignored.
Configuration commands consist of an initial keyword
followed by a list of arguments,
some of which may be optional, separated by whitespace.
Commands may not be continued over multiple lines.
Arguments may be host names,
host addresses written in numeric, dotted-quad form,
integers, floating point numbers (when specifying times in seconds)
and text strings.
.Pp
The rest of this page describes the configuration and control options.
The
.Qq Notes on Configuring NTP and Setting up a NTP Subnet
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
contains an extended discussion of these options.
In addition to the discussion of general
.Sx Configuration Options ,
there are sections describing the following supported functionality
and the options used to control it:
.Bl -bullet -offset indent
.It
.Sx Authentication Support
.It
.Sx Monitoring Support
.It
.Sx Access Control Support
.It
.Sx Automatic NTP Configuration Options
.It
.Sx Reference Clock Support
.It
.Sx Miscellaneous Options
.El
.Pp
Following these is a section describing
.Sx Miscellaneous Options .
While there is a rich set of options available,
the only required option is one or more
.Ic server ,
.Ic peer ,
.Ic broadcast
or
.Ic manycastclient
commands.
.Sh Configuration Support
Following is a description of the configuration commands in
NTPv4.
These commands have the same basic functions as in NTPv3 and
in some cases new functions and new arguments.
There are two
classes of commands, configuration commands that configure a
persistent association with a remote server or peer or reference
clock, and auxiliary commands that specify environmental variables
that control various related operations.
.Ss Configuration Commands
The various modes are determined by the command keyword and the
type of the required IP address.
Addresses are classed by type as
(s) a remote server or peer (IPv4 class A, B and C), (b) the
broadcast address of a local interface, (m) a multicast address (IPv4
class D), or (r) a reference clock address (127.127.x.x).
Note that
only those options applicable to each command are listed below.
Use
of options not listed may not be caught as an error, but may result
in some weird and even destructive behavior.
.Pp
If the Basic Socket Interface Extensions for IPv6 (RFC-2553)
is detected, support for the IPv6 address family is generated
in addition to the default support of the IPv4 address family.
In a few cases, including the reslist billboard generated
by ntpdc, IPv6 addresses are automatically generated.
IPv6 addresses can be identified by the presence of colons
.Dq \&:
in the address field.
IPv6 addresses can be used almost everywhere where
IPv4 addresses can be used,
with the exception of reference clock addresses,
which are always IPv4.
.Pp
Note that in contexts where a host name is expected, a
.Fl 4
qualifier preceding
the host name forces DNS resolution to the IPv4 namespace,
while a
.Fl 6
qualifier forces DNS resolution to the IPv6 namespace.
See IPv6 references for the
equivalent classes for that address family.
.Bl -tag -width indent
.It Xo Ic server Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm burst
.Op Cm iburst
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Xc
.It Xo Ic peer Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Xc
.It Xo Ic broadcast Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm ttl Ar ttl
.Xc
.It Xo Ic manycastclient Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Op Cm ttl Ar ttl
.Xc
.El
.Pp
These four commands specify the time server name or address to
be used and the mode in which to operate.
The
.Ar address
can be
either a DNS name or an IP address in dotted-quad notation.
Additional information on association behavior can be found in the
.Qq Association Management
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
.Bl -tag -width indent
.It Ic server
For type s and r addresses, this command mobilizes a persistent
client mode association with the specified remote server or local
radio clock.
In this mode the local clock can synchronized to the
remote server, but the remote server can never be synchronized to
the local clock.
This command should
.Em not
be used for type
b or m addresses.
.It Ic peer
For type s addresses (only), this command mobilizes a
persistent symmetric-active mode association with the specified
remote peer.
In this mode the local clock can be synchronized to
the remote peer or the remote peer can be synchronized to the local
clock.
This is useful in a network of servers where, depending on
various failure scenarios, either the local or remote peer may be
the better source of time.
This command should NOT be used for type
b, m or r addresses.
.It Ic broadcast
For type b and m addresses (only), this
command mobilizes a persistent broadcast mode association.
Multiple
commands can be used to specify multiple local broadcast interfaces
(subnets) and/or multiple multicast groups.
Note that local
broadcast messages go only to the interface associated with the
subnet specified, but multicast messages go to all interfaces.
In broadcast mode the local server sends periodic broadcast
messages to a client population at the
.Ar address
specified, which is usually the broadcast address on (one of) the
local network(s) or a multicast address assigned to NTP.
The IANA
has assigned the multicast group address IPv4 224.0.1.1 and
IPv6 ff05::101 (site local) exclusively to
NTP, but other nonconflicting addresses can be used to contain the
messages within administrative boundaries.
Ordinarily, this
specification applies only to the local server operating as a
sender; for operation as a broadcast client, see the
.Ic broadcastclient
or
.Ic multicastclient
commands
below.
.It Ic manycastclient
For type m addresses (only), this command mobilizes a
manycast client mode association for the multicast address
specified.
In this case a specific address must be supplied which
matches the address used on the
.Ic manycastserver
command for
the designated manycast servers.
The NTP multicast address
224.0.1.1 assigned by the IANA should NOT be used, unless specific
means are taken to avoid spraying large areas of the Internet with
these messages and causing a possibly massive implosion of replies
at the sender.
The
.Ic manycastserver
command specifies that the local server
is to operate in client mode with the remote servers that are
discovered as the result of broadcast/multicast messages.
The
client broadcasts a request message to the group address associated
with the specified
.Ar address
and specifically enabled
servers respond to these messages.
The client selects the servers
providing the best time and continues as with the
.Ic server
command.
The remaining servers are discarded as if never
heard.
.El
.Pp
Options:
.Bl -tag -width indent
.It Cm autokey
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey scheme
described in
.Sx Authentication Options .
.It Cm burst
when the server is reachable, send a burst of eight packets
instead of the usual one.
The packet spacing is normally 2 s;
however, the spacing between the first and second packets
can be changed with the calldelay command to allow
additional time for a modem or ISDN call to complete.
This is designed to improve timekeeping quality
with the
.Ic server
command and s addresses.
.It Cm iburst
When the server is unreachable, send a burst of eight packets
instead of the usual one.
The packet spacing is normally 2 s;
however, the spacing between the first two packets can be
changed with the calldelay command to allow
additional time for a modem or ISDN call to complete.
This is designed to speed the initial synchronization
acquisition with the
.Ic server
command and s addresses and when
.Xr ntpd 8
is started with the
.Fl q
option.
.It Cm key Ar key
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified
.Ar key
identifier with values from 1 to 65534, inclusive.
The
default is to include no encryption field.
.It Cm minpoll Ar minpoll
.It Cm maxpoll Ar maxpoll
These options specify the minimum and maximum poll intervals
for NTP messages, as a power of 2 in seconds
The maximum poll
interval defaults to 10 (1,024 s), but can be increased by the
.Cm maxpoll
option to an upper limit of 17 (36.4 h).
The
minimum poll interval defaults to 6 (64 s), but can be decreased by
the
.Cm minpoll
option to a lower limit of 4 (16 s).
.It Cm noselect
Marks the server as unused, except for display purposes.
The server is discarded by the selection algorithm.
.It Cm prefer
Marks the server as preferred.
All other things being equal,
this host will be chosen for synchronization among a set of
correctly operating hosts.
See the
.Qq Mitigation Rules and the prefer Keyword
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
for further information.
.It Cm ttl Ar ttl
This option is used only with broadcast server and manycast
client modes.
It specifies the time-to-live
.Ar ttl
to
use on broadcast server and multicast server and the maximum
.Ar ttl
for the expanding ring search with manycast
client packets.
Selection of the proper value, which defaults to
127, is something of a black art and should be coordinated with the
network administrator.
.It Cm version Ar version
Specifies the version number to be used for outgoing NTP
packets.
Versions 1-4 are the choices, with version 4 the
default.
.El
.Ss Auxiliary Commands
.Bl -tag -width indent
.It Ic broadcastclient
This command enables reception of broadcast server messages to
any local interface (type b) address.
Upon receiving a message for
the first time, the broadcast client measures the nominal server
propagation delay using a brief client/server exchange with the
server, then enters the broadcast client mode, in which it
synchronizes to succeeding broadcast messages.
Note that, in order
to avoid accidental or malicious disruption in this mode, both the
server and client should operate using symmetric-key or public-key
authentication as described in
.Sx Authentication Options .
.It Ic manycastserver Ar address ...
This command enables reception of manycast client messages to
the multicast group address(es) (type m) specified.
At least one
address is required, but the NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means are
taken to limit the span of the reply and avoid a possibly massive
implosion at the original sender.
Note that, in order to avoid
accidental or malicious disruption in this mode, both the server
and client should operate using symmetric-key or public-key
authentication as described in
.Sx Authentication Options .
.It Ic multicastclient Ar address ...
This command enables reception of multicast server messages to
the multicast group address(es) (type m) specified.
Upon receiving
a message for the first time, the multicast client measures the
nominal server propagation delay using a brief client/server
exchange with the server, then enters the broadcast client mode, in
which it synchronizes to succeeding multicast messages.
Note that,
in order to avoid accidental or malicious disruption in this mode,
both the server and client should operate using symmetric-key or
public-key authentication as described in
.Sx Authentication Options .
.El
.Sh Authentication Support
Authentication support allows the NTP client to verify that the
server is in fact known and trusted and not an intruder intending
accidentally or on purpose to masquerade as that server.
The NTPv3
specification RFC-1305 defines a scheme which provides
cryptographic authentication of received NTP packets.
Originally,
this was done using the Data Encryption Standard (DES) algorithm
operating in Cipher Block Chaining (CBC) mode, commonly called
DES-CBC.
Subsequently, this was replaced by the RSA Message Digest
5 (MD5) algorithm using a private key, commonly called keyed-MD5.
Either algorithm computes a message digest, or one-way hash, which
can be used to verify the server has the correct private key and
key identifier.
.Pp
NTPv4 retains the NTPv3 scheme, properly described as symmetric key
cryptography and, in addition, provides a new Autokey scheme
based on public key cryptography.
Public key cryptography is generally considered more secure
than symmetric key cryptography, since the security is based
on a private value which is generated by each server and
never revealed.
With Autokey all key distribution and
management functions involve only public values, which
considerably simplifies key distribution and storage.
Public key management is based on X.509 certificates,
which can be provided by commercial services or
produced by utility programs in the OpenSSL software library
or the NTPv4 distribution.
.Pp
While the algorithms for symmetric key cryptography are
included in the NTPv4 distribution, public key cryptography
requires the OpenSSL software library to be installed
before building the NTP distribution.
Directions for doing that
are on the Building and Installing the Distribution page.
.Pp
Authentication is configured separately for each association
using the
.Cm key
or
.Cm autokey
subcommand on the
.Ic peer ,
.Ic server ,
.Ic broadcast
and
.Ic manycastclient
configuration commands as described in
.Sx Configuration Options
page.
The authentication
options described below specify the locations of the key files,
if other than default, which symmetric keys are trusted
and the interval between various operations, if other than default.
.Pp
Authentication is always enabled,
although ineffective if not configured as
described below.
If a NTP packet arrives
including a message authentication
code (MAC), it is accepted only if it
passes all cryptographic checks.
The
checks require correct key ID, key value
and message digest.
If the packet has
been modified in any way or replayed
by an intruder, it will fail one or more
of these checks and be discarded.
Furthermore, the Autokey scheme requires a
preliminary protocol exchange to obtain
the server certificate, verify its
credentials and initialize the protocol
.Pp
The
.Cm auth
flag controls whether new associations or
remote configuration commands require cryptographic authentication.
This flag can be set or reset by the
.Ic enable
and
.Ic disable
commands and also by remote
configuration commands sent by a
.Xr ntpdc 8
program running in
another machine.
If this flag is enabled, which is the default
case, new broadcast client and symmetric passive associations and
remote configuration commands must be cryptographically
authenticated using either symmetric key or public key cryptography.
If this
flag is disabled, these operations are effective
even if not cryptographic
authenticated.
It should be understood
that operating with the
.Ic auth
flag disabled invites a significant vulnerability
where a rogue hacker can
masquerade as a falseticker and seriously
disrupt system timekeeping.
It is
important to note that this flag has no purpose
other than to allow or disallow
a new association in response to new broadcast
and symmetric active messages
and remote configuration commands and, in particular,
the flag has no effect on
the authentication process itself.
.Pp
An attractive alternative where multicast support is available
is manycast mode, in which clients periodically troll
for servers as described in the
.Sx Automatic NTP Configuration Options
page.
Either symmetric key or public key
cryptographic authentication can be used in this mode.
The principle advantage
of manycast mode is that potential servers need not be
configured in advance,
since the client finds them during regular operation,
and the configuration
files for all clients can be identical.
.Pp
The security model and protocol schemes for
both symmetric key and public key
cryptography are summarized below;
further details are in the briefings, papers
and reports at the NTP project page linked from
.Li http://www.ntp.org/ .
.Ss Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly
65,534 keys, each distinguished by a 32-bit key identifier, to
authenticate an association.
The servers and clients involved must
agree on the key and key identifier to
authenticate NTP packets.
Keys and
related information are specified in a key
file, usually called
.Pa ntp.keys ,
which must be distributed and stored using
secure means beyond the scope of the NTP protocol itself.
Besides the keys used
for ordinary NTP associations,
additional keys can be used as passwords for the
.Xr ntpq 8
and
.Xr ntpdc 8
utility programs.
.Pp
When
.Xr ntpd 8
is first started, it reads the key file specified in the
.Ic keys
configuration command and installs the keys
in the key cache.
However,
individual keys must be activated with the
.Ic trusted
command before use.
This
allows, for instance, the installation of possibly
several batches of keys and
then activating or deactivating each batch
remotely using
.Xr ntpdc 8 .
This also provides a revocation capability that can be used
if a key becomes compromised.
The
.Ic requestkey
command selects the key used as the password for the
.Xr ntpdc 8
utility, while the
.Ic controlkey
command selects the key used as the password for the
.Xr ntpq 8
utility.
.Ss Public Key Cryptography
NTPv4 supports the original NTPv3 symmetric key scheme
described in RFC-1305 and in addition the Autokey protocol,
which is based on public key cryptography.
The Autokey Version 2 protocol described on the Autokey Protocol
page verifies packet integrity using MD5 message digests
and verifies the source with digital signatures and any of several
digest/signature schemes.
Optional identity schemes described on the Identity Schemes
page and based on cryptographic challenge/response algorithms
are also available.
Using all of these schemes provides strong security against
replay with or without modification, spoofing, masquerade
and most forms of clogging attacks.
.\" .Pp
.\" The cryptographic means necessary for all Autokey operations
.\" is provided by the OpenSSL software library.
.\" This library is available from http://www.openssl.org/
.\" and can be installed using the procedures outlined
.\" in the Building and Installing the Distribution page.
.\" Once installed,
.\" the configure and build
.\" process automatically detects the library and links
.\" the library routines required.
.Pp
The Autokey protocol has several modes of operation
corresponding to the various NTP modes supported.
Most modes use a special cookie which can be
computed independently by the client and server,
but encrypted in transmission.
All modes use in addition a variant of the S-KEY scheme,
in which a pseudo-random key list is generated and used
in reverse order.
These schemes are described along with an executive summary,
current status, briefing slides and reading list on the
.Sx Autonomous Authentication
page.
.Pp
The specific cryptographic environment used by Autokey servers
and clients is determined by a set of files
and soft links generated by the
.Xr ntp-keygen 8
program.
This includes a required host key file,
required certificate file and optional sign key file,
leapsecond file and identity scheme files.
The
digest/signature scheme is specified in the X.509 certificate
along with the matching sign key.
There are several schemes
available in the OpenSSL software library, each identified
by a specific string such as
.Cm md5WithRSAEncryption ,
which stands for the MD5 message digest with RSA
encryption scheme.
The current NTP distribution supports
all the schemes in the OpenSSL library, including
those based on RSA and DSA digital signatures.
.Pp
NTP secure groups can be used to define cryptographic compartments
and security hierarchies.
It is important that every host
in the group be able to construct a certificate trail to one
or more trusted hosts in the same group.
Each group
host runs the Autokey protocol to obtain the certificates
for all hosts along the trail to one or more trusted hosts.
This requires the configuration file in all hosts to be
engineered so that, even under anticipated failure conditions,
the NTP subnet will form such that every group host can find
a trail to at least one trusted host.
.Ss Naming and Addressing
It is important to note that Autokey does not use DNS to
resolve addresses, since DNS can't be completely trusted
until the name servers have synchronized clocks.
The cryptographic name used by Autokey to bind the host identity
credentials and cryptographic values must be independent
of interface, network and any other naming convention.
The name appears in the host certificate in either or both
the subject and issuer fields, so protection against
DNS compromise is essential.
.Pp
By convention, the name of an Autokey host is the name returned
by the Unix
.Xr gethostname 2
system call or equivalent in other systems.
By the system design
model, there are no provisions to allow alternate names or aliases.
However, this is not to say that DNS aliases, different names
for each interface, etc., are constrained in any way.
.Pp
It is also important to note that Autokey verifies authenticity
using the host name, network address and public keys,
all of which are bound together by the protocol specifically
to deflect masquerade attacks.
For this reason Autokey
includes the source and destinatino IP addresses in message digest
computations and so the same addresses must be available
at both the server and client.
For this reason operation
with network address translation schemes is not possible.
This reflects the intended robust security model where government
and corporate NTP servers are operated outside firewall perimeters.
.Ss Operation
A specific combination of authentication scheme (none,
symmetric key, public key) and identity scheme is called
a cryptotype, although not all combinations are compatible.
There may be management configurations where the clients,
servers and peers may not all support the same cryptotypes.
A secure NTPv4 subnet can be configured in many ways while
keeping in mind the principles explained above and
in this section.
Note however that some cryptotype
combinations may successfully interoperate with each other,
but may not represent good security practice.
.Pp
The cryptotype of an association is determined at the time
of mobilization, either at configuration time or some time
later when a message of appropriate cryptotype arrives.
When mobilized by a
.Ic server
or
.Ic peer
configuration command and no
.Ic key
or
.Ic autokey
subcommands are present, the association is not
authenticated; if the
.Ic key
subcommand is present, the association is authenticated
using the symmetric key ID specified; if the
.Ic autokey
subcommand is present, the association is authenticated
using Autokey.
.Pp
When multiple identity schemes are supported in the Autokey
protocol, the first message exchange determines which one is used.
The client request message contains bits corresponding
to which schemes it has available.
The server response message
contains bits corresponding to which schemes it has available.
Both server and client match the received bits with their own
and select a common scheme.
.Pp
Following the principle that time is a public value,
a server responds to any client packet that matches
its cryptotype capabilities.
Thus, a server receiving
an unauthenticated packet will respond with an unauthenticated
packet, while the same server receiving a packet of a cryptotype
it supports will respond with packets of that cryptotype.
However, unconfigured broadcast or manycast client
associations or symmetric passive associations will not be
mobilized unless the server supports a cryptotype compatible
with the first packet received.
By default, unauthenticated associations will not be mobilized
unless overridden in a decidedly dangerous way.
.Pp
Some examples may help to reduce confusion.
Client Alice has no specific cryptotype selected.
Server Bob has both a symmetric key file and minimal Autokey files.
Alice's unauthenticated messages arrive at Bob, who replies with
unauthenticated messages.
Cathy has a copy of Bob's symmetric
key file and has selected key ID 4 in messages to Bob.
Bob verifies the message with his key ID 4.
If it's the
same key and the message is verified, Bob sends Cathy a reply
authenticated with that key.
If verification fails,
Bob sends Cathy a thing called a crypto-NAK, which tells her
something broke.
She can see the evidence using the ntpq program.
.Pp
Denise has rolled her own host key and certificate.
She also uses one of the identity schemes as Bob.
She sends the first Autokey message to Bob and they
both dance the protocol authentication and identity steps.
If all comes out okay, Denise and Bob continue as described above.
.Pp
It should be clear from the above that Bob can support
all the girls at the same time, as long as he has compatible
authentication and identity credentials.
Now, Bob can act just like the girls in his own choice of servers;
he can run multiple configured associations with multiple different
servers (or the same server, although that might not be useful).
But, wise security policy might preclude some cryptotype
combinations; for instance, running an identity scheme
with one server and no authentication with another might not be wise.
.Ss Key Management
The cryptographic values used by the Autokey protocol are
incorporated as a set of files generated by the
.Xr ntp-keygen 8
utility program, including symmetric key, host key and
public certificate files, as well as sign key, identity parameters
and leapseconds files.
Alternatively, host and sign keys and
certificate files can be generated by the OpenSSL utilities
and certificates can be imported from public certificate
authorities.
Note that symmetric keys are necessary for the
.Xr ntpq 8
and
.Xr ntpdc 8
utility programs.
The remaining files are necessary only for the
Autokey protocol.
.Pp
Certificates imported from OpenSSL or public certificate
authorities have certian limitations.
The certificate should be in ASN.1 syntax, X.509 Version 3
format and encoded in PEM, which is the same format
used by OpenSSL.
The overall length of the certificate encoded
in ASN.1 must not exceed 1024 bytes.
The subject distinguished
name field (CN) is the fully qualified name of the host
on which it is used; the remaining subject fields are ignored.
The certificate extension fields must not contain either
a subject key identifier or a issuer key identifier field;
however, an extended key usage field for a trusted host must
contain the value
.Cm trustRoot ; .
Other extension fields are ignored.
.Ss Authentication Commands
.Bl -tag -width indent
.It Ic autokey Op Ar logsec
Specifies the interval between regenerations of the session key
list used with the Autokey protocol.
Note that the size of the key
list for each association depends on this interval and the current
poll interval.
The default value is 12 (4096 s or about 1.1 hours).
For poll intervals above the specified interval, a session key list
with a single entry will be regenerated for every message
sent.
.It Ic controlkey Ar key
Specifies the key identifier to use with the
.Xr ntpq 8
utility, which uses the standard
protocol defined in RFC-1305.
The
.Ar key
argument is
the key identifier for a trusted key, where the value can be in the
range 1 to 65,534, inclusive.
.It Xo Ic crypto
.Op Cm cert Ar file
.Op Cm leap Ar file
.Op Cm randfile Ar file
.Op Cm host Ar file
.Op Cm sign Ar file
.Op Cm gq Ar file
.Op Cm gqpar Ar file
.Op Cm iffpar Ar file
.Op Cm mvpar Ar file
.Op Cm pw Ar password
.Xc
This command requires the OpenSSL library.
It activates public key
cryptography, selects the message digest and signature
encryption scheme and loads the required private and public
values described above.
If one or more files are left unspecified,
the default names are used as described above.
Unless the complete path and name of the file are specified, the
location of a file is relative to the keys directory specified
in the
.Ic keysdir
command or default
.Pa /usr/local/etc .
Following are the subcommands:
.Bl -tag -width indent
.It Cm cert Ar file
Specifies the location of the required host public certificate file.
This overrides the link
.Pa ntpkey_cert_ Ns Ar hostname
in the keys directory.
.It Cm gqpar Ar file
Specifies the location of the optional GQ parameters file.
This
overrides the link
.Pa ntpkey_gq_ Ns Ar hostname
in the keys directory.
.It Cm host Ar file
Specifies the location of the required host key file.
This overrides
the link
.Pa ntpkey_key_ Ns Ar hostname
in the keys directory.
.It Cm iffpar Ar file
Specifies the location of the optional IFF parameters file.This
overrides the link
.Pa ntpkey_iff_ Ns Ar hostname
in the keys directory.
.It Cm leap Ar file
Specifies the location of the optional leapsecond file.
This overrides the link
.Pa ntpkey_leap
in the keys directory.
.It Cm mvpar Ar file
Specifies the location of the optional MV parameters file.
This
overrides the link
.Pa ntpkey_mv_ Ns Ar hostname
in the keys directory.
.It Cm pw Ar password
Specifies the password to decrypt files containing private keys and
identity parameters.
This is required only if these files have been
encrypted.
.It Cm randfile Ar file
Specifies the location of the random seed file used by the OpenSSL
library.
The defaults are described in the main text above.
.It Cm sign Ar file
Specifies the location of the optional sign key file.
This overrides
the link
.Pa ntpkey_sign_ Ns Ar hostname
in the keys directory.
If this file is
not found, the host key is also the sign key.
.El
.It Ic keys Ar keyfile
Specifies the complete path and location of the MD5 key file
containing the keys and key identifiers used by
.Xr ntpd 8 ,
.Xr ntpq 8
and
.Xr ntpdc
when operating with symmetric key cryptography.
This is the same operation as the
.Fl k
command line option.
.It Ic keysdir Ar path
This command specifies the default directory path for
cryptographic keys, parameters and certificates.
The default is
.Pa /usr/local/etc/ .
.It Ic requestkey Ar key
Specifies the key identifier to use with the
.Xr ntpdc 8
utility program, which uses a
proprietary protocol specific to this implementation of
.Xr ntpd 8 .
The
.Ar key
argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,534, inclusive.
.It Ic revoke Ar logsec
Specifies the interval between re-randomization of certain
cryptographic values used by the Autokey scheme, as a power of 2 in
seconds.
These values need to be updated frequently in order to
deflect brute-force attacks on the algorithms of the scheme;
however, updating some values is a relatively expensive operation.
The default interval is 16 (65,536 s or about 18 hours).
For poll
intervals above the specified interval, the values will be updated
for every message sent.
.It Ic trustedkey Ar key ...
Specifies the key identifiers which are trusted for the
purposes of authenticating peers with symmetric key cryptography,
as well as keys used by the
.Xr ntpq 8
and
.Xr ntpdc 8
programs.
The authentication procedures require that both the local
and remote servers share the same key and key identifier for this
purpose, although different keys can be used with different
servers.
The
.Ar key
arguments are 32-bit unsigned
integers with values from 1 to 65,534.
.El
.Ss Error Codes
The following error codes are reported via the NTP control
and monitoring protocol trap mechanism.
.Bl -tag -width indent
.It 101
.Pq bad field format or length
The packet has invalid version, length or format.
.It 102
.Pq bad timestamp
The packet timestamp is the same or older than the most recent received.
This could be due to a replay or a server clock time step.
.It 103
.Pq bad filestamp
The packet filestamp is the same or older than the most recent received.
This could be due to a replay or a key file generation error.
.It 104
.Pq bad or missing public key
The public key is missing, has incorrect format or is an unsupported type.
.It 105
.Pq unsupported digest type
The server requires an unsupported digest/signature scheme.
.It 106
.Pq mismatched digest types
Not used.
.It 107
.Pq bad signature length
The signature length does not match the current public key.
.It 108
.Pq signature not verified
The message fails the signature check.
It could be bogus or signed by a
different private key.
.It 109
.Pq certificate not verified
The certificate is invalid or signed with the wrong key.
.It 110
.Pq certificate not verified
The certificate is not yet valid or has expired or the signature could not
be verified.
.It 111
.Pq bad or missing cookie
The cookie is missing, corrupted or bogus.
.It 112
.Pq bad or missing leapseconds table
The leapseconds table is missing, corrupted or bogus.
.It 113
.Pq bad or missing certificate
The certificate is missing, corrupted or bogus.
.It 114
.Pq bad or missing identity
The identity key is missing, corrupt or bogus.
.El
.Sh Monitoring Support
.Xr ntpd 8
includes a comprehensive monitoring facility suitable
for continuous, long term recording of server and client
timekeeping performance.
See the
.Ic statistics
command below
for a listing and example of each type of statistics currently
supported.
Statistic files are managed using file generation sets
and scripts in the
.Pa ./scripts
directory of this distribution.
Using
these facilities and
.Ux
.Xr cron 8
jobs, the data can be
automatically summarized and archived for retrospective analysis.
.Ss Monitoring Commands
.Bl -tag -width indent
.It Ic statistics Ar name ...
Enables writing of statistics records.
Currently, four kinds of
.Ar name
statistics are supported.
.Bl -tag -width indent
.It Cm clockstats
Enables recording of clock driver statistics information.
Each update
received from a clock driver appends a line of the following form to
the file generation set named
.Cm clockstats :
.Bd -literal
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and time
(seconds and fraction past UTC midnight).
The next field shows the
clock address in dotted-quad notation.
The final field shows the last
timecode received from the clock in decoded ASCII format, where
meaningful.
In some clock drivers a good deal of additional information
can be gathered and displayed as well.
See information specific to each
clock for further details.
.It Cm cryptostats
This option requires the OpenSSL cryptographic software library.
It
enables recording of cryptographic public key protocol information.
Each message received by the protocol module appends a line of the
following form to the file generation set named
.Cm cryptostats :
.Bd -literal
49213 525.624 127.127.4.1 message
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and time
(seconds and fraction past UTC midnight).
The next field shows the peer
address in dotted-quad notation, The final message field includes the
message type and certain ancillary information.
See the
.Sx Authentication Options
section for further information.
.It Cm loopstats
Enables recording of loop filter statistics information.
Each
update of the local clock outputs a line of the following form to
the file generation set named
.Cm loopstats :
.Bd -literal
50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and
time (seconds and fraction past UTC midnight).
The next five fields
show time offset (seconds), frequency offset (parts per million -
PPM), RMS jitter (seconds), Allan deviation (PPM) and clock
discipline time constant.
.It Cm peerstats
Enables recording of peer statistics information.
This includes
statistics records of all peers of a NTP server and of special
signals, where present and configured.
Each valid update appends a
line of the following form to the current element of a file
generation set named
.Cm peerstats :
.Bd -literal
48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and
time (seconds and fraction past UTC midnight).
The next two fields
show the peer address in dotted-quad notation and status,
respectively.
The status field is encoded in hex in the format
described in Appendix A of the NTP specification RFC 1305.
The final four fields show the offset,
delay, dispersion and RMS jitter, all in seconds.
.It Cm rawstats
Enables recording of raw-timestamp statistics information.
This
includes statistics records of all peers of a NTP server and of
special signals, where present and configured.
Each NTP message
received from a peer or clock driver appends a line of the
following form to the file generation set named
.Cm rawstats :
.Bd -literal
50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and
time (seconds and fraction past UTC midnight).
The next two fields
show the remote peer or clock address followed by the local address
in dotted-quad notation.
The final four fields show the originate,
receive, transmit and final NTP timestamps in order.
The timestamp
values are as received and before processing by the various data
smoothing and mitigation algorithms.
.It Cm sysstats
Enables recording of ntpd statistics counters on a periodic basis.
Each
hour a line of the following form is appended to the file generation
set named
.Cm sysstats :
.Bd -literal
50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147
.Ed
.Pp
The first two fields show the date (Modified Julian Day) and time
(seconds and fraction past UTC midnight).
The remaining ten fields show
the statistics counter values accumulated since the last generated
line.
.Bl -tag -width indent
.It Time since restart Cm 36000
Time in hours since the system was last rebooted.
.It Packets received Cm 81965
Total number of packets received.
.It Packets processed Cm 0
Number of packets received in response to previous packets sent
.It Current version Cm 9546
Number of packets matching the current NTP version.
.It Previous version Cm 56
Number of packets matching the previous NTP version.
.It Bad version Cm 71793
Number of packets matching neither NTP version.
.It Access denied Cm 512
Number of packets denied access for any reason.
.It Bad length or format Cm 540
Number of packets with invalid length, format or port number.
.It Bad authentication Cm 10
Number of packets not verified as authentic.
.It Rate exceeded Cm 147
Number of packets discarded due to rate limitation.
.El
.It Cm statsdir Ar directory_path
Indicates the full path of a directory where statistics files
should be created (see below).
This keyword allows
the (otherwise constant)
.Cm filegen
filename prefix to be modified for file generation sets, which
is useful for handling statistics logs.
.It Cm filegen Ar name Xo
.Op Cm file Ar filename
.Op Cm type Ar typename
.Op Cm link | nolink
.Op Cm enable | disable
.Xc
Configures setting of generation file set name.
Generation
file sets provide a means for handling files that are
continuously growing during the lifetime of a server.
Server statistics are a typical example for such files.
Generation file sets provide access to a set of files used
to store the actual data.
At any time at most one element
of the set is being written to.
The type given specifies
when and how data will be directed to a new element of the set.
This way, information stored in elements of a file set
that are currently unused are available for administrational
operations without the risk of disturbing the operation of ntpd.
(Most important: they can be removed to free space for new data
produced.)
.Pp
Note that this command can be sent from the
.Xr ntpdc 8
program running at a remote location.
.Bl -tag -width indent
.It Cm name
This is the type of the statistics records, as shown in the
.Cm statistics
command.
.It Cm file Ar filename
This is the file name for the statistics records.
Filenames of set
members are built from three concatenated elements
.Ar Cm prefix ,
.Ar Cm filename
and
.Ar Cm suffix :
.Bl -tag -width indent
.It Cm prefix
This is a constant filename path.
It is not subject to
modifications via the
.Ar filegen
option.
It is defined by the
server, usually specified as a compile-time constant.
It may,
however, be configurable for individual file generation sets
via other commands.
For example, the prefix used with
.Ar loopstats
and
.Ar peerstats
generation can be configured using the
.Ar statsdir
option explained above.
.It Cm filename
This string is directly concatenated to the prefix mentioned
above (no intervening
.Ql / ) .
This can be modified using
the file argument to the
.Ar filegen
statement.
No
.Pa ..
elements are
allowed in this component to prevent filenames referring to
parts outside the filesystem hierarchy denoted by
.Ar prefix .
.It Cm suffix
This part is reflects individual elements of a file set.
It is
generated according to the type of a file set.
.El
.It Cm type Ar typename
A file generation set is characterized by its type.
The following
types are supported:
.Bl -tag -width indent
.It Cm none
The file set is actually a single plain file.
.It Cm pid
One element of file set is used per incarnation of a ntpd
server.
This type does not perform any changes to file set
members during runtime, however it provides an easy way of
separating files belonging to different
.Xr ntpd 8
server incarnations.
The set member filename is built by appending a
.Ql \&.
to concatenated
.Ar prefix
and
.Ar filename
strings, and
appending the decimal representation of the process ID of the
.Xr ntpd 8
server process.
.It Cm day
One file generation set element is created per day.
A day is
defined as the period between 00:00 and 24:00 UTC.
The file set
member suffix consists of a
.Ql \&.
and a day specification in
the form
.Cm YYYYMMdd .
.Cm YYYY
is a 4-digit year number (e.g., 1992).
.Cm MM
is a two digit month number.
.Cm dd
is a two digit day number.
Thus, all information written at 10 December 1992 would end up
in a file named
.Ar prefix
.Ar filename Ns .19921210 .
.It Cm week
Any file set member contains data related to a certain week of
a year.
The term week is defined by computing day-of-year
modulo 7.
Elements of such a file generation set are
distinguished by appending the following suffix to the file set
filename base: A dot, a 4-digit year number, the letter
.Cm W ,
and a 2-digit week number.
For example, information from January,
10th 1992 would end up in a file with suffix
.No . Ns Ar 1992W1 .
.It Cm month
One generation file set element is generated per month.
The
file name suffix consists of a dot, a 4-digit year number, and
a 2-digit month.
.It Cm year
One generation file element is generated per year.
The filename
suffix consists of a dot and a 4 digit year number.
.It Cm age
This type of file generation sets changes to a new element of
the file set every 24 hours of server operation.
The filename
suffix consists of a dot, the letter
.Cm a ,
and an 8-digit number.
This number is taken to be the number of seconds the server is
running at the start of the corresponding 24-hour period.
Information is only written to a file generation by specifying
.Cm enable ;
output is prevented by specifying
.Cm disable .
.El
.It Cm link | nolink
It is convenient to be able to access the current element of a file
generation set by a fixed name.
This feature is enabled by
specifying
.Cm link
and disabled using
.Cm nolink .
If link is specified, a
hard link from the current file set element to a file without
suffix is created.
When there is already a file with this name and
the number of links of this file is one, it is renamed appending a
dot, the letter
.Cm C ,
and the pid of the ntpd server process.
When the
number of links is greater than one, the file is unlinked.
This
allows the current file to be accessed by a constant name.
.It Cm enable \&| Cm disable
Enables or disables the recording function.
.El
.El
.El
.Sh Access Control Support
The
.Xr ntpd 8
daemon implements a general purpose address/mask based restriction
list.
The list contains address/match entries sorted first
by increasing address values and then by increasing mask values.
A match occurs when the bitwise AND of the mask and the packet
source address is equal to the bitwise AND of the mask and
address in the list.
The list is searched in order with the
last match found defining the restriction flags associated
with the entry.
Additional information and examples can be found in the
.Qq Notes on Configuring NTP and Setting up a NTP Subnet
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
.Pp
The restriction facility was implemented in conformance
with the access policies for the original NSFnet backbone
time servers.
Later the facility was expanded to deflect
cryptographic and clogging attacks.
While this facility may
be useful for keeping unwanted or broken or malicious clients
from congesting innocent servers, it should not be considered
an alternative to the NTP authentication facilities.
Source address based restrictions are easily circumvented
by a determined cracker.
.Pp
Clients can be denied service because they are explicitly
included in the restrict list created by the restrict command
or implicitly as the result of cryptographic or rate limit
violations.
Cryptographic violations include certificate
or identity verification failure; rate limit violations generally
result from defective NTP implementations that send packets
at abusive rates.
Some violations cause denied service
only for the offending packet, others cause denied service
for a timed period and others cause the denied service for
an indefinite period.
When a client or network is denied access
for an indefinite period, the only way at present to remove
the restrictions is by restarting the server.
.Ss The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no
further action except incrementing statistics counters.
Sometimes a
more proactive response is needed, such as a server message that
explicitly requests the client to stop sending and leave a message
for the system operator.
A special packet format has been created
for this purpose called the "kiss-of-death" (KoD) packet.
KoD packets have the leap bits set unsynchronized and stratum set
to zero and the reference identifier field set to a four-byte
ASCII code.
If the
.Cm noserve
or
.Cm notrust
flag of the matching restrict list entry is set,
the code is "DENY"; if the
.Cm limited
flag is set and the rate limit
is exceeded, the code is "RATE".
Finally, if a cryptographic violation occurs, the code is "CRYP".
.Pp
A client receiving a KoD performs a set of sanity checks to
minimize security exposure, then updates the stratum and
reference identifier peer variables, sets the access
denied (TEST4) bit in the peer flash variable and sends
a message to the log.
As long as the TEST4 bit is set,
the client will send no further packets to the server.
The only way at present to recover from this condition is
to restart the protocol at both the client and server.
This
happens automatically at the client when the association times out.
It will happen at the server only if the server operator cooperates.
.Ss Access Control Commands
.Bl -tag -width indent
.It Xo Ic discard
.Op Cm average Ar avg
.Op Cm minimum Ar min
.Op Cm monitor Ar prob
.Xc
Set the parameters of the
.Cm limited
facility which protects the server from
client abuse.
The
.Cm average
subcommand specifies the minimum average packet
spacing, while the
.Cm minimum
subcommand specifies the minimum packet spacing.
Packets that violate these minima are discarded
and a kiss-o'-death packet returned if enabled.
The default
minimum average and minimum are 5 and 2, respectively.
The monitor subcommand specifies the probability of discard
for packets that overflow the rate-control window.
.It Xo Ic restrict address
.Op Cm mask Ar mask
.Op Ar flag ...
.Xc
The
.Ar address
argument expressed in
dotted-quad form is the address of a host or network.
Alternatively, the
.Ar address
argument can be a valid host DNS name.
The
.Ar mask
argument expressed in dotted-quad form defaults to
.Cm 255.255.255.255 ,
meaning that the
.Ar address
is treated as the address of an individual host.
A default entry (address
.Cm 0.0.0.0 ,
mask
.Cm 0.0.0.0 )
is always included and is always the first entry in the list.
Note that text string
.Cm default ,
with no mask option, may
be used to indicate the default entry.
In the current implementation,
.Cm flag
always
restricts access, i.e., an entry with no flags indicates that free
access to the server is to be given.
The flags are not orthogonal,
in that more restrictive flags will often make less restrictive
ones redundant.
The flags can generally be classed into two
categories, those which restrict time service and those which
restrict informational queries and attempts to do run-time
reconfiguration of the server.
One or more of the following flags
may be specified:
.Bl -tag -width indent
.It Cm ignore
Deny packets of all kinds, including
.Xr ntpq 8
and
.Xr ntpdc 8
queries.
.It Cm kod
If this flag is set when an access violation occurs, a kiss-o'-death
(KoD) packet is sent.
KoD packets are rate limited to no more than one
per second.
If another KoD packet occurs within one second after the
last one, the packet is dropped.
.It Cm limited
Deny service if the packet spacing violates the lower limits specified
in the discard command.
A history of clients is kept using the
monitoring capability of
.Xr ntpd 8 .
Thus, monitoring is always active as
long as there is a restriction entry with the
.Cm limited
flag.
.It Cm lowpriotrap
Declare traps set by matching hosts to be low priority.
The
number of traps a server can maintain is limited (the current limit
is 3).
Traps are usually assigned on a first come, first served
basis, with later trap requestors being denied service.
This flag
modifies the assignment algorithm by allowing low priority traps to
be overridden by later requests for normal priority traps.
.It Cm nomodify
Deny
.Xr ntpq 8
and
.Xr ntpdc 8
queries which attempt to modify the state of the
server (i.e., run time reconfiguration).
Queries which return
information are permitted.
.It Cm noquery
Deny
.Xr ntpq 8
and
.Xr ntpdc 8
queries.
Time service is not affected.
.It Cm nopeer
Deny packets which would result in mobilizing a new association.
This
includes broadcast and symmetric active packets when a configured
association does not exist.
.It Cm noserve
Deny all packets except
.Xr ntpq 8
and
.Xr ntpdc 8
queries.
.It Cm notrap
Decline to provide mode 6 control message trap service to matching
hosts.
The trap service is a subsystem of the ntpdq control message
protocol which is intended for use by remote event logging programs.
.It Cm notrust
Deny service unless the packet is cryptographically authenticated.
.It Cm ntpport
This is actually a match algorithm modifier, rather than a
restriction flag.
Its presence causes the restriction entry to be
matched only if the source port in the packet is the standard NTP
UDP port (123).
Both
.Cm ntpport
and
.Cm non-ntpport
may
be specified.
The
.Cm ntpport
is considered more specific and
is sorted later in the list.
.It Cm version
Deny packets that do not match the current NTP version.
.El
.Pp
Default restriction list entries with the flags ignore, interface,
ntpport, for each of the local host's interface addresses are
inserted into the table at startup to prevent the server
from attempting to synchronize to its own time.
A default entry is also always present, though if it is
otherwise unconfigured; no flags are associated
with the default entry (i.e., everything besides your own
NTP server is unrestricted).
.El
.Sh Automatic NTP Configuration Options
.Ss Manycasting
Manycasting is a automatic discovery and configuration paradigm
new to NTPv4.
It is intended as a means for a multicast client
to troll the nearby network neighborhood to find cooperating
manycast servers, validate them using cryptographic means
and evaluate their time values with respect to other servers
that might be lurking in the vicinity.
The intended result is that each manycast client mobilizes
client associations with some number of the "best"
of the nearby manycast servers, yet automatically reconfigures
to sustain this number of servers should one or another fail.
.Pp
Note that the manycasting paradigm does not coincide
with the anycast paradigm described in RFC-1546,
which is designed to find a single server from a clique
of servers providing the same service.
The manycast paradigm is designed to find a plurality
of redundant servers satisfying defined optimality criteria.
.Pp
Manycasting can be used with either symmetric key
or public key cryptography.
The public key infrastructure (PKI)
offers the best protection against compromised keys
and is generally considered stronger, at least with relatively
large key sizes.
It is implemented using the Autokey protocol and
the OpenSSL cryptographic library available from
.Li http://www.openssl.org/ .
The library can also be used with other NTPv4 modes
as well and is highly recommended, especially for broadcast modes.
.Pp
A persistent manycast client association is configured
using the manycastclient command, which is similar to the
server command but with a multicast (IPv4 class
.Cm D
or IPv6 prefix
.Cm FF )
group address.
The IANA has designated IPv4 address 224.1.1.1
and IPv6 address FF05::101 (site local) for NTP.
When more servers are needed, it broadcasts manycast
client messages to this address at the minimum feasible rate
and minimum feasible time-to-live (TTL) hops, depending
on how many servers have already been found.
There can be as many manycast client associations
as different group address, each one serving as a template
for a future ephemeral unicast client/server association.
.Pp
Manycast servers configured with the
.Ic manycastserver
command listen on the specified group address for manycast
client messages.
Note the distinction between manycast client,
which actively broadcasts messages, and manycast server,
which passively responds to them.
If a manycast server is
in scope of the current TTL and is itself synchronized
to a valid source and operating at a stratum level equal
to or lower than the manycast client, it replies to the
manycast client message with an ordinary unicast server message.
.Pp
The manycast client receiving this message mobilizes
an ephemeral client/server association according to the
matching manycast client template, but only if cryptographically
authenticated and the server stratum is less than or equal
to the client stratum.
Authentication is explicitly required
and either symmetric key or public key (Autokey) can be used.
Then, the client polls the server at its unicast address
in burst mode in order to reliably set the host clock
and validate the source.
This normally results
in a volley of eight client/server at 2-s intervals
during which both the synchronization and cryptographic
protocols run concurrently.
Following the volley,
the client runs the NTP intersection and clustering
algorithms, which act to discard all but the "best"
associations according to stratum and synchronization
distance.
The surviving associations then continue
in ordinary client/server mode.
.Pp
The manycast client polling strategy is designed to reduce
as much as possible the volume of manycast client messages
and the effects of implosion due to near-simultaneous
arrival of manycast server messages.
The strategy is determined by the
.Ic manycastclient ,
.Ic tos
and
.Ic ttl
configuration commands.
The manycast poll interval is
normally eight times the system poll interval,
which starts out at the
.Cm minpoll
value specified in the
.Ic manycastclient ,
command and, under normal circumstances, increments to the
.Cm maxpolll
value specified in this command.
Initially, the TTL is
set at the minimum hops specified by the ttl command.
At each retransmission the TTL is increased until reaching
the maximum hops specified by this command or a sufficient
number client associations have been found.
Further retransmissions use the same TTL.
.Pp
The quality and reliability of the suite of associations
discovered by the manycast client is determined by the NTP
mitigation algorithms and the
.Cm minclock
and
.Cm minsane
values specified in the
.Ic tos
configuration command.
At least
.Cm minsane
candidate servers must be available and the mitigation
algorithms produce at least
.Cm minclock
survivors in order to synchronize the clock.
Byzantine agreement principles require at least four
candidates in order to correctly discard a single falseticker.
For legacy purposes,
.Cm minsane
defaults to 1 and
.Cm minclock
defaults to 3.
For manycast service
.Cm minsane
should be explicitly set to 4, assuming at least that
number of servers are available.
.Pp
If at least
.Cm minclock
servers are found, the manycast poll interval is immediately
set to eight times
.Cm maxpoll .
If less than
.Cm minclock
servers are found when the TTL has reached the maximum hops,
the manycast poll interval is doubled.
For each transmission
after that, the poll interval is doubled again until
reaching the maximum of eight times
.Cm maxpoll .
Further transmissions use the same poll interval and
TTL values.
Note that while all this is going on,
each client/server association found is operating normally
it the system poll interval.
.Pp
Administratively scoped multicast boundaries are normally
specified by the network router configuration and,
in the case of IPv6, the link/site scope prefix.
By default, the increment for TTL hops is 32 starting
from 31; however, the
.Ic ttl
configuration command can be
used to modify the values to match the scope rules.
.Pp
It is often useful to narrow the range of acceptable
servers which can be found by manycast client associations.
Because manycast servers respond only when the client
stratum is equal to or greater than the server stratum,
primary (stratum 1) servers fill find only primary servers
in TTL range, which is probably the most common objective.
However, unless configured otherwise, all manycast clients
in TTL range will eventually find all primary servers
in TTL range, which is probably not the most common
objective in large networks.
The
.Ic tos
command can be used to modify this behavior.
Servers with stratum below
.Cm floor
or above
.Cm ceiling
specified in the
.Ic tos
command are strongly discouraged during the selection
process; however, these servers may be temporally
accepted if the number of servers within TTL range is
less than
.Cm minclock .
.Pp
The above actions occur for each manycast client message,
which repeats at the designated poll interval.
However, once the ephemeral client association is mobilized,
subsequent manycast server replies are discarded,
since that would result in a duplicate association.
If during a poll interval the number of client associations
falls below
.Cm minclock ,
all manycast client prototype associations are reset
to the initial poll interval and TTL hops and operation
resumes from the beginning.
It is important to avoid
frequent manycast client messages, since each one requires
all manycast servers in TTL range to respond.
The result could well be an implosion, either minor or major,
depending on the number of servers in range.
The recommended value for
.Cm maxpoll
is 12 (4,096 s).
.Pp
It is possible and frequently useful to configure a host
as both manycast client and manycast server.
A number of hosts configured this way and sharing a common
group address will automatically organize themselves
in an optimum configuration based on stratum and
synchronization distance.
For example, consider an NTP
subnet of two primary servers and a hundred or more
dependent clients.
With two exceptions, all servers
and clients have identical configuration files including both
.Ic multicastclient
and
.Ic multicastserver
commands using, for instance, multicast group address
239.1.1.1.
The only exception is that each primary server
configuration file must include commands for the primary
reference source such as a GPS receiver.
.Pp
The remaining configuration files for all secondary
servers and clients have the same contents, except for the
.Ic tos
command, which is specific for each stratum level.
For stratum 1 and stratum 2 servers, that command is
not necessary.
For stratum 3 and above servers the
.Cm floor
value is set to the intended stratum number.
Thus, all stratum 3 configuration files are identical,
all stratum 4 files are identical and so forth.
.Pp
Once operations have stabilized in this scenario,
the primary servers will find the primary reference source
and each other, since they both operate at the same
stratum (1), but not with any secondary server or client,
since these operate at a higher stratum.
The secondary
servers will find the servers at the same stratum level.
If one of the primary servers loses its GPS receiver,
it will continue to operate as a client and other clients
will time out the corresponding association and
re-associate accordingly.
.Pp
Some administrators prefer to avoid running
.Xr ntpd 8
continuously and run either
.Xr ntpdate 8
or
.Xr ntpd 8
.Fl q
as a cron job.
In either case the servers must be
configured in advance and the program fails if none are
available when the cron job runs.
A really slick
application of manycast is with
.Xr ntpd 8
.Fl q .
The program wakes up, scans the local landscape looking
for the usual suspects, selects the best from among
the rascals, sets the clock and then departs.
Servers do not have to be configured in advance and
all clients throughout the network can have the same
configuration file.
.Ss Manycast Interactions with Autokey
Each time a manycast client sends a client mode packet
to a multicast group address, all manycast servers
in scope generate a reply including the host name
and status word.
The manycast clients then run
the Autokey protocol, which collects and verifies
all certificates involved.
Following the burst interval
all but three survivors are cast off,
but the certificates remain in the local cache.
It often happens that several complete signing trails
from the client to the primary servers are collected in this way.
.Pp
About once an hour or less often if the poll interval
exceeds this, the client regenerates the Autokey key list.
This is in general transparent in client/server mode.
However, about once per day the server private value
used to generate cookies is refreshed along with all
manycast client associations.
In this case all
cryptographic values including certificates is refreshed.
If a new certificate has been generated since
the last refresh epoch, it will automatically revoke
all prior certificates that happen to be in the
certificate cache.
At the same time, the manycast
scheme starts all over from the beginning and
the expanding ring shrinks to the minimum and increments
from there while collecting all servers in scope.
.Ss Manycast Options
.Bl -tag -width indent
.It Xo Ic tos
.Oo
.Cm ceiling Ar ceiling |
.Cm cohort { 0 | 1 } |
.Cm floor Ar floor |
.Cm minclock Ar minclock |
.Cm minsane Ar minsane
.Oc
.Xc
This command affects the clock selection and clustering
algorithms.
It can be used to select the quality and
quantity of peers used to synchronize the system clock
and is most useful in manycast mode.
The variables operate
as follows:
.Bl -tag -width indent
.It Cm ceiling Ar ceiling
Peers with strata above
.Cm ceiling
will be discarded if there are at least
.Cm minclock
peers remaining.
This value defaults to 15, but can be changed
to any number from 1 to 15.
.It Cm cohort Bro 0 | 1 Brc
This is a binary flag which enables (0) or disables (1)
manycast server replies to manycast clients with the same
stratum level.
This is useful to reduce implosions where
large numbers of clients with the same stratum level
are present.
The default is to enable these replies.
.It Cm floor Ar floor
Peers with strata below
.Cm floor
will be discarded if there are at least
.Cm minclock
peers remaining.
This value defaults to 1, but can be changed
to any number from 1 to 15.
.It Cm minclock Ar minclock
The clustering algorithm repeatedly casts out outlyer
associations until no more than
.Cm minclock
associations remain.
This value defaults to 3,
but can be changed to any number from 1 to the number of
configured sources.
.It Cm minsane Ar minsane
This is the minimum number of candidates available
to the clock selection algorithm in order to produce
one or more truechimers for the clustering algorithm.
If fewer than this number are available, the clock is
undisciplined and allowed to run free.
The default is 1
for legacy purposes.
However, according to principles of
Byzantine agreement,
.Cm minsane
should be at least 4 in order to detect and discard
a single falseticker.
.El
.It Cm ttl Ar hop ...
This command specifies a list of TTL values in increasing
order, up to 8 values can be specified.
In manycast mode these values are used in turn
in an expanding-ring search.
The default is eight
multiples of 32 starting at 31.
.El
.Sh Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio,
satellite and modem reference clocks plus a special pseudo-clock
used for backup or when no other clock source is available.
Detailed descriptions of individual device drivers and options can
be found in the
.Qq Reference Clock Drivers
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
Additional information can be found in the pages linked
there, including the
.Qq Debugging Hints for Reference Clock Drivers
and
.Qq How To Write a Reference Clock Driver
pages
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
In addition, support for a PPS
signal is available as described in the
.Qq Pulse-per-second (PPS) Signal Interfacing
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
Many
drivers support special line discipline/streams modules which can
significantly improve the accuracy using the driver.
These are
described in the
.Qq Line Disciplines and Streams Drivers
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
.Pp
A reference clock will generally (though not always) be a radio
timecode receiver which is synchronized to a source of standard
time such as the services offered by the NRC in Canada and NIST and
USNO in the US.
The interface between the computer and the timecode
receiver is device dependent, but is usually a serial port.
A
device driver specific to each reference clock must be selected and
compiled in the distribution; however, most common radio, satellite
and modem clocks are included by default.
Note that an attempt to
configure a reference clock when the driver has not been compiled
or the hardware port has not been appropriately configured results
in a scalding remark to the system log file, but is otherwise non
hazardous.
.Pp
For the purposes of configuration,
.Xr ntpd 8
treats
reference clocks in a manner analogous to normal NTP peers as much
as possible.
Reference clocks are identified by a syntactically
correct but invalid IP address, in order to distinguish them from
normal NTP peers.
Reference clock addresses are of the form
.Sm off
.Li 127.127. Ar t . Ar u ,
.Sm on
where
.Ar t
is an integer
denoting the clock type and
.Ar u
indicates the unit
number in the range 0-3.
While it may seem overkill, it is in fact
sometimes useful to configure multiple reference clocks of the same
type, in which case the unit numbers must be unique.
.Pp
The
.Ic server
command is used to configure a reference
clock, where the
.Ar address
argument in that command
is the clock address.
The
.Cm key ,
.Cm version
and
.Cm ttl
options are not used for reference clock support.
The
.Cm mode
option is added for reference clock support, as
described below.
The
.Cm prefer
option can be useful to
persuade the server to cherish a reference clock with somewhat more
enthusiasm than other reference clocks or peers.
Further
information on this option can be found in the
.Qq Mitigation Rules and the prefer Keyword
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
page.
The
.Cm minpoll
and
.Cm maxpoll
options have
meaning only for selected clock drivers.
See the individual clock
driver document pages for additional information.
.Pp
The
.Ic fudge
command is used to provide additional
information for individual clock drivers and normally follows
immediately after the
.Ic server
command.
The
.Ar address
argument specifies the clock address.
The
.Cm refid
and
.Cm stratum
options can be used to
override the defaults for the device.
There are two optional
device-dependent time offsets and four flags that can be included
in the
.Ic fudge
command as well.
.Pp
The stratum number of a reference clock is by default zero.
Since the
.Xr ntpd 8
daemon adds one to the stratum of each
peer, a primary server ordinarily displays an external stratum of
one.
In order to provide engineered backups, it is often useful to
specify the reference clock stratum as greater than zero.
The
.Cm stratum
option is used for this purpose.
Also, in cases
involving both a reference clock and a pulse-per-second (PPS)
discipline signal, it is useful to specify the reference clock
identifier as other than the default, depending on the driver.
The
.Cm refid
option is used for this purpose.
Except where noted,
these options apply to all clock drivers.
.Ss Reference Clock Commands
.Bl -tag -width indent
.It Xo Ic server
.Sm off
.Li 127.127. Ar t . Ar u
.Sm on
.Op Cm prefer
.Op Cm mode Ar int
.Op Cm minpoll Ar int
.Op Cm maxpoll Ar int
.Xc
This command can be used to configure reference clocks in
special ways.
The options are interpreted as follows:
.Bl -tag -width indent
.It Cm prefer
Marks the reference clock as preferred.
All other things being
equal, this host will be chosen for synchronization among a set of
correctly operating hosts.
See the
.Qq Mitigation Rules and the prefer Keyword
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
for further information.
.It Cm mode Ar int
Specifies a mode number which is interpreted in a
device-specific fashion.
For instance, it selects a dialing
protocol in the ACTS driver and a device subtype in the
parse
drivers.
.It Cm minpoll Ar int
.It Cm maxpoll Ar int
These options specify the minimum and maximum polling interval
for reference clock messages, as a power of 2 in seconds
For
most directly connected reference clocks, both
.Cm minpoll
and
.Cm maxpoll
default to 6 (64 s).
For modem reference clocks,
.Cm minpoll
defaults to 10 (17.1 m) and
.Cm maxpoll
defaults to 14 (4.5 h).
The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.
.El
.It Xo Ic fudge
.Sm off
.Li 127.127. Ar t . Ar u
.Sm on
.Op Cm time1 Ar sec
.Op Cm time2 Ar sec
.Op Cm stratum Ar int
.Op Cm refid Ar string
.Op Cm mode Ar int
.Op Cm flag1 Cm 0 \&| Cm 1
.Op Cm flag2 Cm 0 \&| Cm 1
.Op Cm flag3 Cm 0 \&| Cm 1
.Op Cm flag4 Cm 0 \&| Cm 1
.Xc
This command can be used to configure reference clocks in
special ways.
It must immediately follow the
.Ic server
command which configures the driver.
Note that the same capability
is possible at run time using the
.Xr ntpdc 8
program.
The options are interpreted as
follows:
.Bl -tag -width indent
.It Cm time1 Ar sec
Specifies a constant to be added to the time offset produced by
the driver, a fixed-point decimal number in seconds.
This is used
as a calibration constant to adjust the nominal time offset of a
particular clock to agree with an external standard, such as a
precision PPS signal.
It also provides a way to correct a
systematic error or bias due to serial port or operating system
latencies, different cable lengths or receiver internal delay.
The
specified offset is in addition to the propagation delay provided
by other means, such as internal DIPswitches.
Where a calibration
for an individual system and driver is available, an approximate
correction is noted in the driver documentation pages.
Note: in order to facilitate calibration when more than one
radio clock or PPS signal is supported, a special calibration
feature is available.
It takes the form of an argument to the
.Ic enable
command described in
.Sx Miscellaneous Options
page and operates as described in the
.Qq Reference Clock Drivers
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
.It Cm time2 Ar secs
Specifies a fixed-point decimal number in seconds, which is
interpreted in a driver-dependent way.
See the descriptions of
specific drivers in the
.Qq Reference Clock Drivers
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
.It Cm stratum Ar int
Specifies the stratum number assigned to the driver, an integer
between 0 and 15.
This number overrides the default stratum number
ordinarily assigned by the driver itself, usually zero.
.It Cm refid Ar string
Specifies an ASCII string of from one to four characters which
defines the reference identifier used by the driver.
This string
overrides the default identifier ordinarily assigned by the driver
itself.
.It Cm mode Ar int
Specifies a mode number which is interpreted in a
device-specific fashion.
For instance, it selects a dialing
protocol in the ACTS driver and a device subtype in the
parse
drivers.
.It Cm flag1 Cm 0 \&| Cm 1
.It Cm flag2 Cm 0 \&| Cm 1
.It Cm flag3 Cm 0 \&| Cm 1
.It Cm flag4 Cm 0 \&| Cm 1
These four flags are used for customizing the clock driver.
The
interpretation of these values, and whether they are used at all,
is a function of the particular clock driver.
However, by
convention
.Cm flag4
is used to enable recording monitoring
data to the
.Cm clockstats
file configured with the
.Ic filegen
command.
Further information on the
.Ic filegen
command can be found in
.Sx Monitoring Options .
.El
.El
.Sh Miscellaneous Options
.Bl -tag -width indent
.It Ic broadcastdelay Ar seconds
The broadcast and multicast modes require a special calibration
to determine the network delay between the local and remote
servers.
Ordinarily, this is done automatically by the initial
protocol exchanges between the client and server.
In some cases,
the calibration procedure may fail due to network or server access
controls, for example.
This command specifies the default delay to
be used under these circumstances.
Typically (for Ethernet), a
number between 0.003 and 0.007 seconds is appropriate.
The default
when this command is not used is 0.004 seconds.
.It Ic calldelay Ar delay
This option controls the delay in seconds between the first and second
packets sent in burst or iburst mode to allow additional time for a modem
or ISDN call to complete.
.It Ic driftfile Ar driftfile
This command specifies the complete path and name of the file used to
record the frequency of the local clock oscillator.
This is the same
operation as the
.Fl f
command line option.
If the file exists, it is read at
startup in order to set the initial frequency and then updated once per
hour with the current frequency computed by the daemon.
If the file name is
specified, but the file itself does not exist, the starts with an initial
frequency of zero and creates the file when writing it for the first time.
If this command is not given, the daemon will always start with an initial
frequency of zero.
.Pp
The file format consists of a single line containing a single
floating point number, which records the frequency offset measured
in parts-per-million (PPM).
The file is updated by first writing
the current drift value into a temporary file and then renaming
this file to replace the old version.
This implies that
.Xr ntpd 8
must have write permission for the directory the
drift file is located in, and that file system links, symbolic or
otherwise, should be avoided.
.It Xo Ic enable
.Oo
.Cm auth | Cm bclient |
.Cm calibrate | Cm kernel |
.Cm monitor | Cm ntp |
.Cm pps | Cm stats
.Oc
.Xc
.It Xo Ic disable
.Oo
.Cm auth | Cm bclient |
.Cm calibrate | Cm kernel |
.Cm monitor | Cm ntp |
.Cm pps | Cm stats
.Oc
.Xc
Provides a way to enable or disable various server options.
Flags not mentioned are unaffected.
Note that all of these flags
can be controlled remotely using the
.Xr ntpdc 8
utility program.
.Bl -tag -width indent
.It Cm auth
Enables the server to synchronize with unconfigured peers only if the
peer has been correctly authenticated using either public key or
private key cryptography.
The default for this flag is
.Ic enable .
.It Cm bclient
Enables the server to listen for a message from a broadcast or
multicast server, as in the
.Ic multicastclient
command with default
address.
The default for this flag is
.Ic disable .
.It Cm calibrate
Enables the calibrate feature for reference clocks.
The default for
this flag is
.Ic disable .
.It Cm kernel
Enables the kernel time discipline, if available.
The default for this
flag is
.Ic enable
if support is available, otherwise
.Ic disable .
.It Cm monitor
Enables the monitoring facility.
See the
.Xr ntpdc 8
program
and the
.Ic monlist
command or further information.
The
default for this flag is
.Ic enable .
.It Cm ntp
Enables time and frequency discipline.
In effect, this switch opens and
closes the feedback loop, which is useful for testing.
The default for
this flag is
.Ic enable .
.It Cm pps
Enables the pulse-per-second (PPS) signal when frequency and time is
disciplined by the precision time kernel modifications.
See the
.Qq A Kernel Model for Precision Timekeeping
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
page for further information.
The default for this flag is
.Ic disable .
.It Cm stats
Enables the statistics facility.
See the
.Sx Monitoring Options
section for further information.
The default for this flag is
.Ic disable .
.El
.It Ic includefile Ar includefile
This command allows additional configuration commands
to be included from a separate file.
Include files may
be nested to a depth of five; upon reaching the end of any
include file, command processing resumes in the previous
configuration file.
This option is useful for sites that run
.Xr ntpd 8
on multiple hosts, with (mostly) common options (e.g., a
restriction list).
.It Ic logconfig Ar configkeyword
This command controls the amount and type of output written to
the system
.Xr syslog 3
facility or the alternate
.Ic logfile
log file.
By default, all output is turned on.
All
.Ar configkeyword
keywords can be prefixed with
.Ql = ,
.Ql +
and
.Ql - ,
where
.Ql =
sets the
.Xr syslog 3
priority mask,
.Ql +
adds and
.Ql -
removes
messages.
.Xr syslog 3
messages can be controlled in four
classes
.Po
.Cm clock ,
.Cm peer ,
.Cm sys
and
.Cm sync
.Pc .
Within these classes four types of messages can be
controlled: informational messages
.Po
.Cm info
.Pc ,
event messages
.Po
.Cm events
.Pc ,
statistics messages
.Po
.Cm statistics
.Pc
and
status messages
.Po
.Cm status
.Pc .
.Pp
Configuration keywords are formed by concatenating the message class with
the event class.
The
.Cm all
prefix can be used instead of a message class.
A
message class may also be followed by the
.Cm all
keyword to enable/disable all
messages of the respective message class.Thus, a minimal log configuration
could look like this:
.Bd -literal
logconfig =syncstatus +sysevents
.Ed
.Pp
This would just list the synchronizations state of
.Xr ntpd 8
and the major system events.
For a simple reference server, the
following minimum message configuration could be useful:
.Bd -literal
logconfig =syncall +clockall
.Ed
.Pp
This configuration will list all clock information and
synchronization information.
All other events and messages about
peers, system events and so on is suppressed.
.It Ic logfile Ar logfile
This command specifies the location of an alternate log file to
be used instead of the default system
.Xr syslog 3
facility.
This is the same operation as the -l command line option.
.It Ic setvar Ar variable Op Cm default
This command adds an additional system variable.
These
variables can be used to distribute additional information such as
the access policy.
If the variable of the form
.Sm off
.Va name = Ar value
.Sm on
is followed by the
.Cm default
keyword, the
variable will be listed as part of the default system variables
.Po
.Xr ntpq 8
.Ic rv
command
.Pc ) .
These additional variables serve
informational purposes only.
They are not related to the protocol
other that they can be listed.
The known protocol variables will
always override any variables defined via the
.Ic setvar
mechanism.
There are three special variables that contain the names
of all variable of the same group.
The
.Va sys_var_list
holds
the names of all system variables.
The
.Va peer_var_list
holds
the names of all peer variables and the
.Va clock_var_list
holds the names of the reference clock variables.
.It Xo Ic tinker
.Oo
.Cm allan Ar allan |
.Cm dispersion Ar dispersion |
.Cm freq Ar freq |
.Cm huffpuff Ar huffpuff |
.Cm panic Ar panic |
.Cm step Ar srep |
.Cm stepout Ar stepout
.Oc
.Xc
This command can be used to alter several system variables in
very exceptional circumstances.
It should occur in the
configuration file before any other configuration options.
The
default values of these variables have been carefully optimized for
a wide range of network speeds and reliability expectations.
In
general, they interact in intricate ways that are hard to predict
and some combinations can result in some very nasty behavior.
Very
rarely is it necessary to change the default values; but, some
folks cannot resist twisting the knobs anyway and this command is
for them.
Emphasis added: twisters are on their own and can expect
no help from the support group.
.Pp
The variables operate as follows:
.Bl -tag -width indent
.It Cm allan Ar allan
The argument becomes the new value for the minimum Allan
intercept, which is a parameter of the PLL/FLL clock discipline
algorithm.
The value in log2 seconds defaults to 7 (1024 s), which is also the lower
limit.
.It Cm dispersion Ar dispersion
The argument becomes the new value for the dispersion increase rate,
normally .000015 s/s.
.It Cm freq Ar freq
The argument becomes the initial value of the frequency offset in
parts-per-million.
This overrides the value in the frequency file, if
present, and avoids the initial training state if it is not.
.It Cm huffpuff Ar huffpuff
The argument becomes the new value for the experimental
huff-n'-puff filter span, which determines the most recent interval
the algorithm will search for a minimum delay.
The lower limit is
900 s (15 m), but a more reasonable value is 7200 (2 hours).
There
is no default, since the filter is not enabled unless this command
is given.
.It Cm panic Ar panic
The argument is the panic threshold, normally 1000 s.
If set to zero,
the panic sanity check is disabled and a clock offset of any value will
be accepted.
.It Cm step Ar step
The argument is the step threshold, which by default is 0.128 s.
It can
be set to any positive number in seconds.
If set to zero, step
adjustments will never occur.
Note: The kernel time discipline is
disabled if the step threshold is set to zero or greater than the
default.
.It Cm stepout Ar stepout
The argument is the stepout timeout, which by default is 900 s.
It can
be set to any positive number in seconds.
If set to zero, the stepout
pulses will not be suppressed.
.El
.It Xo Ic trap Ar host_address
.Op Cm port Ar port_number
.Op Cm interface Ar interface_address
.Xc
This command configures a trap receiver at the given host
address and port number for sending messages with the specified
local interface address.
If the port number is unspecified, a value
of 18447 is used.
If the interface address is not specified, the
message is sent with a source address of the local interface the
message is sent through.
Note that on a multihomed host the
interface used may vary from time to time with routing changes.
.Pp
The trap receiver will generally log event messages and other
information from the server in a log file.
While such monitor
programs may also request their own trap dynamically, configuring a
trap receiver will ensure that no messages are lost when the server
is started.
.It Cm hop Ar ...
This command specifies a list of TTL values in increasing order, up to 8
values can be specified.
In manycast mode these values are used in turn in
an expanding-ring search.
The default is eight multiples of 32 starting at
31.
.El
.Sh FILES
.Bl -tag -width /etc/ntp.drift -compact
.It Pa /etc/ntp.conf
the default name of the configuration file
.It Pa ntp.keys
private MD5 keys
.It Pa ntpkey
RSA private key
.It Pa ntpkey_ Ns Ar host
RSA public key
.It Pa ntp_dh
Diffie-Hellman agreement parameters
.El
.Sh SEE ALSO
.Xr rc.conf 5 ,
.Xr ntpd 8 ,
.Xr ntpdc 8 ,
.Xr ntpq 8
.Pp
In addition to the manual pages provided,
comprehensive documentation is available on the world wide web
at
.Li http://www.ntp.org/ .
A snapshot of this documentation is available in HTML format in
.Pa /usr/share/doc/ntp .
.Rs
.%A David L. Mills
.%T Network Time Protocol (Version 3)
.%O RFC1305
.Re
.Sh BUGS
The syntax checking is not picky; some combinations of
ridiculous and even hilarious options and modes may not be
detected.
.Pp
The
.Pa ntpkey_ Ns Ar host
files are really digital
certificates.
These should be obtained via secure directory
services when they become universally available.
|