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
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
4243
4244
4245
4246
4247
4248
4249
4250
4251
4252
4253
4254
4255
4256
4257
4258
4259
4260
4261
4262
4263
4264
4265
4266
4267
4268
4269
4270
4271
4272
4273
4274
4275
4276
4277
4278
4279
4280
4281
4282
4283
4284
4285
4286
4287
4288
4289
4290
4291
4292
4293
4294
4295
4296
4297
4298
4299
4300
4301
4302
4303
4304
4305
4306
4307
4308
4309
4310
4311
4312
4313
4314
4315
4316
4317
4318
4319
4320
4321
4322
4323
4324
4325
4326
4327
4328
4329
4330
4331
4332
4333
4334
4335
4336
4337
4338
4339
4340
4341
4342
4343
4344
4345
4346
4347
4348
4349
4350
4351
4352
4353
4354
4355
4356
4357
4358
4359
4360
4361
4362
4363
4364
4365
4366
4367
4368
4369
4370
4371
4372
4373
4374
4375
4376
4377
4378
4379
4380
4381
4382
4383
4384
4385
4386
4387
4388
4389
4390
4391
4392
4393
4394
4395
4396
4397
4398
4399
4400
4401
4402
4403
4404
4405
4406
4407
4408
4409
4410
4411
4412
4413
4414
4415
4416
4417
4418
4419
4420
4421
4422
4423
4424
4425
4426
4427
4428
4429
4430
4431
4432
4433
4434
4435
4436
4437
4438
4439
4440
4441
4442
4443
4444
4445
4446
4447
4448
4449
4450
4451
4452
4453
4454
4455
4456
4457
4458
4459
4460
4461
4462
4463
4464
4465
4466
4467
4468
4469
4470
4471
4472
4473
4474
4475
4476
4477
4478
4479
4480
4481
4482
4483
4484
4485
4486
4487
4488
4489
4490
4491
4492
4493
4494
4495
4496
4497
4498
4499
4500
4501
4502
4503
4504
4505
4506
4507
4508
4509
4510
4511
4512
4513
4514
4515
4516
4517
4518
4519
4520
4521
4522
4523
4524
4525
4526
4527
4528
4529
4530
4531
4532
4533
4534
4535
4536
4537
4538
4539
4540
4541
4542
4543
4544
4545
4546
4547
4548
4549
4550
4551
4552
4553
4554
4555
4556
4557
4558
4559
4560
4561
4562
4563
4564
4565
4566
4567
4568
4569
4570
4571
4572
4573
4574
4575
4576
4577
4578
4579
4580
4581
4582
4583
4584
4585
4586
4587
4588
4589
4590
4591
4592
4593
4594
4595
4596
4597
4598
4599
4600
4601
4602
4603
4604
4605
4606
4607
4608
4609
4610
4611
4612
4613
4614
4615
4616
4617
4618
4619
4620
4621
4622
4623
4624
4625
4626
4627
4628
4629
4630
4631
4632
4633
4634
4635
4636
4637
4638
4639
4640
4641
4642
4643
4644
4645
4646
4647
4648
4649
4650
4651
4652
4653
4654
4655
4656
4657
4658
4659
4660
4661
4662
4663
4664
4665
4666
4667
4668
4669
4670
4671
4672
4673
4674
4675
4676
4677
4678
4679
4680
4681
4682
4683
4684
4685
4686
4687
4688
4689
4690
4691
4692
4693
4694
4695
4696
4697
4698
4699
4700
4701
4702
4703
4704
4705
4706
4707
4708
4709
4710
4711
4712
4713
4714
4715
4716
4717
4718
4719
4720
4721
4722
4723
4724
4725
4726
4727
4728
4729
4730
4731
4732
4733
4734
4735
4736
4737
4738
4739
4740
4741
4742
4743
4744
4745
4746
4747
4748
4749
4750
4751
4752
4753
4754
4755
4756
4757
4758
4759
4760
4761
4762
4763
4764
4765
4766
4767
4768
4769
4770
4771
4772
4773
4774
4775
4776
4777
4778
4779
4780
4781
4782
4783
4784
4785
4786
4787
4788
4789
4790
4791
4792
4793
4794
4795
4796
4797
4798
4799
4800
4801
4802
4803
4804
4805
4806
4807
4808
4809
4810
4811
4812
4813
4814
4815
4816
4817
4818
4819
4820
4821
4822
4823
4824
4825
4826
4827
4828
4829
4830
4831
4832
4833
4834
4835
4836
4837
4838
4839
4840
4841
4842
4843
4844
4845
4846
4847
4848
4849
4850
4851
4852
4853
4854
4855
4856
4857
4858
4859
4860
4861
4862
4863
4864
4865
4866
4867
4868
4869
4870
4871
4872
4873
4874
4875
4876
4877
4878
4879
4880
4881
4882
4883
4884
4885
4886
4887
4888
4889
4890
4891
4892
4893
4894
4895
4896
4897
4898
4899
4900
4901
4902
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
4919
4920
4921
4922
4923
4924
4925
4926
4927
4928
4929
4930
4931
4932
4933
4934
4935
4936
4937
4938
4939
4940
4941
4942
4943
4944
4945
4946
4947
4948
4949
4950
4951
4952
4953
4954
4955
4956
4957
4958
4959
4960
4961
4962
4963
4964
4965
4966
4967
4968
4969
4970
4971
4972
4973
4974
4975
4976
4977
4978
4979
4980
4981
4982
4983
4984
4985
4986
4987
4988
4989
4990
4991
4992
4993
|
<?xml version="1.0" encoding="ISO-8859-7" standalone="no"?>
<!--
Ôï Åã÷åéñßäéï ôïõ FreeBSD: ÁóöÜëåéá
The FreeBSD Greek Documentation Project
$FreeBSD$
%SOURCE% en_US.ISO8859-1/books/handbook/security/chapter.sgml
%SRCID% 1.1
-->
<chapter id="security">
<chapterinfo>
<authorgroup>
<author>
<firstname>Matthew</firstname>
<surname>Dillon</surname>
<contrib>Ôï ìåãáëýôåñï ìÝñïò áõôïý ôïõ êåöáëáßïõ ðñïÝñ÷åôáé áðü ôçí
óåëßäá ôïõ manual ôçò security(7) áðü ôïí </contrib>
</author>
</authorgroup>
</chapterinfo>
<title>ÁóöÜëåéá</title>
<indexterm><primary>áóöÜëåéá</primary></indexterm>
<sect1 id="security-synopsis">
<title>Óýíïøç</title>
<para>Ôï êåöÜëáéï áõôü ðáñÝ÷åé ìéá âáóéêÞ åéóáãùãÞ óôéò Ýííïéåò ôçò
áóöÜëåéáò óõóôÞìáôïò, êÜðïéïõò ãåíéêÜ êáëïýò êáíüíåò, êáé ïñéóìÝíá
ðñï÷ùñçìÝíá èÝìáôá ó÷åôéêÜ ìå ôï &os;. ÁñêåôÜ áðü ôá èÝìáôá ðïõ
êáëýðôïíôáé åäþ, ìðïñïýí íá åöáñìïóôïýí ôï ßäéï êáëÜ ôüóï óôï ßäéï ôï
óýóôçìá, üóï êáé ãéá áóöÜëåéá ìÝóù Internet. Ôï Internet äåí åßíáé
ðëÝïí Ýíá <quote>öéëéêü</quote> ìÝñïò óôï ïðïßï êáèÝíáò èÝëåé íá åßíáé
ï åõãåíéêüò óáò ãåßôïíáò. Ç áíÜãêç áóöÜëéóçò ôïõ óõóôÞìáôïò óáò åßíáé
åðéôáêôéêÞ ãéá íá ðñïóôáôÝøåôå ôá äåäïìÝíá óáò,ôçí ðíåõìáôéêÞ óáò
éäéïêôçóßá, ôï ÷ñüíï óáò, êáé ðïëëÜ ðåñéóóüôåñá áðü ôá ÷Ýñéá ôùí ÷Üêåñò
êáé ôùí ïìïßùí ôïõò.</para>
<para>Ôï &os; ðáñÝ÷åé ìéá óåéñÜ áðü âïçèçôéêÜ ðñïãñÜììáôá êáé ìç÷áíéóìïýò
ãéá íá åîáóöáëßóåé ôçí áêåñáéüôçôá êáé ôçí áóöÜëåéá ôïõ óõóôÞìáôïò óáò
êáé ôïõ äéêôýïõ.</para>
<para>Áöïý äéáâÜóåôå áõôü ôï êåöÜëáéï, èá îÝñåôå:</para>
<itemizedlist>
<listitem>
<para>ÂáóéêÝò Ýííïéåò ãéá ôçí áóöÜëåéá, óå ó÷Ýóç ìå ôï &os;.</para>
</listitem>
<listitem>
<para>Óôïé÷åßá ó÷åôéêÜ ìå ôïõò äéÜöïñïõò ìç÷áíéóìïýò êñõðôïãñÜöçóçò
ðïõ åßíáé äéáèÝóéìïé óôï &os;, üðùò ôï <acronym>DES</acronym> êáé
ôï <acronym>MD5</acronym>.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå ôï óýóôçìá óáò ãéá êùäéêïýò ìéáò ÷ñÞóçò.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå <acronym>TCP</acronym> Wrappers ãéá ÷ñÞóç ìå
ôçí <command>inetd</command>.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå ôïí <application>KerberosIV</application> óå
&os; åêäüóåéò ðñéí ôç 5.0.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå ôïí <application>Kerberos5</application> óôï
&os;.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå ôï IPsec êáé íá äçìéïõñãÞóåôå Ýíá
<acronym>VPN</acronym> ìåôáîý ìç÷áíçìÜôùí &os;/&windows;.</para>
</listitem>
<listitem>
<para>Ðùò íá ñõèìßóåôå êáé íá ÷ñçóéìïðïéÞóåôå ôçí êáôÜ &os; õëïðïßçóç
<acronym>SSH</acronym> ôïõ <application>OpenSSH</application>
</para>
</listitem>
<listitem>
<para>Ôé åßíáé ôá <acronym>ACL</acronym>s óôï óýóôçìá áñ÷åßùí êáé ðùò
íá ôá ÷ñçóéìïðïéÞóåôå.</para>
</listitem>
<listitem>
<para>Ðùò íá ÷ñçóéìïðïéÞóåôå ôï âïçèçôéêü ðñüãñáììá
<application>Portaudit</application> ãéá íá åëÝãîåôå ëïãéóìéêü
ôñßôïõ êáôáóêåõáóôÞ ðïõ Ý÷åé åãêáôáóôáèåß ìÝóù ôçò óõëëïãÞò Ports.
</para>
</listitem>
<listitem>
<para>Ðùò íá ÷ñçóéìïðïéÞóåôå ôéò äçìïóéåýóåéò security advisories
ôïõ &os;.</para>
</listitem>
<listitem>
<para>Èá Ý÷åôå ìéá éäÝá ãéá ôï ôé åßíáé ôï Process Accounting êáé ðùò
íá ôï åíåñãïðïéÞóåôå óôï &os;.</para>
</listitem>
</itemizedlist>
<para>Ðñéí äéáâÜóåôå áõôü ôï êåöÜëáéï, èá ðñÝðåé:</para>
<itemizedlist>
<listitem>
<para>Íá êáôáíïåßôå âáóéêÝò Ýííïéåò ôïõ &os; êáé ôïõ Internet.</para>
</listitem>
</itemizedlist>
<para>Ðñüóèåôá èÝìáôá ó÷åôéêÜ ìå ôçí áóöÜëåéá êáëýðôïíôáé óå ïëüêëçñï ôï
âéâëßï. Ãéá ðáñÜäåéãìá, ï Õðï÷ñåùôéêüò ¸ëåã÷ïò Ðñüóâáóçò óõæçôåßôáé
óôï <xref linkend="mac"/> êáé ôá Internet Firewalls óõæçôïýíôáé óôï
<xref linkend="firewalls"/>.</para>
</sect1>
<sect1 id="security-intro">
<title>Introduction</title>
<para>Security is a function that begins and ends with the system
administrator. While all BSD &unix; multi-user systems have some
inherent security, the job of building and maintaining additional
security mechanisms to keep those users <quote>honest</quote> is
probably one of the single largest undertakings of the sysadmin.
Machines are only as secure as you make them, and security concerns
are ever competing with the human necessity for convenience. &unix;
systems, in general, are capable of running a huge number of
simultaneous processes and many of these processes operate as
servers — meaning that external entities can connect and talk
to them. As yesterday's mini-computers and mainframes become
today's desktops, and as computers become networked and
inter-networked, security becomes an even bigger issue.</para>
<para>System security also pertains to dealing with various forms of
attack, including attacks that attempt to crash, or otherwise make a
system unusable, but do not attempt to compromise the
<username>root</username> account (<quote>break root</quote>).
Security concerns
can be split up into several categories:</para>
<orderedlist>
<listitem>
<para>Denial of service attacks.</para>
</listitem>
<listitem>
<para>User account compromises.</para>
</listitem>
<listitem>
<para>Root compromise through accessible servers.</para>
</listitem>
<listitem>
<para>Root compromise via user accounts.</para>
</listitem>
<listitem>
<para>Backdoor creation.</para>
</listitem>
</orderedlist>
<indexterm>
<primary>DoS attacks</primary>
<see>Denial of Service (DoS)</see>
</indexterm>
<indexterm>
<primary>security</primary>
<secondary>DoS attacks</secondary>
<see>Denial of Service (DoS)</see>
</indexterm>
<indexterm><primary>Denial of Service (DoS)</primary></indexterm>
<para>A denial of service attack is an action that deprives the
machine of needed resources. Typically, DoS attacks are
brute-force mechanisms that attempt to crash or otherwise make a
machine unusable by overwhelming its servers or network stack. Some
DoS attacks try to take advantage of bugs in the networking
stack to crash a machine with a single packet. The latter can only
be fixed by applying a bug fix to the kernel. Attacks on servers
can often be fixed by properly specifying options to limit the load
the servers incur on the system under adverse conditions.
Brute-force network attacks are harder to deal with. A
spoofed-packet attack, for example, is nearly impossible to stop,
short of cutting your system off from the Internet. It may not be
able to take your machine down, but it can saturate your
Internet connection.</para>
<indexterm>
<primary>security</primary>
<secondary>account compromises</secondary>
</indexterm>
<para>A user account compromise is even more common than a DoS
attack. Many sysadmins still run standard
<application>telnetd</application>, <application>rlogind</application>,
<application>rshd</application>,
and <application>ftpd</application> servers on their machines.
These servers, by default, do
not operate over encrypted connections. The result is that if you
have any moderate-sized user base, one or more of your users logging
into your system from a remote location (which is the most common
and convenient way to login to a system) will have his or her
password sniffed. The attentive system admin will analyze his
remote access logs looking for suspicious source addresses even for
successful logins.</para>
<para>One must always assume that once an attacker has access to a
user account, the attacker can break <username>root</username>.
However, the reality is that in a well secured and maintained system,
access to a user account does not necessarily give the attacker
access to <username>root</username>. The distinction is important
because without access to <username>root</username> the attacker
cannot generally hide his tracks and may, at best, be able to do
nothing more than mess with the user's files, or crash the machine.
User account compromises are very common because users tend not to
take the precautions that sysadmins take.</para>
<indexterm>
<primary>security</primary>
<secondary>backdoors</secondary>
</indexterm>
<para>System administrators must keep in mind that there are
potentially many ways to break <username>root</username> on a machine.
The attacker may know the <username>root</username> password,
the attacker may find a bug in a root-run server and be able
to break <username>root</username> over a network
connection to that server, or the attacker may know of a bug in
a suid-root program that allows the attacker to break
<username>root</username> once he has broken into a user's account.
If an attacker has found a way to break <username>root</username>
on a machine, the attacker may not have a need
to install a backdoor. Many of the <username>root</username> holes
found and closed to date involve a considerable amount of work
by the attacker to cleanup after himself, so most attackers install
backdoors. A backdoor provides the attacker with a way to easily
regain <username>root</username> access to the system, but it
also gives the smart system administrator a convenient way
to detect the intrusion.
Making it impossible for an attacker to install a backdoor may
actually be detrimental to your security, because it will not
close off the hole the attacker found to break in the first
place.</para>
<para>Security remedies should always be implemented with a
multi-layered <quote>onion peel</quote> approach and can be
categorized as follows:</para>
<orderedlist>
<listitem>
<para>Securing <username>root</username> and staff accounts.</para>
</listitem>
<listitem>
<para>Securing <username>root</username>–run servers
and suid/sgid binaries.</para>
</listitem>
<listitem>
<para>Securing user accounts.</para>
</listitem>
<listitem>
<para>Securing the password file.</para>
</listitem>
<listitem>
<para>Securing the kernel core, raw devices, and
file systems.</para>
</listitem>
<listitem>
<para>Quick detection of inappropriate changes made to the
system.</para>
</listitem>
<listitem>
<para>Paranoia.</para>
</listitem>
</orderedlist>
<para>The next section of this chapter will cover the above bullet
items in greater depth.</para>
</sect1>
<sect1 id="securing-freebsd">
<title>Securing &os;</title>
<indexterm>
<primary>security</primary>
<secondary>securing &os;</secondary>
</indexterm>
<note>
<title>Command vs. Protocol</title>
<para>Throughout this document, we will use
<application>bold</application> text to refer to an
application, and a <command>monospaced</command> font to refer
to specific commands. Protocols will use a normal font. This
typographical distinction is useful for instances such as ssh,
since it is
a protocol as well as command.</para>
</note>
<para>The sections that follow will cover the methods of securing your
&os; system that were mentioned in the <link
linkend="security-intro">last section</link> of this chapter.</para>
<sect2 id="securing-root-and-staff">
<title>Securing the <username>root</username> Account and
Staff Accounts</title>
<indexterm>
<primary><command>su</command></primary>
</indexterm>
<para>First off, do not bother securing staff accounts if you have
not secured the <username>root</username> account.
Most systems have a password assigned to the <username>root</username>
account. The first thing you do is assume
that the password is <emphasis>always</emphasis> compromised.
This does not mean that you should remove the password. The
password is almost always necessary for console access to the
machine. What it does mean is that you should not make it
possible to use the password outside of the console or possibly
even with the &man.su.1; command. For example, make sure that
your ptys are specified as being insecure in the
<filename>/etc/ttys</filename> file so that direct
<username>root</username> logins
via <command>telnet</command> or <command>rlogin</command> are
disallowed. If using other login services such as
<application>sshd</application>, make sure that direct
<username>root</username> logins are disabled there as well.
You can do this by editing
your <filename>/etc/ssh/sshd_config</filename> file, and making
sure that <literal>PermitRootLogin</literal> is set to
<literal>NO</literal>. Consider every access method —
services such as FTP often fall through the cracks.
Direct <username>root</username> logins should only be allowed
via the system console.</para>
<indexterm>
<primary><groupname>wheel</groupname></primary>
</indexterm>
<para>Of course, as a sysadmin you have to be able to get to
<username>root</username>, so we open up a few holes.
But we make sure these holes require additional password
verification to operate. One way to make <username>root</username>
accessible is to add appropriate staff accounts to the
<groupname>wheel</groupname> group (in
<filename>/etc/group</filename>). The staff members placed in the
<groupname>wheel</groupname> group are allowed to
<command>su</command> to <username>root</username>.
You should never give staff
members native <groupname>wheel</groupname> access by putting them in the
<groupname>wheel</groupname> group in their password entry. Staff
accounts should be placed in a <groupname>staff</groupname> group, and
then added to the <groupname>wheel</groupname> group via the
<filename>/etc/group</filename> file. Only those staff members
who actually need to have <username>root</username> access
should be placed in the
<groupname>wheel</groupname> group. It is also possible, when using
an authentication method such as Kerberos, to use Kerberos'
<filename>.k5login</filename> file in the <username>root</username>
account to allow a &man.ksu.1; to <username>root</username>
without having to place anyone at all in the
<groupname>wheel</groupname> group. This may be the better solution
since the <groupname>wheel</groupname> mechanism still allows an
intruder to break <username>root</username> if the intruder
has gotten hold of your
password file and can break into a staff account. While having
the <groupname>wheel</groupname> mechanism is better than having
nothing at all, it is not necessarily the safest option.</para>
<!-- XXX:
This will need updating depending on the outcome of PR bin/71147.
Personally I know what I'd like to see, which puts this in definite
need of a rewrite, but we'll have to wait and see. ceri@
-->
<para>An indirect way to secure staff accounts, and ultimately
<username>root</username> access is to use an alternative
login access method and
do what is known as <quote>starring</quote> out the encrypted
password for the staff accounts. Using the &man.vipw.8;
command, one can replace each instance of an encrypted password
with a single <quote><literal>*</literal></quote> character.
This command will update the <filename>/etc/master.passwd</filename>
file and user/password database to disable password-authenticated
logins.</para>
<para>A staff account entry such as:</para>
<programlisting>foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh</programlisting>
<para>Should be changed to this:</para>
<programlisting>foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh</programlisting>
<para>This change will prevent normal logins from occurring,
since the encrypted password will never match
<quote><literal>*</literal></quote>. With this done,
staff members must use
another mechanism to authenticate themselves such as
&man.kerberos.1; or &man.ssh.1; using a public/private key
pair. When using something like Kerberos, one generally must
secure the machines which run the Kerberos servers and your
desktop workstation. When using a public/private key pair
with ssh, one must generally secure
the machine used to login <emphasis>from</emphasis> (typically
one's workstation). An additional layer of protection can be
added to the key pair by password protecting the key pair when
creating it with &man.ssh-keygen.1;. Being able to
<quote>star</quote> out the passwords for staff accounts also
guarantees that staff members can only login through secure
access methods that you have set up. This forces all staff
members to use secure, encrypted connections for all of their
sessions, which closes an important hole used by many
intruders: sniffing the network from an unrelated,
less secure machine.</para>
<para>The more indirect security mechanisms also assume that you are
logging in from a more restrictive server to a less restrictive
server. For example, if your main box is running all sorts of
servers, your workstation should not be running any. In order for
your workstation to be reasonably secure you should run as few
servers as possible, up to and including no servers at all, and
you should run a password-protected screen blanker. Of course,
given physical access to a workstation an attacker can break any
sort of security you put on it. This is definitely a problem that
you should consider, but you should also consider the fact that the
vast majority of break-ins occur remotely, over a network, from
people who do not have physical access to your workstation or
servers.</para>
<indexterm><primary>KerberosIV</primary></indexterm>
<para>Using something like Kerberos also gives you the ability to
disable or change the password for a staff account in one place,
and have it immediately affect all the machines on which the staff
member may have an account. If a staff member's account gets
compromised, the ability to instantly change his password on all
machines should not be underrated. With discrete passwords,
changing a password on N machines can be a mess. You can also
impose re-passwording restrictions with Kerberos: not only can a
Kerberos ticket be made to timeout after a while, but the Kerberos
system can require that the user choose a new password after a
certain period of time (say, once a month).</para>
</sect2>
<sect2>
<title>Securing Root-run Servers and SUID/SGID Binaries</title>
<indexterm>
<primary><command>ntalk</command></primary>
</indexterm>
<indexterm>
<primary><command>comsat</command></primary>
</indexterm>
<indexterm>
<primary><command>finger</command></primary>
</indexterm>
<indexterm>
<primary>sandboxes</primary>
</indexterm>
<indexterm>
<primary><application>sshd</application></primary>
</indexterm>
<indexterm>
<primary><application>telnetd</application></primary>
</indexterm>
<indexterm>
<primary><application>rshd</application></primary>
</indexterm>
<indexterm>
<primary><application>rlogind</application></primary>
</indexterm>
<para>The prudent sysadmin only runs the servers he needs to, no
more, no less. Be aware that third party servers are often the
most bug-prone. For example, running an old version of
<application>imapd</application> or
<application>popper</application> is like giving a universal
<username>root</username> ticket out to the entire world.
Never run a server that you have not checked out carefully.
Many servers do not need to be run as <username>root</username>.
For example, the <application>ntalk</application>,
<application>comsat</application>, and
<application>finger</application> daemons can be run in special
user <firstterm>sandboxes</firstterm>. A sandbox is not perfect,
unless you go through a large amount of trouble, but the onion
approach to security still stands: If someone is able to break
in through a server running in a sandbox, they still have to
break out of the sandbox. The more layers the attacker must
break through, the lower the likelihood of his success. Root
holes have historically been found in virtually every server
ever run as <username>root</username>, including basic system servers.
If you are running a machine through which people only login via
<application>sshd</application> and never login via
<application>telnetd</application> or
<application>rshd</application> or
<application>rlogind</application>, then turn off those
services!</para>
<para>&os; now defaults to running
<application>ntalkd</application>,
<application>comsat</application>, and
<application>finger</application> in a sandbox. Another program
which may be a candidate for running in a sandbox is &man.named.8;.
<filename>/etc/defaults/rc.conf</filename> includes the arguments
necessary to run <application>named</application> in a sandbox in a
commented-out form. Depending on whether you are installing a new
system or upgrading an existing system, the special user accounts
used by these sandboxes may not be installed. The prudent
sysadmin would research and implement sandboxes for servers
whenever possible.</para>
<indexterm>
<primary><application>sendmail</application></primary>
</indexterm>
<para>There are a number of other servers that typically do not run
in sandboxes: <application>sendmail</application>,
<application>popper</application>,
<application>imapd</application>, <application>ftpd</application>,
and others. There are alternatives to some of these, but
installing them may require more work than you are willing to
perform (the convenience factor strikes again). You may have to
run these servers as <username>root</username> and rely on other
mechanisms to detect break-ins that might occur through them.</para>
<para>The other big potential <username>root</username> holes in a
system are the
suid-root and sgid binaries installed on the system. Most of
these binaries, such as <application>rlogin</application>, reside
in <filename>/bin</filename>, <filename>/sbin</filename>,
<filename>/usr/bin</filename>, or <filename>/usr/sbin</filename>.
While nothing is 100% safe, the system-default suid and sgid
binaries can be considered reasonably safe. Still,
<username>root</username> holes are occasionally found in these
binaries. A <username>root</username> hole was found in
<literal>Xlib</literal> in 1998 that made
<application>xterm</application> (which is typically suid)
vulnerable. It is better to be safe than sorry and the prudent
sysadmin will restrict suid binaries, that only staff should run,
to a special group that only staff can access, and get rid of
(<command>chmod 000</command>) any suid binaries that nobody uses.
A server with no display generally does not need an
<application>xterm</application> binary. Sgid binaries can be
almost as dangerous. If an intruder can break an sgid-kmem binary,
the intruder might be able to read <filename>/dev/kmem</filename>
and thus read the encrypted password file, potentially compromising
any passworded account. Alternatively an intruder who breaks
group <literal>kmem</literal> can monitor keystrokes sent through
ptys, including ptys used by users who login through secure
methods. An intruder that breaks the <groupname>tty</groupname>
group can write to
almost any user's tty. If a user is running a terminal program or
emulator with a keyboard-simulation feature, the intruder can
potentially generate a data stream that causes the user's terminal
to echo a command, which is then run as that user.</para>
</sect2>
<sect2 id="secure-users">
<title>Securing User Accounts</title>
<para>User accounts are usually the most difficult to secure. While
you can impose draconian access restrictions on your staff and
<quote>star</quote> out their passwords, you may not be able to
do so with any general user accounts you might have. If you do
have sufficient control, then you may win out and be able to secure
the user accounts properly. If not, you simply have to be more
vigilant in your monitoring of those accounts. Use of
ssh and Kerberos for user accounts is
more problematic, due to the extra administration and technical
support required, but still a very good solution compared to a
encrypted password file.</para>
</sect2>
<sect2>
<title>Securing the Password File</title>
<para>The only sure fire way is to star out as many
passwords as you can and use ssh or
Kerberos for access to those accounts. Even though the encrypted
password file (<filename>/etc/spwd.db</filename>) can only be read
by <username>root</username>, it may be possible for an intruder
to obtain read access to that file even if the attacker cannot
obtain root-write access.</para>
<para>Your security scripts should always check for and report
changes to the password file (see the <link
linkend="security-integrity">Checking file integrity</link> section
below).</para>
</sect2>
<sect2>
<title>Securing the Kernel Core, Raw Devices, and
File systems</title>
<para>If an attacker breaks <username>root</username> he can do
just about anything, but
there are certain conveniences. For example, most modern kernels
have a packet sniffing device driver built in. Under &os; it
is called the <devicename>bpf</devicename> device. An intruder
will commonly attempt to run a packet sniffer on a compromised
machine. You do not need to give the intruder the capability and
most systems do not have the need for the
<devicename>bpf</devicename> device compiled in.</para>
<indexterm>
<primary><command>sysctl</command></primary>
</indexterm>
<para>But even if you turn off the <devicename>bpf</devicename>
device, you still have
<filename>/dev/mem</filename> and
<filename>/dev/kmem</filename>
to worry about. For that matter, the intruder can still write to
raw disk devices. Also, there is another kernel feature called
the module loader, &man.kldload.8;. An enterprising intruder can
use a KLD module to install his own <devicename>bpf</devicename>
device, or other sniffing
device, on a running kernel. To avoid these problems you have to
run the kernel at a higher secure level, at least securelevel 1.
The securelevel can be set with a <command>sysctl</command> on
the <varname>kern.securelevel</varname> variable. Once you have
set the securelevel to 1, write access to raw devices will be
denied and special <command>chflags</command> flags,
such as <literal>schg</literal>,
will be enforced. You must also ensure that the
<literal>schg</literal> flag is set on critical startup binaries,
directories, and script files — everything that gets run up
to the point where the securelevel is set. This might be overdoing
it, and upgrading the system is much more difficult when you
operate at a higher secure level. You may compromise and run the
system at a higher secure level but not set the
<literal>schg</literal> flag for every system file and directory
under the sun. Another possibility is to simply mount
<filename>/</filename> and <filename>/usr</filename> read-only.
It should be noted that being too draconian in what you attempt to
protect may prevent the all-important detection of an
intrusion.</para>
</sect2>
<sect2 id="security-integrity">
<title>Checking File Integrity: Binaries, Configuration Files,
Etc.</title>
<para>When it comes right down to it, you can only protect your core
system configuration and control files so much before the
convenience factor rears its ugly head. For example, using
<command>chflags</command> to set the <literal>schg</literal> bit
on most of the files in <filename>/</filename> and
<filename>/usr</filename> is probably counterproductive, because
while it may protect the files, it also closes a detection window.
The last layer of your security onion is perhaps the most
important — detection. The rest of your security is pretty
much useless (or, worse, presents you with a false sense of
security) if you cannot detect potential intrusions. Half the job
of the onion is to slow down the attacker, rather than stop him, in
order to be able to catch him in the act.</para>
<para>The best way to detect an intrusion is to look for modified,
missing, or unexpected files. The best way to look for modified
files is from another (often centralized) limited-access system.
Writing your security scripts on the extra-secure limited-access
system makes them mostly invisible to potential attackers, and this
is important. In order to take maximum advantage you generally
have to give the limited-access box significant access to the
other machines in the business, usually either by doing a
read-only NFS export of the other machines to the limited-access
box, or by setting up ssh key-pairs to
allow the limited-access box to ssh to
the other machines. Except for its network traffic, NFS is the
least visible method — allowing you to monitor the
file systems on each client box virtually undetected. If your
limited-access server is connected to the client boxes through a
switch, the NFS method is often the better choice. If your
limited-access server is connected to the client boxes through a
hub, or through several layers of routing, the NFS method may be
too insecure (network-wise) and using
ssh may be the better choice even with
the audit-trail tracks that ssh
lays.</para>
<para>Once you have given a limited-access box at least read access to the
client systems it is supposed to monitor, you must write scripts
to do the actual monitoring. Given an NFS mount, you can write
scripts out of simple system utilities such as &man.find.1; and
&man.md5.1;. It is best to physically md5 the client-box files
at least once a day, and to test control files such as those
found in <filename>/etc</filename> and
<filename>/usr/local/etc</filename> even more often. When
mismatches are found, relative to the base md5 information the
limited-access machine knows is valid, it should scream at a
sysadmin to go check it out. A good security script will also
check for inappropriate suid binaries and for new or deleted files
on system partitions such as <filename>/</filename> and
<filename>/usr</filename>.</para>
<para>When using ssh rather than NFS,
writing the security script is much more difficult. You
essentially have to <command>scp</command> the scripts to the client
box in order to
run them, making them visible, and for safety you also need to
<command>scp</command> the binaries (such as find) that those
scripts use. The <application>ssh</application> client on the
client box may already be compromised. All in all, using
ssh may be necessary when running over
insecure links, but it is also a lot harder to deal with.</para>
<para>A good security script will also check for changes to user and
staff members access configuration files:
<filename>.rhosts</filename>, <filename>.shosts</filename>,
<filename>.ssh/authorized_keys</filename> and so forth,
files that might fall outside the purview of the
<literal>MD5</literal> check.</para>
<para>If you have a huge amount of user disk space, it may take too
long to run through every file on those partitions. In this case,
setting mount flags to disallow suid binaries and devices on those
partitions is a good idea. The <literal>nodev</literal> and
<literal>nosuid</literal> options (see &man.mount.8;) are what you
want to look into. You should probably scan them anyway, at least
once a week, since the object of this layer is to detect a break-in
attempt, whether or not the attempt succeeds.</para>
<para>Process accounting (see &man.accton.8;) is a relatively
low-overhead feature of the operating system which might help
as a post-break-in evaluation mechanism. It is especially
useful in tracking down how an intruder has actually broken into
a system, assuming the file is still intact after the break-in has
occured.</para>
<para>Finally, security scripts should process the log files, and the
logs themselves should be generated in as secure a manner as
possible — remote syslog can be very useful. An intruder
will try to cover his tracks, and log files are critical to the
sysadmin trying to track down the time and method of the initial
break-in. One way to keep a permanent record of the log files is
to run the system console to a serial port and collect the
information to a secure machine monitoring the consoles.</para>
</sect2>
<sect2>
<title>Paranoia</title>
<para>A little paranoia never hurts. As a rule, a sysadmin can add
any number of security features, as long as they do not affect
convenience, and can add security features that
<emphasis>do</emphasis> affect convenience with some added thought.
Even more importantly, a security administrator should mix it up a
bit — if you use recommendations such as those given by this
document verbatim, you give away your methodologies to the
prospective attacker who also has access to this document.</para>
</sect2>
<sect2>
<title>Denial of Service Attacks</title>
<indexterm><primary>Denial of Service (DoS)</primary></indexterm>
<para>This section covers Denial of Service attacks. A DoS attack
is typically a packet attack. While there is not much you can do
about modern spoofed packet attacks that saturate your network,
you can generally limit the damage by ensuring that the attacks
cannot take down your servers by:</para>
<orderedlist>
<listitem>
<para>Limiting server forks.</para>
</listitem>
<listitem>
<para>Limiting springboard attacks (ICMP response attacks, ping
broadcast, etc.).</para>
</listitem>
<listitem>
<para>Overloading the Kernel Route Cache.</para>
</listitem>
</orderedlist>
<para>A common DoS attack scenario is attacking a forking server and
making it spawning so many child processes that the host system
eventually runs out of memory, file descriptors, etc. and then
grinds to a halt. <application>inetd</application>
(see &man.inetd.8;) has several
options to limit this sort of attack. It should be noted that
while it is possible to prevent a machine from going down, it is
not generally possible to prevent a service from being disrupted
by the attack. Read the <application>inetd</application> manual
page carefully and pay
specific attention to the <option>-c</option>, <option>-C</option>,
and <option>-R</option> options. Note that spoofed-IP attacks
will circumvent the <option>-C</option> option to
<application>inetd</application>, so
typically a combination of options must be used. Some standalone
servers have self-fork-limitation parameters.</para>
<para><application>Sendmail</application> has its
<option>-OMaxDaemonChildren</option> option, which tends to work
much better than trying to use <application>Sendmail</application>'s load limiting options
due to the load lag. You should specify a
<literal>MaxDaemonChildren</literal> parameter, when you start
<application>sendmail</application>; high enough to handle your
expected load, but not so high that the computer cannot handle that
number of <application>Sendmail</application> instances without falling on
its face. It is also prudent to run <application>Sendmail</application> in queued mode
(<option>-ODeliveryMode=queued</option>) and to run the daemon
(<command>sendmail -bd</command>) separate from the queue-runs
(<command>sendmail -q15m</command>). If you still want real-time
delivery you can run the queue at a much lower interval, such as
<option>-q1m</option>, but be sure to specify a reasonable
<literal>MaxDaemonChildren</literal> option for
<emphasis>that</emphasis> <application>Sendmail</application> to prevent cascade failures.</para>
<para><application>Syslogd</application> can be attacked directly
and it is strongly recommended that you use the <option>-s</option>
option whenever possible, and the <option>-a</option> option
otherwise.</para>
<para>You should also be fairly careful with connect-back services
such as <application>TCP Wrapper</application>'s reverse-identd,
which can be attacked directly. You generally do not want to use
the reverse-ident feature of
<application>TCP Wrapper</application> for this reason.</para>
<para>It is a very good idea to protect internal services from
external access by firewalling them off at your border routers.
The idea here is to prevent saturation attacks from outside your
LAN, not so much to protect internal services from network-based
<username>root</username> compromise.
Always configure an exclusive firewall, i.e.,
<quote>firewall everything <emphasis>except</emphasis> ports A, B,
C, D, and M-Z</quote>. This way you can firewall off all of your
low ports except for certain specific services such as
<application>named</application> (if you are primary for a zone),
<application>ntalkd</application>,
<application>sendmail</application>, and other Internet-accessible
services. If you try to configure the firewall the other way
— as an inclusive or permissive firewall, there is a good
chance that you will forget to <quote>close</quote> a couple of
services, or that you will add a new internal service and forget
to update the firewall. You can still open up the high-numbered
port range on the firewall, to allow permissive-like operation,
without compromising your low ports. Also take note that &os;
allows you to control the range of port numbers used for dynamic
binding, via the various <varname>net.inet.ip.portrange</varname>
<command>sysctl</command>'s (<command>sysctl -a | fgrep
portrange</command>), which can also ease the complexity of your
firewall's configuration. For example, you might use a normal
first/last range of 4000 to 5000, and a hiport range of 49152 to
65535, then block off everything under 4000 in your firewall
(except for certain specific Internet-accessible ports, of
course).</para>
<para>Another common DoS attack is called a springboard attack
— to attack a server in a manner that causes the server to
generate responses which overloads the server, the local
network, or some other machine. The most common attack of this
nature is the <emphasis>ICMP ping broadcast attack</emphasis>.
The attacker spoofs ping packets sent to your LAN's broadcast
address with the source IP address set to the actual machine they
wish to attack. If your border routers are not configured to
stomp on ping packets to broadcast addresses, your LAN winds up
generating sufficient responses to the spoofed source address to
saturate the victim, especially when the attacker uses the same
trick on several dozen broadcast addresses over several dozen
different networks at once. Broadcast attacks of over a hundred
and twenty megabits have been measured. A second common
springboard attack is against the ICMP error reporting system.
By constructing packets that generate ICMP error responses, an
attacker can saturate a server's incoming network and cause the
server to saturate its outgoing network with ICMP responses. This
type of attack can also crash the server by running it out of
memory, especially if the server cannot drain the ICMP responses
it generates fast enough.
Use the <application>sysctl</application>
variable <literal>net.inet.icmp.icmplim</literal> to limit these attacks.
The last major class of springboard
attacks is related to certain internal
<application>inetd</application> services such as the
udp echo service. An attacker simply spoofs a UDP packet with the
source address being server A's echo port, and the destination
address being server B's echo port, where server A and B are both
on your LAN. The two servers then bounce this one packet back and
forth between each other. The attacker can overload both servers
and their LANs simply by injecting a few packets in this manner.
Similar problems exist with the internal
<application>chargen</application> port. A
competent sysadmin will turn off all of these inetd-internal test
services.</para>
<para>Spoofed packet attacks may also be used to overload the kernel
route cache. Refer to the <varname>net.inet.ip.rtexpire</varname>,
<varname>rtminexpire</varname>, and <varname>rtmaxcache</varname>
<command>sysctl</command> parameters. A spoofed packet attack
that uses a random source IP will cause the kernel to generate a
temporary cached route in the route table, viewable with
<command>netstat -rna | fgrep W3</command>. These routes
typically timeout in 1600 seconds or so. If the kernel detects
that the cached route table has gotten too big it will dynamically
reduce the <varname>rtexpire</varname> but will never decrease it
to less than <varname>rtminexpire</varname>. There are two
problems:</para>
<orderedlist>
<listitem>
<para>The kernel does not react quickly enough when a lightly
loaded server is suddenly attacked.</para>
</listitem>
<listitem>
<para>The <varname>rtminexpire</varname> is not low enough for
the kernel to survive a sustained attack.</para>
</listitem>
</orderedlist>
<para>If your servers are connected to the Internet via a T3 or
better, it may be prudent to manually override both
<varname>rtexpire</varname> and <varname>rtminexpire</varname>
via &man.sysctl.8;. Never set either parameter to zero (unless
you want to crash the machine). Setting both
parameters to 2 seconds should be sufficient to protect the route
table from attack.</para>
</sect2>
<sect2>
<title>Access Issues with Kerberos and SSH</title>
<indexterm><primary><command>ssh</command></primary></indexterm>
<indexterm><primary>KerberosIV</primary></indexterm>
<para>There are a few issues with both Kerberos and
ssh that need to be addressed if
you intend to use them. Kerberos 5 is an excellent
authentication protocol, but there are bugs in the kerberized
<application>telnet</application> and
<application>rlogin</application> applications that make them
unsuitable for dealing with binary streams. Also, by default
Kerberos does not encrypt a session unless you use the
<option>-x</option> option. <application>ssh</application>
encrypts everything by default.</para>
<para>Ssh works quite well in every
respect except that it forwards encryption keys by default. What
this means is that if you have a secure workstation holding keys
that give you access to the rest of the system, and you
ssh to an insecure machine, your keys
are usable. The actual keys themselves are not exposed, but
ssh installs a forwarding port for the
duration of your login, and if an attacker has broken
<username>root</username> on the
insecure machine he can utilize that port to use your keys to gain
access to any other machine that your keys unlock.</para>
<para>We recommend that you use ssh in
combination with Kerberos whenever possible for staff logins.
<application>Ssh</application> can be compiled with Kerberos
support. This reduces your reliance on potentially exposed
ssh keys while at the same time
protecting passwords via Kerberos. Ssh
keys should only be used for automated tasks from secure machines
(something that Kerberos is unsuited to do). We also recommend that
you either turn off key-forwarding in the
ssh configuration, or that you make use
of the <literal>from=IP/DOMAIN</literal> option that
ssh allows in its
<filename>authorized_keys</filename> file to make the key only
usable to entities logging in from specific machines.</para>
</sect2>
</sect1>
<sect1 id="crypt">
<sect1info>
<authorgroup>
<author>
<firstname>Bill</firstname>
<surname>Swingle</surname>
<contrib>Parts rewritten and updated by </contrib>
</author>
</authorgroup>
<!-- 21 Mar 2000 -->
</sect1info>
<title>DES, MD5, and Crypt</title>
<indexterm>
<primary>security</primary>
<secondary>crypt</secondary>
</indexterm>
<indexterm><primary>crypt</primary></indexterm>
<indexterm><primary>DES</primary></indexterm>
<indexterm><primary>MD5</primary></indexterm>
<para>Every user on a &unix; system has a password associated with
their account. It seems obvious that these passwords need to be
known only to the user and the actual operating system. In
order to keep these passwords secret, they are encrypted with
what is known as a <quote>one-way hash</quote>, that is, they can
only be easily encrypted but not decrypted. In other words, what
we told you a moment ago was obvious is not even true: the
operating system itself does not <emphasis>really</emphasis> know
the password. It only knows the <emphasis>encrypted</emphasis>
form of the password. The only way to get the
<quote>plain-text</quote> password is by a brute force search of the
space of possible passwords.</para>
<para>Unfortunately the only secure way to encrypt passwords when
&unix; came into being was based on DES, the Data Encryption
Standard. This was not such a problem for users resident in
the US, but since the source code for DES could not be exported
outside the US, &os; had to find a way to both comply with
US law and retain compatibility with all the other &unix;
variants that still used DES.</para>
<para>The solution was to divide up the encryption libraries
so that US users could install the DES libraries and use
DES but international users still had an encryption method
that could be exported abroad. This is how &os; came to
use MD5 as its default encryption method. MD5 is believed to
be more secure than DES, so installing DES is offered primarily
for compatibility reasons.</para>
<sect2>
<title>Recognizing Your Crypt Mechanism</title>
<para>Currently the library supports DES, MD5 and Blowfish hash
functions. By default &os; uses MD5 to encrypt
passwords.</para>
<para>It is pretty easy to identify which encryption method
&os; is set up to use. Examining the encrypted passwords in
the <filename>/etc/master.passwd</filename> file is one way.
Passwords encrypted with the MD5 hash are longer than those
encrypted with the DES hash and also begin with the characters
<literal>$1$</literal>. Passwords starting with
<literal>$2a$</literal> are encrypted with the
Blowfish hash function. DES password strings do not
have any particular identifying characteristics, but they are
shorter than MD5 passwords, and are coded in a 64-character
alphabet which does not include the <literal>$</literal>
character, so a relatively short string which does not begin with
a dollar sign is very likely a DES password.</para>
<para>The password format used for new passwords is controlled
by the <literal>passwd_format</literal> login capability in
<filename>/etc/login.conf</filename>, which takes values of
<literal>des</literal>, <literal>md5</literal> or
<literal>blf</literal>. See the &man.login.conf.5; manual page
for more information about login capabilities.</para>
</sect2>
</sect1>
<sect1 id="one-time-passwords">
<title>One-time Passwords</title>
<indexterm><primary>one-time passwords</primary></indexterm>
<indexterm>
<primary>security</primary>
<secondary>one-time passwords</secondary>
</indexterm>
<para>By default, &os; includes support for OPIE (One-time Passwords
In Everything), which uses the MD5 hash by default.</para>
<para>There are three different sorts of passwords which we will discuss
below. The first is your usual &unix; style or
Kerberos password; we will call this a <quote>&unix; password</quote>.
The second sort is the one-time password which is generated by the OPIE
&man.opiekey.1; program and accepted by the
&man.opiepasswd.1; program
and the login prompt; we will
call this a <quote>one-time password</quote>. The final sort of
password is the secret password which you give to the
<command>opiekey</command> program (and
sometimes the
<command>opiepasswd</command> programs)
which it uses to generate
one-time passwords; we will call it a <quote>secret password</quote>
or just unqualified <quote>password</quote>.</para>
<para>The secret password does not have anything to do with your &unix;
password; they can be the same but this is not recommended.
OPIE secret passwords are not limited to 8 characters like old
&unix; passwords<footnote><para>Under &os; the standard login
password may be up to 128 characters in length.</para></footnote>,
they can be as long as you like. Passwords of six or
seven word long phrases are fairly common. For the most part, the
OPIE system operates completely independently of the &unix;
password system.</para>
<para>Besides the password, there are two other pieces of data that
are important to OPIE. One is what is known as the
<quote>seed</quote> or <quote>key</quote>, consisting of two letters
and five digits. The other is what is called the <quote>iteration
count</quote>, a number between 1 and 100. OPIE creates the
one-time password by concatenating the seed and the secret password,
then applying the MD5 hash as many times as specified by the
iteration count and turning the result into six short English words.
These six English words are your one-time password. The
authentication system (primarily PAM) keeps
track of the last one-time password used, and the user is
authenticated if the hash of the user-provided password is equal to
the previous password. Because a one-way hash is used it is
impossible to generate future one-time passwords if a successfully
used password is captured; the iteration count is decremented after
each successful login to keep the user and the login program in
sync. When the iteration count gets down to 1, OPIE must be
reinitialized.</para>
<para>There are a few programs involved in each system
which we will discuss below. The
<command>opiekey</command> program accepts an iteration
count, a seed, and a secret password, and generates a one-time
password or a consecutive list of one-time passwords. The
<command>opiepasswd</command>
program is used to initialize OPIE,
and to change passwords, iteration counts, or seeds; it
takes either a secret passphrase, or an iteration count,
seed, and a one-time password. The
<command>opieinfo</command> program will examine the
relevant credentials files
(<filename>/etc/opiekeys</filename>) and print out the invoking user's
current iteration count and seed.</para>
<para>There are four different sorts of operations we will cover. The
first is using
<command>opiepasswd</command> over a secure connection to set up
one-time-passwords for the first time, or to change your password
or seed. The second operation is using
<command>opiepasswd</command> over an insecure connection, in
conjunction with <command>opiekey</command>
over a secure connection, to do the same. The third is using
<command>opiekey</command> to log in over
an insecure connection. The fourth is using
<command>opiekey</command> to generate a number of keys which
can be written down or printed out to carry with you when going to
some location without secure connections to anywhere.</para>
<sect2>
<title>Secure Connection Initialization</title>
<para>To initialize OPIE for the first time, execute the
<command>opiepasswd</command> command:</para>
<screen>&prompt.user; <userinput>opiepasswd -c</userinput>
[grimreaper] ~ $ opiepasswd -f -c
Adding unfurl:
Only use this method from the console; NEVER from remote. If you are using
telnet, xterm, or a dial-in, type ^C now or exit with no password.
Then run opiepasswd without the -c parameter.
Using MD5 to compute responses.
Enter new secret pass phrase:
Again new secret pass phrase:
ID unfurl OTP key is 499 to4268
MOS MALL GOAT ARM AVID COED
</screen>
<para>At the <prompt>Enter new secret pass phrase:</prompt> or
<prompt>Enter secret password:</prompt> prompts, you
should enter a password or phrase. Remember, this is not the
password that you will use to login with, this is used to generate
your one-time login keys. The <quote>ID</quote> line gives the
parameters of your particular instance: your login name, the
iteration count, and seed. When logging in the system
will remember these parameters and present them back to you so you
do not have to remember them. The last line gives the particular
one-time password which corresponds to those parameters and your
secret password; if you were to re-login immediately, this
one-time password is the one you would use.</para>
</sect2>
<sect2>
<title>Insecure Connection Initialization</title>
<para>To initialize or change your secret password over an
insecure connection, you will need to already have a secure
connection to some place where you can run
<command>opiekey</command>; this might be in the form of a shell
prompt on a machine you
trust. You will also need to make up an iteration count (100 is
probably a good value), and you may make up your own seed or use a
randomly-generated one. Over on the insecure connection (to the
machine you are initializing), use <command>opiepasswd</command>:</para>
<screen>&prompt.user; <userinput>opiepasswd</userinput>
Updating unfurl:
You need the response from an OTP generator.
Old secret pass phrase:
otp-md5 498 to4268 ext
Response: GAME GAG WELT OUT DOWN CHAT
New secret pass phrase:
otp-md5 499 to4269
Response: LINE PAP MILK NELL BUOY TROY
ID mark OTP key is 499 gr4269
LINE PAP MILK NELL BUOY TROY
</screen>
<para>To accept the default seed press <keycap>Return</keycap>.
Then before entering an
access password, move over to your secure connection and give it
the same parameters:</para>
<screen>&prompt.user; <userinput>opiekey 498 to4268</userinput>
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT
</screen>
<para>Now switch back over to the insecure connection, and copy the
one-time password generated over to the relevant program.</para>
</sect2>
<sect2>
<title>Generating a Single One-time Password</title>
<para>Once you have initialized OPIE and login, you will be
presented with a prompt like this:</para>
<screen>&prompt.user; <userinput>telnet example.com</userinput>
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.
FreeBSD/i386 (example.com) (ttypa)
login: <userinput><username></userinput>
otp-md5 498 gr4269 ext
Password: </screen>
<para>As a side note, the OPIE prompts have a useful feature
(not shown here): if you press <keycap>Return</keycap>
at the password prompt, the
prompter will turn echo on, so you can see what you are
typing. This can be extremely useful if you are attempting to
type in a password by hand, such as from a printout.</para>
<indexterm><primary>MS-DOS</primary></indexterm>
<indexterm><primary>Windows</primary></indexterm>
<indexterm><primary>MacOS</primary></indexterm>
<para>At this point you need to generate your one-time password to
answer this login prompt. This must be done on a trusted system
that you can run
<command>opiekey</command> on. (There are versions of these for DOS,
&windows; and &macos; as well.) They need the iteration count and
the seed as command line options. You can cut-and-paste these
right from the login prompt on the machine that you are logging
in to.</para>
<para>On the trusted system:</para>
<screen>&prompt.user; <userinput>opiekey 498 to4268</userinput>
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT</screen>
<para>Now that you have your one-time password you can continue
logging in.</para>
</sect2>
<sect2>
<title>Generating Multiple One-time Passwords</title>
<para>Sometimes you have to go places where you do not have
access to a trusted machine or secure connection. In this case,
it is possible to use the
<command>opiekey</command> command to
generate a number of one-time passwords beforehand to be printed
out and taken with you. For example:</para>
<screen>&prompt.user; <userinput>opiekey -n 5 30 zz99999</userinput>
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase: <userinput><secret password></userinput>
26: JOAN BORE FOSS DES NAY QUIT
27: LATE BIAS SLAY FOLK MUCH TRIG
28: SALT TIN ANTI LOON NEAL USE
29: RIO ODIN GO BYE FURY TIC
30: GREW JIVE SAN GIRD BOIL PHI</screen>
<para>The <option>-n 5</option> requests five keys in sequence, the
<option>30</option> specifies what the last iteration number
should be. Note that these are printed out in
<emphasis>reverse</emphasis> order of eventual use. If you are
really paranoid, you might want to write the results down by hand;
otherwise you can cut-and-paste into <command>lpr</command>. Note
that each line shows both the iteration count and the one-time
password; you may still find it handy to scratch off passwords as
you use them.</para>
</sect2>
<sect2>
<title>Restricting Use of &unix; Passwords</title>
<para>OPIE can restrict the use of &unix; passwords based on the IP
address of a login session. The relevant file
is <filename>/etc/opieaccess</filename>, which is present by default.
Please check &man.opieaccess.5;
for more information on this file and which security considerations
you should be aware of when using it.</para>
<para>Here is a sample <filename>opieaccess</filename> file:</para>
<programlisting>permit 192.168.0.0 255.255.0.0</programlisting>
<para>This line allows users whose IP source address (which is
vulnerable to spoofing) matches the specified value and mask,
to use &unix; passwords at any time.</para>
<para>If no rules in <filename>opieaccess</filename> are matched,
the default is to deny non-OPIE logins.</para>
</sect2>
</sect1>
<sect1 id="tcpwrappers">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Written by: </contrib>
</author>
</authorgroup>
</sect1info>
<title>TCP Wrappers</title>
<indexterm><primary>TCP Wrappers</primary></indexterm>
<para>Anyone familiar with &man.inetd.8; has probably heard
of <acronym>TCP</acronym> Wrappers at some point. But few
individuals seem to fully comprehend its usefulness in a
network environment. It seems that everyone wants to
install a firewall to handle network connections. While a
firewall has a wide variety of uses, there are some things
that a firewall not handle such as sending text back to the
connection originator. The <acronym>TCP</acronym> software
does this and much more. In the next few sections many of
the <acronym>TCP</acronym> Wrappers features will be discussed,
and, when applicable, example configuration lines will be
provided.</para>
<para>The <acronym>TCP</acronym> Wrappers software extends the
abilities of <command>inetd</command> to provide support for
every server daemon under its control. Using this method it
is possible to provide logging support, return messages to
connections, permit a daemon to only accept internal connections,
etc. While some of these features can be provided by implementing
a firewall, this will add not only an extra layer of protection
but go beyond the amount of control a firewall can
provide.</para>
<para>The added functionality of <acronym>TCP</acronym> Wrappers
should not be considered a replacement for a good firewall.
<acronym>TCP</acronym> Wrappers can be used in conjunction
with a firewall or other security enhancements though and
it can serve nicely as an extra layer of protection
for the system.</para>
<para>Since this is an extension to the configuration of
<command>inetd</command>, the reader is expected have
read the <link linkend="network-inetd">inetd configuration</link>
section.</para>
<note>
<para>While programs run by &man.inetd.8; are not exactly
<quote>daemons</quote>, they have traditionally been called
daemons. This is the term we will use in this section too.</para>
</note>
<sect2>
<title>Initial Configuration</title>
<para>The only requirement of using <acronym>TCP</acronym>
Wrappers in &os; is to ensure the <command>inetd</command>
server is started from <filename>rc.conf</filename> with the
<option>-Ww</option> option; this is the default setting. Of
course, proper configuration of
<filename>/etc/hosts.allow</filename> is also expected, but
&man.syslogd.8; will throw messages in the system logs in
these cases.</para>
<note>
<para>Unlike other implementations of <acronym>TCP</acronym>
Wrappers, the use of <filename>hosts.deny</filename> has
been deprecated. All configuration options should be placed
in <filename>/etc/hosts.allow</filename>.</para>
</note>
<para>In the simplest configuration, daemon connection policies
are set to either be permitted or blocked depending on the
options in <filename>/etc/hosts.allow</filename>. The default
configuration in &os; is to allow a connection to every daemon
started with <command>inetd</command>. Changing this will be
discussed only after the basic configuration is covered.</para>
<para>Basic configuration usually takes the form of
<literal>daemon : address : action</literal>. Where
<literal>daemon</literal> is the daemon name which
<command>inetd</command> started. The
<literal>address</literal> can be a valid hostname, an
<acronym>IP</acronym> address or an IPv6 address enclosed in
brackets ([ ]). The action field can be either allow
or deny to grant or deny access appropriately. Keep in mind
that configuration works off a first rule match semantic,
meaning that the configuration file is scanned in ascending
order for a matching rule. When a match is found the rule
is applied and the search process will halt.</para>
<para>Several other options exist but they will be explained
in a later section. A simple configuration line may easily be
constructed from that information alone. For example, to
allow <acronym>POP</acronym>3 connections via the
<filename role="package">mail/qpopper</filename> daemon,
the following lines should be appended to
<filename>hosts.allow</filename>:</para>
<programlisting># This line is required for POP3 connections:
qpopper : ALL : allow</programlisting>
<para>After adding this line, <command>inetd</command> will need
restarted. This can be accomplished by use of the &man.kill.1;
command, or with the <parameter>restart</parameter> parameter
with <filename>/etc/rc.d/inetd</filename>.</para>
</sect2>
<sect2>
<title>Advanced Configuration</title>
<para><acronym>TCP</acronym> Wrappers has advanced
options too; they will allow for more control over the
way connections are handled. In some cases it may be
a good idea to return a comment to certain hosts or
daemon connections. In other cases, perhaps a log file
should be recorded or an email sent to the administrator.
Other situations may require the use of a service for local
connections only. This is all possible through the use of
configuration options known as <literal>wildcards</literal>,
expansion characters and external command execution. The
next two sections are written to cover these situations.</para>
<sect3>
<title>External Commands</title>
<para>Suppose that a situation occurs where a connection
should be denied yet a reason should be sent to the
individual who attempted to establish that connection. How
could it be done? That action can be made possible by
using the <option>twist</option> option. When a connection
attempt is made, <option>twist</option> will be called to
execute a shell command or script. An example already exists
in the <filename>hosts.allow</filename> file:</para>
<programlisting># The rest of the daemons are protected.
ALL : ALL \
: severity auth.info \
: twist /bin/echo "You are not welcome to use %d from %h."</programlisting>
<para>This example shows that the message,
<quote>You are not allowed to use <literal>daemon</literal>
from <literal>hostname</literal>.</quote> will be returned
for any daemon not previously configured in the access file.
This is extremely useful for sending a reply back to the
connection initiator right after the established connection
is dropped. Note that any message returned
<emphasis>must</emphasis> be wrapped in quote
<literal>"</literal> characters; there are no exceptions to
this rule.</para>
<warning>
<para>It may be possible to launch a denial of service attack
on the server if an attacker, or group of attackers could
flood these daemons with connection requests.</para>
</warning>
<para>Another possibility is to use the <option>spawn</option>
option in these cases. Like <option>twist</option>, the
<option>spawn</option> implicitly denies the connection and
may be used to run external shell commands or scripts.
Unlike <option>twist</option>, <option>spawn</option> will
not send a reply back to the individual who established the
connection. For an example, consider the following
configuration line:</para>
<programlisting># We do not allow connections from example.com:
ALL : .example.com \
: spawn (/bin/echo %a from %h attempted to access %d >> \
/var/log/connections.log) \
: deny</programlisting>
<para>This will deny all connection attempts from the
<hostid role="fqdn">*.example.com</hostid> domain;
simultaneously logging the hostname, <acronym>IP</acronym>
address and the daemon which they attempted to access in the
<filename>/var/log/connections.log</filename> file.</para>
<para>Aside from the already explained substitution characters
above, e.g. %a, a few others exist. See the
&man.hosts.access.5; manual page for the complete list.</para>
</sect3>
<sect3>
<title>Wildcard Options</title>
<para>Thus far the <literal>ALL</literal> example has been used
continuously throughout the examples. Other options exist
which could extend the functionality a bit further. For
instance, <literal>ALL</literal> may be used to match every
instance of either a daemon, domain or an
<acronym>IP</acronym> address. Another wildcard available is
<literal>PARANOID</literal> which may be used to match any
host which provides an <acronym>IP</acronym> address that may
be forged. In other words, <literal>paranoid</literal> may
be used to define an action to be taken whenever a connection
is made from an <acronym>IP</acronym> address that differs
from its hostname. The following example may shed some more
light on this discussion:</para>
<programlisting># Block possibly spoofed requests to sendmail:
sendmail : PARANOID : deny</programlisting>
<para>In that example all connection requests to
<command>sendmail</command> which have an
<acronym>IP</acronym> address that varies from its hostname
will be denied.</para>
<caution>
<para>Using the <literal>PARANOID</literal> may severely
cripple servers if the client or server has a broken
<acronym>DNS</acronym> setup. Administrator discretion
is advised.</para>
</caution>
<para>To learn more about wildcards and their associated
functionality, see the &man.hosts.access.5; manual
page.</para>
<para>Before any of the specific configuration lines above will
work, the first configuration line should be commented out
in <filename>hosts.allow</filename>. This was noted at the
beginning of this section.</para>
</sect3>
</sect2>
</sect1>
<sect1 id="kerberosIV">
<sect1info>
<authorgroup>
<author>
<firstname>Mark</firstname>
<surname>Murray</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Mark</firstname>
<surname>Dapoz</surname>
<contrib>Based on a contribution by </contrib>
</author>
</authorgroup>
</sect1info>
<title><application>KerberosIV</application></title>
<para>Kerberos is a network add-on system/protocol that allows users to
authenticate themselves through the services of a secure server.
Services such as remote login, remote copy, secure inter-system file
copying and other high-risk tasks are made considerably safer and more
controllable.</para>
<para>The following instructions can be used as a guide on how to set up
Kerberos as distributed for &os;. However, you should refer to the
relevant manual pages for a complete description.</para>
<sect2>
<title>Installing <application>KerberosIV</application></title>
<indexterm><primary>MIT</primary></indexterm>
<indexterm>
<primary>KerberosIV</primary>
<secondary>installing</secondary>
</indexterm>
<para>Kerberos is an optional component of &os;. The easiest
way to install this software is by selecting the <literal>krb4</literal> or
<literal>krb5</literal> distribution in <application>sysinstall</application>
during the initial installation of &os;. This will install
the <quote>eBones</quote> (KerberosIV) or <quote>Heimdal</quote> (Kerberos5)
implementation of Kerberos. These implementations are
included because they are developed outside the USA/Canada and
were thus available to system owners outside those countries
during the era of restrictive export controls on cryptographic
code from the USA.</para>
<para>Alternatively, the MIT implementation of Kerberos is
available from the Ports Collection as
<filename role="package">security/krb5</filename>.</para>
</sect2>
<sect2>
<title>Creating the Initial Database</title>
<para>This is done on the Kerberos server only. First make sure that
you do not have any old Kerberos databases around. You should change
to the directory <filename>/etc/kerberosIV</filename> and check that
only the following files are present:</para>
<screen>&prompt.root; <userinput>cd /etc/kerberosIV</userinput>
&prompt.root; <userinput>ls</userinput>
README krb.conf krb.realms</screen>
<para>If any additional files (such as <filename>principal.*</filename>
or <filename>master_key</filename>) exist, then use the
<command>kdb_destroy</command> command to destroy the old Kerberos
database, or if Kerberos is not running, simply delete the extra
files.</para>
<para>You should now edit the <filename>krb.conf</filename> and
<filename>krb.realms</filename> files to define your Kerberos realm.
In this case the realm will be <literal>EXAMPLE.COM</literal> and the
server is <hostid role="fqdn">grunt.example.com</hostid>. We edit
or create the <filename>krb.conf</filename> file:</para>
<screen>&prompt.root; <userinput>cat krb.conf</userinput>
EXAMPLE.COM
EXAMPLE.COM grunt.example.com admin server
CS.BERKELEY.EDU okeeffe.berkeley.edu
ATHENA.MIT.EDU kerberos.mit.edu
ATHENA.MIT.EDU kerberos-1.mit.edu
ATHENA.MIT.EDU kerberos-2.mit.edu
ATHENA.MIT.EDU kerberos-3.mit.edu
LCS.MIT.EDU kerberos.lcs.mit.edu
TELECOM.MIT.EDU bitsy.mit.edu
ARC.NASA.GOV trident.arc.nasa.gov</screen>
<para>In this case, the other realms do not need to be there. They are
here as an example of how a machine may be made aware of multiple
realms. You may wish to not include them for simplicity.</para>
<para>The first line names the realm in which this system works. The
other lines contain realm/host entries. The first item on a line is a
realm, and the second is a host in that realm that is acting as a
<quote>key distribution center</quote>. The words <literal>admin
server</literal> following a host's name means that host also
provides an administrative database server. For further explanation
of these terms, please consult the Kerberos manual pages.</para>
<para>Now we have to add <hostid role="fqdn">grunt.example.com</hostid>
to the <literal>EXAMPLE.COM</literal> realm and also add an entry to
put all hosts in the <hostid role="domainname">.example.com</hostid>
domain in the <literal>EXAMPLE.COM</literal> realm. The
<filename>krb.realms</filename> file would be updated as
follows:</para>
<screen>&prompt.root; <userinput>cat krb.realms</userinput>
grunt.example.com EXAMPLE.COM
.example.com EXAMPLE.COM
.berkeley.edu CS.BERKELEY.EDU
.MIT.EDU ATHENA.MIT.EDU
.mit.edu ATHENA.MIT.EDU</screen>
<para>Again, the other realms do not need to be there. They are here as
an example of how a machine may be made aware of multiple realms. You
may wish to remove them to simplify things.</para>
<para>The first line puts the <emphasis>specific</emphasis> system into
the named realm. The rest of the lines show how to default systems of
a particular subdomain to a named realm.</para>
<para>Now we are ready to create the database. This only needs to run
on the Kerberos server (or Key Distribution Center). Issue the
<command>kdb_init</command> command to do this:</para>
<screen>&prompt.root; <userinput>kdb_init</userinput>
<prompt>Realm name [default ATHENA.MIT.EDU ]:</prompt> <userinput>EXAMPLE.COM</userinput>
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
<prompt>Enter Kerberos master key:</prompt> </screen>
<para>Now we have to save the key so that servers on the local machine
can pick it up. Use the <command>kstash</command> command to do
this:</para>
<screen>&prompt.root; <userinput>kstash</userinput>
<prompt>Enter Kerberos master key:</prompt>
Current Kerberos master key version is 1.
Master key entered. BEWARE!</screen>
<para>This saves the encrypted master password in
<filename>/etc/kerberosIV/master_key</filename>.</para>
</sect2>
<sect2>
<title>Making It All Run</title>
<indexterm>
<primary>KerberosIV</primary>
<secondary>initial startup</secondary>
</indexterm>
<para>Two principals need to be added to the database for
<emphasis>each</emphasis> system that will be secured with Kerberos.
Their names are <literal>kpasswd</literal> and <literal>rcmd</literal>.
These two principals are made for each system, with the instance being
the name of the individual system.</para>
<para>These daemons, <application>kpasswd</application> and
<application>rcmd</application> allow other systems to change Kerberos
passwords and run commands like &man.rcp.1;,
&man.rlogin.1; and &man.rsh.1;.</para>
<para>Now let us add these entries:</para>
<screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...
<prompt>Enter Kerberos master key:</prompt>
Current Kerberos master key version is 1.
Master key entered. BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.
<prompt>Principal name:</prompt> <userinput>passwd</userinput>
<prompt>Instance:</prompt> <userinput>grunt</userinput>
<Not found>, <prompt>Create [y] ?</prompt> <userinput>y</userinput>
Principal: passwd, Instance: grunt, kdc_key_ver: 1
<prompt>New Password:</prompt> <---- enter RANDOM here
Verifying password
<prompt>New Password:</prompt> <---- enter RANDOM here
<prompt>Random password [y] ?</prompt> <userinput>y</userinput>
Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt> <userinput>rcmd</userinput>
<prompt>Instance:</prompt> <userinput>grunt</userinput>
<Not found>, <prompt>Create [y] ?</prompt>
Principal: rcmd, Instance: grunt, kdc_key_ver: 1
<prompt>New Password:</prompt> <---- enter RANDOM here
Verifying password
<prompt>New Password:</prompt> <---- enter RANDOM here
<prompt>Random password [y] ?</prompt>
Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt> <---- null entry here will cause an exit</screen>
</sect2>
<sect2>
<title>Creating the Server File</title>
<para>We now have to extract all the instances which define the
services on each machine. For this we use the
<command>ext_srvtab</command> command. This will create a file
which must be copied or moved <emphasis>by secure
means</emphasis> to each Kerberos client's
<filename>/etc</filename> directory. This file must
be present on each server and client, and is crucial to the
operation of Kerberos.</para>
<screen>&prompt.root; <userinput>ext_srvtab grunt</userinput>
<prompt>Enter Kerberos master key:</prompt>
Current Kerberos master key version is 1.
Master key entered. BEWARE!
Generating 'grunt-new-srvtab'....</screen>
<para>Now, this command only generates a temporary file which must be
renamed to <filename>srvtab</filename> so that all the servers can pick
it up. Use the &man.mv.1; command to move it into place on
the original system:</para>
<screen>&prompt.root; <userinput>mv grunt-new-srvtab srvtab</userinput></screen>
<para>If the file is for a client system, and the network is not deemed
safe, then copy the
<filename><replaceable>client</replaceable>-new-srvtab</filename> to
removable media and transport it by secure physical means. Be sure to
rename it to <filename>srvtab</filename> in the client's
<filename>/etc</filename> directory, and make sure it is
mode 600:</para>
<screen>&prompt.root; <userinput>mv grumble-new-srvtab srvtab</userinput>
&prompt.root; <userinput>chmod 600 srvtab</userinput></screen>
</sect2>
<sect2>
<title>Populating the Database</title>
<para>We now have to add some user entries into the database. First
let us create an entry for the user <username>jane</username>. Use the
<command>kdb_edit</command> command to do this:</para>
<screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...
<prompt>Enter Kerberos master key:</prompt>
Current Kerberos master key version is 1.
Master key entered. BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.
<prompt>Principal name:</prompt> <userinput>jane</userinput>
<prompt>Instance:</prompt>
<Not found>, <prompt>Create [y] ?</prompt> <userinput>y</userinput>
Principal: jane, Instance: , kdc_key_ver: 1
<prompt>New Password:</prompt> <---- enter a secure password here
Verifying password
<prompt>New Password:</prompt> <---- re-enter the password here
Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt> <---- null entry here will cause an exit</screen>
</sect2>
<sect2>
<title>Testing It All Out</title>
<para>First we have to start the Kerberos daemons. Note that if you
have correctly edited your <filename>/etc/rc.conf</filename> then this
will happen automatically when you reboot. This is only necessary on
the Kerberos server. Kerberos clients will automatically get what
they need from the <filename>/etc/kerberosIV</filename>
directory.</para>
<screen>&prompt.root; <userinput>kerberos &</userinput>
Kerberos server starting
Sleep forever on error
Log file is /var/log/kerberos.log
Current Kerberos master key version is 1.
Master key entered. BEWARE!
Current Kerberos master key version is 1
Local realm: EXAMPLE.COM
&prompt.root; <userinput>kadmind -n &</userinput>
KADM Server KADM0.0A initializing
Please do not use 'kill -9' to kill this job, use a
regular kill instead
Current Kerberos master key version is 1.
Master key entered. BEWARE!</screen>
<para>Now we can try using the <command>kinit</command> command to get a
ticket for the ID <username>jane</username> that we created
above:</para>
<screen>&prompt.user; <userinput>kinit jane</userinput>
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane"
<prompt>Password:</prompt> </screen>
<para>Try listing the tokens using <command>klist</command> to see if we
really have them:</para>
<screen>&prompt.user; <userinput>klist</userinput>
Ticket file: /tmp/tkt245
Principal: jane@EXAMPLE.COM
Issued Expires Principal
Apr 30 11:23:22 Apr 30 19:23:22 krbtgt.EXAMPLE.COM@EXAMPLE.COM</screen>
<para>Now try changing the password using &man.passwd.1; to
check if the <application>kpasswd</application> daemon can get
authorization to the Kerberos database:</para>
<screen>&prompt.user; <userinput>passwd</userinput>
realm EXAMPLE.COM
<prompt>Old password for jane:</prompt>
<prompt>New Password for jane:</prompt>
Verifying password
<prompt>New Password for jane:</prompt>
Password changed.</screen>
</sect2>
<sect2>
<title>Adding <command>su</command> Privileges</title>
<para>Kerberos allows us to give <emphasis>each</emphasis> user
who needs <username>root</username> privileges their own
<emphasis>separate</emphasis> &man.su.1; password.
We could now add an ID which is authorized to
&man.su.1; to <username>root</username>. This is
controlled by having an instance of <username>root</username>
associated with a principal. Using <command>kdb_edit</command>
we can create the entry <literal>jane.root</literal> in the
Kerberos database:</para>
<screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...
<prompt>Enter Kerberos master key:</prompt>
Current Kerberos master key version is 1.
Master key entered. BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.
<prompt>Principal name:</prompt> <userinput>jane</userinput>
<prompt>Instance:</prompt> <userinput>root</userinput>
<Not found>, Create [y] ? y
Principal: jane, Instance: root, kdc_key_ver: 1
<prompt>New Password:</prompt> <---- enter a SECURE password here
Verifying password
<prompt>New Password:</prompt> <---- re-enter the password here
Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt> <userinput>12</userinput> <--- Keep this short!
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt> <---- null entry here will cause an exit</screen>
<para>Now try getting tokens for it to make sure it works:</para>
<screen>&prompt.root; <userinput>kinit jane.root</userinput>
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane.root"
<prompt>Password:</prompt></screen>
<para>Now we need to add the user to <username>root</username>'s
<filename>.klogin</filename> file:</para>
<screen>&prompt.root; <userinput>cat /root/.klogin</userinput>
jane.root@EXAMPLE.COM</screen>
<para>Now try doing the &man.su.1;:</para>
<screen>&prompt.user; <userinput>su</userinput>
<prompt>Password:</prompt></screen>
<para>and take a look at what tokens we have:</para>
<screen>&prompt.root; <userinput>klist</userinput>
Ticket file: /tmp/tkt_root_245
Principal: jane.root@EXAMPLE.COM
Issued Expires Principal
May 2 20:43:12 May 3 04:43:12 krbtgt.EXAMPLE.COM@EXAMPLE.COM</screen>
</sect2>
<sect2>
<title>Using Other Commands</title>
<para>In an earlier example, we created a principal called
<literal>jane</literal> with an instance <literal>root</literal>.
This was based on a user with the same name as the principal, and this
is a Kerberos default; that a
<literal><principal>.<instance></literal> of the form
<literal><username>.</literal><username>root</username> will allow
that <literal><username></literal> to &man.su.1; to
<username>root</username> if the necessary entries are in the
<filename>.klogin</filename> file in <username>root</username>'s
home directory:</para>
<screen>&prompt.root; <userinput>cat /root/.klogin</userinput>
jane.root@EXAMPLE.COM</screen>
<para>Likewise, if a user has in their own home directory lines of the
form:</para>
<screen>&prompt.user; <userinput>cat ~/.klogin</userinput>
jane@EXAMPLE.COM
jack@EXAMPLE.COM</screen>
<para>This allows anyone in the <literal>EXAMPLE.COM</literal> realm
who has authenticated themselves as <username>jane</username> or
<username>jack</username> (via <command>kinit</command>, see above)
to access to <username>jane</username>'s
account or files on this system (<hostid>grunt</hostid>) via
&man.rlogin.1;, &man.rsh.1; or
&man.rcp.1;.</para>
<para>For example, <username>jane</username> now logs into another system using
Kerberos:</para>
<screen>&prompt.user; <userinput>kinit</userinput>
MIT Project Athena (grunt.example.com)
<prompt>Password:</prompt>
&prompt.user; <userinput>rlogin grunt</userinput>
Last login: Mon May 1 21:14:47 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
The Regents of the University of California. All rights reserved.
FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995</screen>
<para>Or <username>jack</username> logs into <username>jane</username>'s account on the same machine
(<username>jane</username> having
set up the <filename>.klogin</filename> file as above, and the person
in charge of Kerberos having set up principal
<emphasis>jack</emphasis> with a null instance):</para>
<screen>&prompt.user; <userinput>kinit</userinput>
&prompt.user; <userinput>rlogin grunt -l jane</userinput>
MIT Project Athena (grunt.example.com)
<prompt>Password:</prompt>
Last login: Mon May 1 21:16:55 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
The Regents of the University of California. All rights reserved.
FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995</screen>
</sect2>
</sect1>
<sect1 id="kerberos5">
<sect1info>
<authorgroup>
<author>
<firstname>Tillman</firstname>
<surname>Hodgson</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Mark</firstname>
<surname>Murray</surname>
<contrib>Based on a contribution by </contrib>
</author>
</authorgroup>
</sect1info>
<title><application>Kerberos5</application></title>
<para>Every &os; release beyond &os;-5.1 includes support
only for <application>Kerberos5</application>. Hence
<application>Kerberos5</application> is the only version
included, and its configuration is similar in many aspects
to that of <application>KerberosIV</application>. The following
information only applies to
<application>Kerberos5</application> in post &os;-5.0
releases. Users who wish to use the
<application>KerberosIV</application> package may install the
<filename role="package">security/krb4</filename> port.</para>
<para><application>Kerberos</application> is a network add-on
system/protocol that allows users to authenticate themselves
through the services of a secure server. Services such as remote
login, remote copy, secure inter-system file copying and other
high-risk tasks are made considerably safer and more
controllable.</para>
<para><application>Kerberos</application> can be described as an
identity-verifying proxy system. It can also be described as a
trusted third-party authentication system.
<application>Kerberos</application> provides only one
function — the secure authentication of users on the network.
It does not provide authorization functions (what users are
allowed to do) or auditing functions (what those users did).
After a client and server have used
<application>Kerberos</application> to prove their identity, they
can also encrypt all of their communications to assure privacy
and data integrity as they go about their business.</para>
<para>Therefore it is highly recommended that
<application>Kerberos</application> be used with other security
methods which provide authorization and audit services.</para>
<para>The following instructions can be used as a guide on how to set
up <application>Kerberos</application> as distributed for &os;.
However, you should refer to the relevant manual pages for a complete
description.</para>
<para>For purposes of demonstrating a <application>Kerberos</application>
installation, the various name spaces will be handled as follows:</para>
<itemizedlist>
<listitem>
<para>The <acronym>DNS</acronym> domain (<quote>zone</quote>)
will be example.org.</para>
</listitem>
<listitem>
<para>The <application>Kerberos</application> realm will be
EXAMPLE.ORG.</para>
</listitem>
</itemizedlist>
<note>
<para>Please use real domain names when setting up
<application>Kerberos</application> even if you intend to run
it internally. This avoids <acronym>DNS</acronym> problems
and assures inter-operation with other
<application>Kerberos</application> realms.</para>
</note>
<sect2>
<title>History</title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>history</secondary>
</indexterm>
<para><application>Kerberos</application> was created by
<acronym>MIT</acronym> as a solution to network security problems.
The <application>Kerberos</application> protocol uses strong
cryptography so that a client can prove its identity to a server
(and vice versa) across an insecure network connection.</para>
<para><application>Kerberos</application> is both the name of a
network authentication protocol and an adjective to describe
programs that implement the program
(<application>Kerberos</application> telnet, for example). The
current version of the protocol is version 5, described in
<acronym>RFC</acronym> 1510.</para>
<para>Several free implementations of this protocol are available,
covering a wide range of operating systems. The Massachusetts
Institute of Technology (<acronym>MIT</acronym>), where
<application>Kerberos</application> was originally developed,
continues to develop their <application>Kerberos</application>
package. It is commonly used in the <acronym>US</acronym>
as a cryptography product, as such it
has historically been affected by <acronym>US</acronym> export
regulations. The <acronym>MIT</acronym>
<application>Kerberos</application> is available as a port
(<filename role="package">security/krb5</filename>). Heimdal
<application>Kerberos</application> is another version 5
implementation, and was explicitly developed outside of the
<acronym>US</acronym> to avoid export
regulations (and is thus often included in non-commercial &unix;
variants). The Heimdal <application>Kerberos</application>
distribution is available as a port
(<filename role="package">security/heimdal</filename>), and a
minimal installation of it is included in the base &os;
install.</para>
<para>In order to reach the widest audience, these instructions assume
the use of the Heimdal distribution included in &os;.</para>
</sect2>
<sect2>
<title>Setting up a Heimdal <acronym>KDC</acronym></title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>Key Distribution Center</secondary>
</indexterm>
<para>The Key Distribution Center (<acronym>KDC</acronym>) is the
centralized authentication service that
<application>Kerberos</application> provides — it is the
computer that issues <application>Kerberos</application> tickets.
The <acronym>KDC</acronym> is considered <quote>trusted</quote> by
all other computers in the <application>Kerberos</application>
realm, and thus has heightened security concerns.</para>
<para>Note that while running the <application>Kerberos</application>
server requires very few computing resources, a dedicated machine
acting only as a <acronym>KDC</acronym> is recommended for security
reasons.</para>
<para>To begin setting up a <acronym>KDC</acronym>, ensure that your
<filename>/etc/rc.conf</filename> file contains the correct
settings to act as a <acronym>KDC</acronym> (you may need to adjust
paths to reflect your own system):</para>
<programlisting>kerberos5_server_enable="YES"
kadmind5_server_enable="YES"</programlisting>
<para>Next we will set up your <application>Kerberos</application>
config file, <filename>/etc/krb5.conf</filename>:</para>
<programlisting>[libdefaults]
default_realm = EXAMPLE.ORG
[realms]
EXAMPLE.ORG = {
kdc = kerberos.example.org
admin_server = kerberos.example.org
}
[domain_realm]
.example.org = EXAMPLE.ORG</programlisting>
<para>Note that this <filename>/etc/krb5.conf</filename> file implies
that your <acronym>KDC</acronym> will have the fully-qualified
hostname of <hostid role="fqdn">kerberos.example.org</hostid>.
You will need to add a CNAME (alias) entry to your zone file to
accomplish this if your <acronym>KDC</acronym> has a different
hostname.</para>
<note>
<para>For large networks with a properly configured
<acronym>BIND</acronym> <acronym>DNS</acronym> server, the
above example could be trimmed to:</para>
<programlisting>[libdefaults]
default_realm = EXAMPLE.ORG</programlisting>
<para>With the following lines being appended to the
<hostid role="fqdn">example.org</hostid> zonefile:</para>
<programlisting>_kerberos._udp IN SRV 01 00 88 kerberos.example.org.
_kerberos._tcp IN SRV 01 00 88 kerberos.example.org.
_kpasswd._udp IN SRV 01 00 464 kerberos.example.org.
_kerberos-adm._tcp IN SRV 01 00 749 kerberos.example.org.
_kerberos IN TXT EXAMPLE.ORG</programlisting></note>
<note>
<para>For clients to be able to find the
<application>Kerberos</application> services, you
<emphasis>must</emphasis> have either a fully configured
<filename>/etc/krb5.conf</filename> or a minimally configured
<filename>/etc/krb5.conf</filename> <emphasis>and</emphasis> a
properly configured DNS server.</para>
</note>
<para>Next we will create the <application>Kerberos</application>
database. This database contains the keys of all principals encrypted
with a master password. You are not
required to remember this password, it will be stored in a file
(<filename>/var/heimdal/m-key</filename>). To create the master
key, run <command>kstash</command> and enter a password.</para>
<para>Once the master key has been created, you can initialize the
database using the <command>kadmin</command> program with the
<literal>-l</literal> option (standing for <quote>local</quote>).
This option instructs <command>kadmin</command> to modify the
database files directly rather than going through the
<command>kadmind</command> network service. This handles the
chicken-and-egg problem of trying to connect to the database
before it is created. Once you have the <command>kadmin</command>
prompt, use the <command>init</command> command to create your
realms initial database.</para>
<para>Lastly, while still in <command>kadmin</command>, create your
first principal using the <command>add</command> command. Stick
to the defaults options for the principal for now, you can always
change them later with the <command>modify</command> command.
Note that you can use the <literal>?</literal> command at any
prompt to see the available options.</para>
<para>A sample database creation session is shown below:</para>
<screen>&prompt.root; <userinput>kstash</userinput>
Master key: <userinput>xxxxxxxx</userinput>
Verifying password - Master key: <userinput>xxxxxxxx</userinput>
&prompt.root; <userinput>kadmin -l</userinput>
kadmin> <userinput>init EXAMPLE.ORG</userinput>
Realm max ticket life [unlimited]:
kadmin> <userinput>add tillman</userinput>
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
Password: <userinput>xxxxxxxx</userinput>
Verifying password - Password: <userinput>xxxxxxxx</userinput></screen>
<para>Now it is time to start up the <acronym>KDC</acronym> services.
Run <command>/etc/rc.d/kerberos start</command> and
<command>/etc/rc.d/kadmind start</command> to bring up the
services. Note that you will not have any kerberized daemons running
at this point but you should be able to confirm the that the
<acronym>KDC</acronym> is functioning by obtaining and listing a
ticket for the principal (user) that you just created from the
command-line of the <acronym>KDC</acronym> itself:</para>
<screen>&prompt.user; <userinput>kinit <replaceable>tillman</replaceable></userinput>
tillman@EXAMPLE.ORG's Password:
&prompt.user; <userinput>klist</userinput>
Credentials cache: FILE:<filename>/tmp/krb5cc_500</filename>
Principal: tillman@EXAMPLE.ORG
Issued Expires Principal
Aug 27 15:37:58 Aug 28 01:37:58 krbtgt/EXAMPLE.ORG@EXAMPLE.ORG</screen>
<para>The ticket can then be revoked when you have
finished:</para>
<screen>&prompt.user; <userinput>k5destroy</userinput></screen>
</sect2>
<sect2>
<title><application>Kerberos</application> enabling a server with
Heimdal services</title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>enabling services</secondary>
</indexterm>
<para>First, we need a copy of the <application>Kerberos</application>
configuration file, <filename>/etc/krb5.conf</filename>. To do
so, simply copy it over to the client computer from the
<acronym>KDC</acronym> in a secure fashion (using network utilities,
such as &man.scp.1;, or physically via a
floppy disk).</para>
<para>Next you need a <filename>/etc/krb5.keytab</filename> file.
This is the major difference between a server providing
<application>Kerberos</application> enabled daemons and a
workstation — the server must have a
<filename>keytab</filename> file. This file
contains the server's host key, which allows it and the
<acronym>KDC</acronym> to verify each others identity. It
must be transmitted to the server in a secure fashion, as the
security of the server can be broken if the key is made public.
This explicitly means that transferring it via a clear text
channel, such as <acronym>FTP</acronym>, is a very bad idea.</para>
<para>Typically, you transfer to the <filename>keytab</filename>
to the server using the <command>kadmin</command> program.
This is handy because you also need to create the host principal
(the <acronym>KDC</acronym> end of the
<filename>krb5.keytab</filename>) using
<command>kadmin</command>.</para>
<para>Note that you must have already obtained a ticket and that this
ticket must be allowed to use the <command>kadmin</command>
interface in the <filename>kadmind.acl</filename>. See the section
titled <quote>Remote administration</quote> in the Heimdal info
pages (<command>info heimdal</command>) for details on designing
access control lists. If you do not want to enable remote
<command>kadmin</command> access, you can simply securely connect
to the <acronym>KDC</acronym> (via local console,
&man.ssh.1; or <application>Kerberos</application>
&man.telnet.1;) and perform administration locally
using <command>kadmin -l</command>.</para>
<para>After installing the <filename>/etc/krb5.conf</filename> file,
you can use <command>kadmin</command> from the
<application>Kerberos</application> server. The
<command>add --random-key</command> command will let you add the
server's host principal, and the <command>ext</command> command
will allow you to extract the server's host principal to its own
keytab. For example:</para>
<screen>&prompt.root; <userinput>kadmin</userinput>
kadmin><userinput> add --random-key host/myserver.example.org</userinput>
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
kadmin><userinput> ext host/myserver.example.org</userinput>
kadmin><userinput> exit</userinput></screen>
<para>Note that the <command>ext</command> command (short for
<quote>extract</quote>) stores the extracted key in
<filename>/etc/krb5.keytab</filename> by default.</para>
<para>If you do not have <command>kadmind</command> running on the
<acronym>KDC</acronym> (possibly for security reasons) and thus
do not have access to <command>kadmin</command> remotely, you
can add the host principal
(<username>host/myserver.EXAMPLE.ORG</username>) directly on the
<acronym>KDC</acronym> and then extract it to a temporary file
(to avoid over-writing the <filename>/etc/krb5.keytab</filename>
on the <acronym>KDC</acronym>) using something like this:</para>
<screen>&prompt.root; <userinput>kadmin</userinput>
kadmin><userinput> ext --keytab=/tmp/example.keytab host/myserver.example.org</userinput>
kadmin><userinput> exit</userinput></screen>
<para>You can then securely copy the keytab to the server
computer (using <command>scp</command> or a floppy, for
example). Be sure to specify a non-default keytab name
to avoid over-writing the keytab on the
<acronym>KDC</acronym>.</para>
<para>At this point your server can communicate with the
<acronym>KDC</acronym> (due to its <filename>krb5.conf</filename>
file) and it can prove its own identity (due to the
<filename>krb5.keytab</filename> file). It is now ready for
you to enable some <application>Kerberos</application> services.
For this example we will enable the <command>telnet</command>
service by putting a line like this into your
<filename>/etc/inetd.conf</filename> and then restarting the
&man.inetd.8; service with
<command>/etc/rc.d/inetd restart</command>:</para>
<programlisting>telnet stream tcp nowait root /usr/libexec/telnetd telnetd -a user</programlisting>
<para>The critical bit is that the <command>-a</command>
(for authentication) type is set to user. Consult the
&man.telnetd.8; manual page for more details.</para>
</sect2>
<sect2>
<title><application>Kerberos</application> enabling a client with Heimdal</title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>configure clients</secondary>
</indexterm>
<para>Setting up a client computer is almost trivially easy. As
far as <application>Kerberos</application> configuration goes,
you only need the <application>Kerberos</application>
configuration file, located at <filename>/etc/krb5.conf</filename>.
Simply securely copy it over to the client computer from the
<acronym>KDC</acronym>.</para>
<para>Test your client computer by attempting to use
<command>kinit</command>, <command>klist</command>, and
<command>kdestroy</command> from the client to obtain, show, and
then delete a ticket for the principal you created above. You
should also be able to use <application>Kerberos</application>
applications to connect to <application>Kerberos</application>
enabled servers, though if that does not work and obtaining a
ticket does the problem is likely with the server and not with
the client or the <acronym>KDC</acronym>.</para>
<para>When testing an application like <command>telnet</command>,
try using a packet sniffer (such as &man.tcpdump.1;)
to confirm that your password is not sent in the clear. Try
using <command>telnet</command> with the <literal>-x</literal>
option, which encrypts the entire data stream (similar to
<command>ssh</command>).</para>
<para>Various non-core <application>Kerberos</application> client
applications are also installed by default. This is where the
<quote>minimal</quote> nature of the base Heimdal installation is
felt: <command>telnet</command> is the only
<application>Kerberos</application> enabled service.</para>
<para>The Heimdal port adds some of the missing client applications:
<application>Kerberos</application> enabled versions of
<command>ftp</command>, <command>rsh</command>,
<command>rcp</command>, <command>rlogin</command>, and a few
other less common programs. The <acronym>MIT</acronym> port also
contains a full suite of <application>Kerberos</application>
client applications.</para>
</sect2>
<sect2>
<title>User configuration files: <filename>.k5login</filename> and <filename>.k5users</filename></title>
<indexterm>
<primary><filename>.k5login</filename></primary>
</indexterm>
<indexterm>
<primary><filename>.k5users</filename></primary>
</indexterm>
<para>Users within a realm typically have their
<application>Kerberos</application> principal (such as
<username>tillman@EXAMPLE.ORG</username>) mapped to a local
user account (such as a local account named
<username>tillman</username>). Client applications such as
<command>telnet</command> usually do not require a user name
or a principal.</para>
<para>Occasionally, however, you want to grant access to a local
user account to someone who does not have a matching
<application>Kerberos</application> principal. For example,
<username>tillman@EXAMPLE.ORG</username> may need access to the
local user account <username>webdevelopers</username>. Other
principals may also need access to that local account.</para>
<para>The <filename>.k5login</filename> and
<filename>.k5users</filename> files, placed in a users home
directory, can be used similar to a powerful combination of
<filename>.hosts</filename> and <filename>.rhosts</filename>,
solving this problem. For example, if a
<filename>.k5login</filename> with the following
contents:</para>
<screen>tillman@example.org
jdoe@example.org</screen>
<para>Were to be placed into the home directory of the local user
<username>webdevelopers</username> then both principals listed
would have access to that account without requiring a shared
password.</para>
<para>Reading the manual pages for these commands is recommended.
Note that the <command>ksu</command> manual page covers
<filename>.k5users</filename>.</para>
</sect2>
<sect2>
<title><application>Kerberos</application> Tips, Tricks, and Troubleshooting</title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>troubleshooting</secondary>
</indexterm>
<itemizedlist>
<listitem>
<para>When using either the Heimdal or <acronym>MIT</acronym>
<application>Kerberos</application> ports ensure that your
<envar>PATH</envar> environment variable lists the
<application>Kerberos</application> versions of the client
applications before the system versions.</para>
</listitem>
<listitem>
<para>Do all the computers in your realm have synchronized
time settings? If not, authentication may fail.
<xref linkend="network-ntp"/> describes how to synchronize
clocks using <acronym>NTP</acronym>.</para>
</listitem>
<listitem>
<para><acronym>MIT</acronym> and Heimdal inter-operate nicely.
Except for <command>kadmin</command>, the protocol for
which is not standardized.</para>
</listitem>
<listitem>
<para>If you change your hostname, you also need to change your
<username>host/</username> principal and update your keytab.
This also applies to special keytab entries like the
<username>www/</username> principal used for Apache's
<filename role="package">www/mod_auth_kerb</filename>.</para>
</listitem>
<listitem>
<para>All hosts in your realm must be resolvable (both forwards
and reverse) in <acronym>DNS</acronym> (or
<filename>/etc/hosts</filename> as a minimum). CNAMEs
will work, but the A and PTR records must be correct and in
place. The error message is not very intuitive:
<errorname>Kerberos5 refuses authentication because Read req
failed: Key table entry not found</errorname>.</para>
</listitem>
<listitem>
<para>Some operating systems that may being acting as clients
to your <acronym>KDC</acronym> do not set the permissions
for <command>ksu</command> to be setuid
<username>root</username>. This means that
<command>ksu</command> does not work, which is a good
security idea but annoying. This is not a
<acronym>KDC</acronym> error.</para>
</listitem>
<listitem>
<para>With <acronym>MIT</acronym>
<application>Kerberos</application>, if you want to allow a
principal to have a ticket life longer than the default ten
hours, you must use <command>modify_principal</command> in
<command>kadmin</command> to change the maxlife of both the
principal in question and the <username>krbtgt</username>
principal. Then the principal can use the
<literal>-l</literal> option with <command>kinit</command>
to request a ticket with a longer lifetime.</para>
</listitem>
<listitem>
<note><para>If you run a packet sniffer on your
<acronym>KDC</acronym> to add in troubleshooting and then
run <command>kinit</command> from a workstation, you will
notice that your <acronym>TGT</acronym> is sent
immediately upon running <command>kinit</command> —
even before you type your password! The explanation is
that the <application>Kerberos</application> server freely
transmits a <acronym>TGT</acronym> (Ticket Granting
Ticket) to any unauthorized request; however, every
<acronym>TGT</acronym> is encrypted in a key derived from
the user's password. Therefore, when a user types their
password it is not being sent to the <acronym>KDC</acronym>,
it is being used to decrypt the <acronym>TGT</acronym> that
<command>kinit</command> already obtained. If the decryption
process results in a valid ticket with a valid time stamp,
the user has valid <application>Kerberos</application>
credentials. These credentials include a session key for
establishing secure communications with the
<application>Kerberos</application> server in the future, as
well as the actual ticket-granting ticket, which is actually
encrypted with the <application>Kerberos</application>
server's own key. This second layer of encryption is
unknown to the user, but it is what allows the
<application>Kerberos</application> server to verify
the authenticity of each <acronym>TGT</acronym>.</para></note>
</listitem>
<listitem>
<para>If you want to use long ticket lifetimes (a week, for
example) and you are using <application>OpenSSH</application>
to connect to the machine where your ticket is stored, make
sure that <application>Kerberos</application>
<option>TicketCleanup</option> is set to <literal>no</literal>
in your <filename>sshd_config</filename> or else your tickets
will be deleted when you log out.</para>
</listitem>
<listitem>
<para>Remember that host principals can have a longer ticket
lifetime as well. If your user principal has a lifetime of a
week but the host you are connecting to has a lifetime of nine
hours, you will have an expired host principal in your cache
and the ticket cache will not work as expected.</para>
</listitem>
<listitem>
<para>When setting up a <filename>krb5.dict</filename> file to
prevent specific bad passwords from being used (the manual page
for <command>kadmind</command> covers this briefly), remember
that it only applies to principals that have a password policy
assigned to them. The <filename>krb5.dict</filename> files
format is simple: one string per line. Creating a symbolic
link to <filename>/usr/share/dict/words</filename> might be
useful.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
<title>Differences with the <acronym>MIT</acronym> port</title>
<para>The major difference between the <acronym>MIT</acronym>
and Heimdal installs relates to the <command>kadmin</command>
program which has a different (but equivalent) set of commands
and uses a different protocol. This has a large implications
if your <acronym>KDC</acronym> is <acronym>MIT</acronym> as you
will not be able to use the Heimdal <command>kadmin</command>
program to administer your <acronym>KDC</acronym> remotely
(or vice versa, for that matter).</para>
<para>The client applications may also take slightly different
command line options to accomplish the same tasks. Following
the instructions on the <acronym>MIT</acronym>
<application>Kerberos</application> web site
(<ulink url="http://web.mit.edu/Kerberos/www/"></ulink>)
is recommended. Be careful of path issues: the
<acronym>MIT</acronym> port installs into
<filename>/usr/local/</filename> by default, and the
<quote>normal</quote> system applications may be run instead
of <acronym>MIT</acronym> if your <envar>PATH</envar>
environment variable lists the system directories first.</para>
<note><para>With the <acronym>MIT</acronym>
<filename role="package">security/krb5</filename> port
that is provided by &os;, be sure to read the
<filename>/usr/local/share/doc/krb5/README.FreeBSD</filename>
file installed by the port if you want to understand why logins
via <command>telnetd</command> and <command>klogind</command>
behave somewhat oddly. Most importantly, correcting the
<quote>incorrect permissions on cache file</quote> behavior
requires that the <command>login.krb5</command> binary be used
for authentication so that it can properly change ownership for
the forwarded credentials.</para></note>
<para>The <filename>rc.conf</filename> must also be modified
to contain the following configuration:</para>
<programlisting>kerberos5_server="/usr/local/sbin/krb5kdc"
kadmind5_server="/usr/local/sbin/kadmind"
kerberos5_server_enable="YES"
kadmind5_server_enable="YES"</programlisting>
<para>This is done because the applications for
<acronym>MIT</acronym> kerberos installs binaries in the
<filename role="directory">/usr/local</filename>
hierarchy.</para>
</sect2>
<sect2>
<title>Mitigating limitations found in <application>Kerberos</application></title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>limitations and shortcomings</secondary>
</indexterm>
<sect3>
<title><application>Kerberos</application> is an all-or-nothing approach</title>
<para>Every service enabled on the network must be modified to
work with <application>Kerberos</application> (or be otherwise
secured against network attacks) or else the users credentials
could be stolen and re-used. An example of this would be
<application>Kerberos</application> enabling all remote shells
(via <command>rsh</command> and <command>telnet</command>, for
example) but not converting the <acronym>POP3</acronym> mail
server which sends passwords in plain text.</para>
</sect3>
<sect3>
<title><application>Kerberos</application> is intended for single-user workstations</title>
<para>In a multi-user environment,
<application>Kerberos</application> is less secure.
This is because it stores the tickets in the
<filename>/tmp</filename> directory, which is readable by all
users. If a user is sharing a computer with several other
people simultaneously (i.e. multi-user), it is possible that
the user's tickets can be stolen (copied) by another
user.</para>
<para>This can be overcome with the <literal>-c</literal>
filename command-line option or (preferably) the
<envar>KRB5CCNAME</envar> environment variable, but this
is rarely done. In principal, storing the ticket in the users
home directory and using simple file permissions can mitigate
this problem.</para>
</sect3>
<sect3>
<title>The KDC is a single point of failure</title>
<para>By design, the <acronym>KDC</acronym> must be as secure as
the master password database is contained on it. The
<acronym>KDC</acronym> should have absolutely no other
services running on it and should be physically secured. The
danger is high because <application>Kerberos</application>
stores all passwords encrypted with the same key (the
<quote>master</quote> key), which in turn is stored as a file
on the <acronym>KDC</acronym>.</para>
<para>As a side note, a compromised master key is not quite as
bad as one might normally fear. The master key is only used
to encrypt the <application>Kerberos</application> database
and as a seed for the random number generator. As long as
access to your <acronym>KDC</acronym> is secure, an attacker
cannot do much with the master key.</para>
<para>Additionally, if the <acronym>KDC</acronym> is unavailable
(perhaps due to a denial of service attack or network problems)
the network services are unusable as authentication can not be
performed, a recipe for a denial-of-service attack. This can
alleviated with multiple <acronym>KDC</acronym>s (a single
master and one or more slaves) and with careful implementation
of secondary or fall-back authentication
(<acronym>PAM</acronym> is excellent for this).</para>
</sect3>
<sect3>
<title><application>Kerberos</application> Shortcomings</title>
<para><application>Kerberos</application> allows users, hosts
and services to authenticate between themselves. It does not
have a mechanism to authenticate the <acronym>KDC</acronym>
to the users, hosts or services. This means that a trojanned
<command>kinit</command> (for example) could record all user
names and passwords. Something like
<filename role="package">security/tripwire</filename> or
other file system integrity checking tools can alleviate
this.</para>
</sect3>
</sect2>
<sect2>
<title>Resources and further information</title>
<indexterm>
<primary>Kerberos5</primary>
<secondary>external resources</secondary>
</indexterm>
<itemizedlist>
<listitem>
<para><ulink
url="http://www.faqs.org/faqs/Kerberos-faq/general/preamble.html">
The <application>Kerberos</application> FAQ</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://web.mit.edu/Kerberos/www/dialogue.html">Designing
an Authentication System: a Dialog in Four Scenes</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://www.ietf.org/rfc/rfc1510.txt?number=1510">RFC 1510,
The <application>Kerberos</application> Network Authentication Service
(V5)</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://web.mit.edu/Kerberos/www/"><acronym>MIT</acronym>
<application>Kerberos</application> home page</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://www.pdc.kth.se/heimdal/">Heimdal
<application>Kerberos</application> home page</ulink></para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id="openssl">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Written by: </contrib>
</author>
</authorgroup>
</sect1info>
<title>OpenSSL</title>
<indexterm>
<primary>security</primary>
<secondary>OpenSSL</secondary>
</indexterm>
<para>One feature that many users overlook is the
<application>OpenSSL</application> toolkit included
in &os;. <application>OpenSSL</application> provides an
encryption transport layer on top of the normal communications
layer; thus allowing it to be intertwined with many network
applications and services.</para>
<para>Some uses of <application>OpenSSL</application> may include
encrypted authentication of mail clients, web based transactions
such as credit card payments and more. Many ports such as
<filename role="package">www/apache13-ssl</filename>, and
<filename role="package">mail/sylpheed-claws</filename>
will offer compilation support for building with
<application>OpenSSL</application>.</para>
<note>
<para>In most cases the Ports Collection will attempt to build
the <filename role="package">security/openssl</filename> port
unless the <makevar>WITH_OPENSSL_BASE</makevar> make variable
is explicitly set to <quote>yes</quote>.</para>
</note>
<para>The version of <application>OpenSSL</application> included
in &os; supports Secure Sockets Layer v2/v3 (SSLv2/SSLv3),
Transport Layer Security v1 (TLSv1) network security protocols
and can be used as a general cryptographic library.</para>
<note>
<para>While <application>OpenSSL</application> supports the
<acronym>IDEA</acronym> algorithm, it is disabled by default
due to United States patents. To use it, the license should
be reviewed and, if the restrictions are acceptable, the
<makevar>MAKE_IDEA</makevar> variable must be set in
<filename>make.conf</filename>.</para>
</note>
<para>One of the most common uses of
<application>OpenSSL</application> is to provide certificates for
use with software applications. These certificates ensure
that the credentials of the company or individual are valid
and not fraudulent. If the certificate in question has
not been verified by one of the several <quote>Certificate Authorities</quote>,
or <acronym>CA</acronym>s, a warning is usually produced. A
Certificate Authority is a company, such as <ulink url="http://www.verisign.com">VeriSign</ulink>, which will
sign certificates in order to validate credentials of individuals
or companies. This process has a cost associated with it and
is definitely not a requirement for using certificates; however,
it can put some of the more paranoid users at ease.</para>
<sect2>
<title>Generating Certificates</title>
<indexterm>
<primary>OpenSSL</primary>
<secondary>certificate generation</secondary>
</indexterm>
<para>To generate a certificate, the following command is
available:</para>
<screen>&prompt.root; <userinput>openssl req -new -nodes -out req.pem -keyout cert.pem</userinput>
Generating a 1024 bit RSA private key
................++++++
.......................................++++++
writing new private key to 'cert.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:<userinput><replaceable>US</replaceable></userinput>
State or Province Name (full name) [Some-State]:<userinput><replaceable>PA</replaceable></userinput>
Locality Name (eg, city) []:<userinput><replaceable>Pittsburgh</replaceable></userinput>
Organization Name (eg, company) [Internet Widgits Pty Ltd]:<userinput><replaceable>My Company</replaceable></userinput>
Organizational Unit Name (eg, section) []:<userinput><replaceable>Systems Administrator</replaceable></userinput>
Common Name (eg, YOUR name) []:<userinput><replaceable>localhost.example.org</replaceable></userinput>
Email Address []:<userinput><replaceable>trhodes@FreeBSD.org</replaceable></userinput>
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:<userinput><replaceable>SOME PASSWORD</replaceable></userinput>
An optional company name []:<userinput><replaceable>Another Name</replaceable></userinput></screen>
<para>Notice the response directly after the
<quote>Common Name</quote> prompt shows a domain name.
This prompt requires a server name to be entered for
verification purposes; placing anything but a domain name
would yield a useless certificate. Other options, for
instance expire time, alternate encryption algorithms, etc.
are available. A complete list may be obtained by viewing
the &man.openssl.1; manual page.</para>
<para>Two files should now exist in
the directory in which the aforementioned command was issued.
The certificate request, <filename>req.pem</filename>, may be
sent to a certificate authority who will validate the credentials
that you entered, sign the request and return the certificate to
you. The second file created will be named <filename>cert.pem</filename>
and is the private key for the certificate and should be
protected at all costs; if this falls in the hands of others it
can be used to impersonate you (or your server).</para>
<para>In cases where a signature from a <acronym>CA</acronym> is
not required, a self signed certificate can be created. First,
generate the <acronym>RSA</acronym> key:</para>
<screen>&prompt.root; <userinput>openssl dsaparam -rand -genkey -out <filename>myRSA.key</filename> 1024</userinput></screen>
<para>Next, generate the <acronym>CA</acronym> key:</para>
<screen>&prompt.root; <userinput>openssl gendsa -des3 -out <filename>myca.key</filename> <filename>myRSA.key</filename></userinput></screen>
<para>Use this key to create the certificate:</para>
<screen>&prompt.root; <userinput>openssl req -new -x509 -days 365 -key <filename>myca.key</filename> -out <filename>new.crt</filename></userinput></screen>
<para>Two new files should appear in the directory: a certificate
authority signature file, <filename>myca.key</filename> and the
certificate itself, <filename>new.crt</filename>. These should
be placed in a directory, preferably under
<filename class="directory">/etc</filename>, which is readable
only by <username>root</username>. Permissions of 0700 should be fine for this and
they can be set with the <command>chmod</command>
utility.</para>
</sect2>
<sect2>
<title>Using Certificates, an Example</title>
<para>So what can these files do? A good use would be to
encrypt connections to the <application>Sendmail</application>
<acronym>MTA</acronym>. This would dissolve the use of clear
text authentication for users who send mail via the local
<acronym>MTA</acronym>.</para>
<note>
<para>This is not the best use in the world as some
<acronym>MUA</acronym>s will present the user with an
error if they have not installed the certificate locally.
Refer to the documentation included with the software for
more information on certificate installation.</para>
</note>
<para>The following lines should be placed inside the
local <filename>.mc</filename> file:</para>
<programlisting>dnl SSL Options
define(`confCACERT_PATH',`/etc/certs')dnl
define(`confCACERT',`/etc/certs/new.crt')dnl
define(`confSERVER_CERT',`/etc/certs/new.crt')dnl
define(`confSERVER_KEY',`/etc/certs/myca.key')dnl
define(`confTLS_SRV_OPTIONS', `V')dnl</programlisting>
<para>Where <filename class="directory">/etc/certs/</filename>
is the directory to be used for storing the certificate
and key files locally. The last few requirements are a rebuild
of the local <filename>.cf</filename> file. This is easily
achieved by typing <command>make</command>
<parameter>install</parameter> within the
<filename class="directory">/etc/mail</filename>
directory. Follow that up with <command>make</command>
<parameter>restart</parameter> which should start the
<application>Sendmail</application> daemon.</para>
<para>If all went well there will be no error messages in the
<filename>/var/log/maillog</filename> file and
<application>Sendmail</application> will show up in the process
list.</para>
<para>For a simple test, simply connect to the mail server
using the &man.telnet.1; utility:</para>
<screen>&prompt.root; <userinput>telnet <replaceable>example.com</replaceable> 25</userinput>
Trying 192.0.34.166...
Connected to <hostid role="fqdn">example.com</hostid>.
Escape character is '^]'.
220 <hostid role="fqdn">example.com</hostid> ESMTP Sendmail 8.12.10/8.12.10; Tue, 31 Aug 2004 03:41:22 -0400 (EDT)
<userinput>ehlo <replaceable>example.com</replaceable></userinput>
250-example.com Hello example.com [192.0.34.166], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP
<userinput>quit</userinput>
221 2.0.0 <hostid role="fqdn">example.com</hostid> closing connection
Connection closed by foreign host.</screen>
<para>If the <quote>STARTTLS</quote> line appears in the output
then everything is working correctly.</para>
</sect2>
</sect1>
<sect1 id="ipsec">
<sect1info>
<authorgroup>
<author>
<firstname>Nik</firstname>
<surname>Clayton</surname>
<affiliation>
<address><email>nik@FreeBSD.org</email></address>
</affiliation>
<contrib>Written by </contrib>
</author>
</authorgroup>
</sect1info>
<title>VPN over IPsec</title>
<indexterm>
<primary>IPsec</primary>
</indexterm>
<para>Creating a VPN between two networks, separated by the
Internet, using FreeBSD gateways.</para>
<sect2>
<sect2info>
<authorgroup>
<author>
<firstname>Hiten M.</firstname>
<surname>Pandya</surname>
<affiliation>
<address><email>hmp@FreeBSD.org</email></address>
</affiliation>
<contrib>Written by </contrib>
</author>
</authorgroup>
</sect2info>
<title>Understanding IPsec</title>
<para>This section will guide you through the process of setting
up IPsec, and to use it in an environment which consists of
FreeBSD and <application>µsoft.windows; 2000/XP</application>
machines, to make them communicate securely. In order to set up
IPsec, it is necessary that you are familiar with the concepts
of building a custom kernel (see
<xref linkend="kernelconfig"/>).</para>
<para><emphasis>IPsec</emphasis> is a protocol which sits on top
of the Internet Protocol (IP) layer. It allows two or more
hosts to communicate in a secure manner (hence the name). The
FreeBSD IPsec <quote>network stack</quote> is based on the
<ulink url="http://www.kame.net/">KAME</ulink> implementation,
which has support for both protocol families, IPv4 and
IPv6.</para>
<note>
<para>FreeBSD contains a <quote>hardware
accelerated</quote> IPsec stack, known as <quote>Fast
IPsec</quote>, that was obtained from OpenBSD. It employs
cryptographic hardware (whenever possible) via the
&man.crypto.4; subsystem to optimize the performance of IPsec.
This subsystem is new, and does not support all the features
that are available in the KAME version of IPsec. However, in
order to enable hardware-accelerated IPsec, the following
kernel option has to be added to your kernel configuration
file:</para>
<indexterm>
<primary>kernel options</primary>
<secondary>FAST_IPSEC</secondary>
</indexterm>
<screen>
options FAST_IPSEC # new IPsec (cannot define w/ IPSEC)
</screen>
<para> Note, that it is not currently possible to use the
<quote>Fast IPsec</quote> subsystem in lieu of the KAME
implementation of IPsec. Consult the &man.fast.ipsec.4;
manual page for more information.</para>
</note>
<note>
<para>To let firewalls properly track state for &man.gif.4;
tunnels too, you have to enable the
<option>IPSEC_FILTERGIF</option> in your kernel
configuration:</para>
<screen>
options IPSEC_FILTERGIF #filter ipsec packets from a tunnel
</screen>
</note>
<indexterm>
<primary>IPsec</primary>
<secondary>ESP</secondary>
</indexterm>
<indexterm>
<primary>IPsec</primary>
<secondary>AH</secondary>
</indexterm>
<para>IPsec consists of two sub-protocols:</para>
<itemizedlist>
<listitem>
<para><emphasis>Encapsulated Security Payload
(ESP)</emphasis>, protects the IP packet data from third
party interference, by encrypting the contents using
symmetric cryptography algorithms (like Blowfish,
3DES).</para>
</listitem>
<listitem>
<para><emphasis>Authentication Header (AH)</emphasis>,
protects the IP packet header from third party interference
and spoofing, by computing a cryptographic checksum and
hashing the IP packet header fields with a secure hashing
function. This is then followed by an additional header
that contains the hash, to allow the information in the
packet to be authenticated.</para>
</listitem>
</itemizedlist>
<para><acronym>ESP</acronym> and <acronym>AH</acronym> can
either be used together or separately, depending on the
environment.</para>
<indexterm>
<primary>VPN</primary>
</indexterm>
<indexterm>
<primary>virtual private network</primary>
<see>VPN</see>
</indexterm>
<para>IPsec can either be used to directly encrypt the traffic
between two hosts (known as <emphasis>Transport
Mode</emphasis>); or to build <quote>virtual tunnels</quote>
between two subnets, which could be used for secure
communication between two corporate networks (known as
<emphasis>Tunnel Mode</emphasis>). The latter is more commonly
known as a <emphasis>Virtual Private Network (VPN)</emphasis>.
The &man.ipsec.4; manual page should be consulted for detailed
information on the IPsec subsystem in FreeBSD.</para>
<para>To add IPsec support to your kernel, add the following
options to your kernel configuration file:</para>
<indexterm>
<primary>kernel options</primary>
<secondary>IPSEC</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>IPSEC_ESP</secondary>
</indexterm>
<screen>
options IPSEC #IP security
options IPSEC_ESP #IP security (crypto; define w/ IPSEC)
</screen>
<indexterm>
<primary>kernel options</primary>
<secondary>IPSEC_DEBUG</secondary>
</indexterm>
<para>If IPsec debugging support is desired, the following
kernel option should also be added:</para>
<screen>
options IPSEC_DEBUG #debug for IP security
</screen>
</sect2>
<sect2>
<title>The Problem</title>
<para>There is no standard for what constitutes a VPN. VPNs can
be implemented using a number of different technologies, each of
which have their own strengths and weaknesses. This section
presents a scenario, and the strategies used for implementing a
VPN for this scenario.</para>
</sect2>
<sect2>
<title>The Scenario: Two networks, connected to the Internet, to
behave as one</title>
<indexterm>
<primary>VPN</primary>
<secondary>creating</secondary>
</indexterm>
<para>The premise is as follows:</para>
<itemizedlist>
<listitem>
<para>You have at least two sites</para>
</listitem>
<listitem>
<para>Both sites are using IP internally</para>
</listitem>
<listitem>
<para>Both sites are connected to the Internet, through a
gateway that is running FreeBSD.</para>
</listitem>
<listitem>
<para>The gateway on each network has at least one public IP
address.</para>
</listitem>
<listitem>
<para>The internal addresses of the two networks can be
public or private IP addresses, it does not matter. You can
be running NAT on the gateway machine if necessary.</para>
</listitem>
<listitem>
<para>The internal IP addresses of the two networks
<emphasis>do not collide</emphasis>. While I expect it is
theoretically possible to use a combination of VPN
technology and NAT to get this to work, I expect it to be a
configuration nightmare.</para>
</listitem>
</itemizedlist>
<para>If you find that you are trying to connect two networks,
both of which, internally, use the same private IP address range
(e.g. both of them use <hostid
role="ipaddr">192.168.1.x</hostid>), then one of the networks will
have to be renumbered.</para>
<para>The network topology might look something like this:</para>
<mediaobject>
<imageobject>
<imagedata fileref="security/ipsec-network" align="center"/>
</imageobject>
<textobject>
<literallayout class="monospaced">Network #1 [ Internal Hosts ] Private Net, 192.168.1.2-254
[ Win9x/NT/2K ]
[ UNIX ]
|
|
.---[fxp1]---. Private IP, 192.168.1.1
| FreeBSD |
`---[fxp0]---' Public IP, A.B.C.D
|
|
-=-=- Internet -=-=-
|
|
.---[fxp0]---. Public IP, W.X.Y.Z
| FreeBSD |
`---[fxp1]---' Private IP, 192.168.2.1
|
|
Network #2 [ Internal Hosts ]
[ Win9x/NT/2K ] Private Net, 192.168.2.2-254
[ UNIX ]</literallayout>
</textobject>
</mediaobject>
<para>Notice the two public IP addresses. I will use the letters to
refer to them in the rest of this article. Anywhere you see those
letters in this article, replace them with your own public IP
addresses. Note also that internally, the two gateway
machines have .1 IP addresses, and that the two networks have
different private IP addresses (<hostid
role="ipaddr">192.168.1.x</hostid> and <hostid
role="ipaddr">192.168.2.x</hostid> respectively). All the
machines on the private networks have been configured to use the
<hostid role="ipaddr">.1</hostid> machine as their default
gateway.</para>
<para>The intention is that, from a network point of view, each
network should view the machines on the other network as though
they were directly attached the same router -- albeit a slightly
slow router with an occasional tendency to drop packets.</para>
<para>This means that (for example), machine <hostid
role="ipaddr">192.168.1.20</hostid> should be able to run</para>
<programlisting>ping 192.168.2.34</programlisting>
<para>and have it work, transparently. &windows; machines should
be able to see the machines on the other network, browse file
shares, and so on, in exactly the same way that they can browse
machines on the local network.</para>
<para>And the whole thing has to be secure. This means that
traffic between the two networks has to be encrypted.</para>
<para>Creating a VPN between these two networks is a multi-step
process. The stages are as follows:</para>
<orderedlist>
<listitem>
<para>Create a <quote>virtual</quote> network link between the two
networks, across the Internet. Test it, using tools like
&man.ping.8;, to make sure it works.</para>
</listitem>
<listitem>
<para>Apply security policies to ensure that traffic between
the two networks is transparently encrypted and decrypted as
necessary. Test this, using tools like &man.tcpdump.1;, to
ensure that traffic is encrypted.</para>
</listitem>
<listitem>
<para>Configure additional software on the FreeBSD gateways,
to allow &windows; machines to see one another across the
VPN.</para>
</listitem>
</orderedlist>
<sect3>
<title>Step 1: Creating and testing a <quote>virtual</quote>
network link</title>
<para>Suppose that you were logged in to the gateway machine on
network #1 (with public IP address <hostid
role="ipaddr">A.B.C.D</hostid>, private IP address <hostid
role="ipaddr">192.168.1.1</hostid>), and you ran <command>ping
192.168.2.1</command>, which is the private address of the machine
with IP address <hostid role="ipaddr">W.X.Y.Z</hostid>. What
needs to happen in order for this to work?</para>
<orderedlist>
<listitem>
<para>The gateway machine needs to know how to reach <hostid
role="ipaddr">192.168.2.1</hostid>. In other words, it needs
to have a route to <hostid
role="ipaddr">192.168.2.1</hostid>.</para>
</listitem>
<listitem>
<para>Private IP addresses, such as those in the <hostid
role="ipaddr">192.168.x</hostid> range are not supposed to
appear on the Internet at large. Instead, each packet you
send to <hostid role="ipaddr">192.168.2.1</hostid> will need
to be wrapped up inside another packet. This packet will need
to appear to be from <hostid role="ipaddr">A.B.C.D</hostid>,
and it will have to be sent to <hostid
role="ipaddr">W.X.Y.Z</hostid>. This process is called
<firstterm>encapsulation</firstterm>.</para>
</listitem>
<listitem>
<para>Once this packet arrives at <hostid
role="ipaddr">W.X.Y.Z</hostid> it will need to
<quote>unencapsulated</quote>, and delivered to <hostid
role="ipaddr">192.168.2.1</hostid>.</para>
</listitem>
</orderedlist>
<para>You can think of this as requiring a <quote>tunnel</quote>
between the two networks. The two <quote>tunnel mouths</quote> are the IP
addresses <hostid role="ipaddr">A.B.C.D</hostid> and <hostid
role="ipaddr">W.X.Y.Z</hostid>, and the tunnel must be told the
addresses of the private IP addresses that will be allowed to pass
through it. The tunnel is used to transfer traffic with private
IP addresses across the public Internet.</para>
<para>This tunnel is created by using the generic interface, or
<devicename>gif</devicename> devices on FreeBSD. As you can
imagine, the <devicename>gif</devicename> interface on each
gateway host must be configured with four IP addresses; two for
the public IP addresses, and two for the private IP
addresses.</para>
<para>Support for the gif device must be compiled in to the
&os; kernel on both machines. You can do this by adding the
line:</para>
<programlisting>device gif</programlisting>
<para>to the kernel configuration files on both machines, and
then compile, install, and reboot as normal.</para>
<para>Configuring the tunnel is a two step process. First the
tunnel must be told what the outside (or public) IP addresses
are, using &man.ifconfig.8;. Then the private IP addresses must be
configured using &man.ifconfig.8;.</para>
<para>On the gateway machine on network #1 you would run the
following commands to configure the tunnel.</para>
<screen>&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> create</userinput>
&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> tunnel <replaceable>A.B.C.D</replaceable> <replaceable>W.X.Y.Z</replaceable></userinput>
&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> inet <replaceable>192.168.1.1</replaceable> <replaceable>192.168.2.1</replaceable> netmask <replaceable>0xffffffff</replaceable></userinput>
</screen>
<para>On the other gateway machine you run the same commands,
but with the order of the IP addresses reversed.</para>
<screen>&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> create</userinput>
&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> tunnel <replaceable>W.X.Y.Z</replaceable> <replaceable>A.B.C.D</replaceable></userinput>
&prompt.root; <userinput>ifconfig <replaceable>gif0</replaceable> inet <replaceable>192.168.2.1</replaceable> <replaceable>192.168.1.1</replaceable> netmask <replaceable>0xffffffff</replaceable></userinput>
</screen>
<para>You can then run:</para>
<programlisting>ifconfig gif0</programlisting>
<para>to see the configuration. For example, on the network #1
gateway, you would see this:</para>
<screen>&prompt.root; <userinput>ifconfig gif0</userinput>
gif0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
tunnel inet A.B.C.D --> W.X.Y.Z
inet 192.168.1.1 --> 192.168.2.1 netmask 0xffffffff
</screen>
<para>As you can see, a tunnel has been created between the
physical addresses <hostid role="ipaddr">A.B.C.D</hostid> and
<hostid role="ipaddr">W.X.Y.Z</hostid>, and the traffic allowed
through the tunnel is that between <hostid
role="ipaddr">192.168.1.1</hostid> and <hostid
role="ipaddr">192.168.2.1</hostid>.</para>
<para>This will also have added an entry to the routing table
on both machines, which you can examine with the command <command>netstat -rn</command>.
This output is from the gateway host on network #1.</para>
<screen>&prompt.root; <userinput>netstat -rn</userinput>
Routing tables
Internet:
Destination Gateway Flags Refs Use Netif Expire
...
192.168.2.1 192.168.1.1 UH 0 0 gif0
...
</screen>
<para>As the <quote>Flags</quote> value indicates, this is a
host route, which means that each gateway knows how to reach the
other gateway, but they do not know how to reach the rest of
their respective networks. That problem will be fixed
shortly.</para>
<para>It is likely that you are running a firewall on both
machines. This will need to be circumvented for your VPN
traffic. You might want to allow all traffic between both
networks, or you might want to include firewall rules that
protect both ends of the VPN from one another.</para>
<para>It greatly simplifies testing if you configure the
firewall to allow all traffic through the VPN. You can always
tighten things up later. If you are using &man.ipfw.8; on the
gateway machines then a command like</para>
<programlisting>ipfw add 1 allow ip from any to any via gif0</programlisting>
<para>will allow all traffic between the two end points of the
VPN, without affecting your other firewall rules. Obviously
you will need to run this command on both gateway hosts.</para>
<para>This is sufficient to allow each gateway machine to ping
the other. On <hostid role="ipaddr">192.168.1.1</hostid>, you
should be able to run</para>
<programlisting>ping 192.168.2.1</programlisting>
<para>and get a response, and you should be able to do the same
thing on the other gateway machine.</para>
<para>However, you will not be able to reach internal machines
on either network yet. This is because of the routing --
although the gateway machines know how to reach one another,
they do not know how to reach the network behind each one.</para>
<para>To solve this problem you must add a static route on each
gateway machine. The command to do this on the first gateway
would be:</para>
<programlisting>route add 192.168.2.0 192.168.2.1 netmask 0xffffff00
</programlisting>
<para>This says <quote>In order to reach the hosts on the
network <hostid role="ipaddr">192.168.2.0</hostid>, send the
packets to the host <hostid
role="ipaddr">192.168.2.1</hostid></quote>. You will need to
run a similar command on the other gateway, but with the
<hostid role="ipaddr">192.168.1.x</hostid> addresses
instead.</para>
<para>IP traffic from hosts on one network will now be able to
reach hosts on the other network.</para>
<para>That has now created two thirds of a VPN between the two
networks, in as much as it is <quote>virtual</quote> and it is a
<quote>network</quote>. It is not private yet. You can test
this using &man.ping.8; and &man.tcpdump.1;. Log in to the
gateway host and run</para>
<programlisting>tcpdump dst host 192.168.2.1</programlisting>
<para>In another log in session on the same host run</para>
<programlisting>ping 192.168.2.1</programlisting>
<para>You will see output that looks something like this:</para>
<programlisting>
16:10:24.018080 192.168.1.1 > 192.168.2.1: icmp: echo request
16:10:24.018109 192.168.1.1 > 192.168.2.1: icmp: echo reply
16:10:25.018814 192.168.1.1 > 192.168.2.1: icmp: echo request
16:10:25.018847 192.168.1.1 > 192.168.2.1: icmp: echo reply
16:10:26.028896 192.168.1.1 > 192.168.2.1: icmp: echo request
16:10:26.029112 192.168.1.1 > 192.168.2.1: icmp: echo reply
</programlisting>
<para>As you can see, the ICMP messages are going back and forth
unencrypted. If you had used the <option>-s</option> parameter to
&man.tcpdump.1; to grab more bytes of data from the packets you
would see more information.</para>
<para>Obviously this is unacceptable. The next section will
discuss securing the link between the two networks so that
all traffic is automatically encrypted.</para>
<itemizedlist>
<title>Summary:</title>
<listitem>
<para>Configure both kernels with <quote>device gif</quote>.</para>
</listitem>
<listitem>
<para>Edit <filename>/etc/rc.conf</filename> on gateway host
#1 and add the following lines (replacing IP addresses as
necessary).</para>
<programlisting>gif_interfaces="gif0"
gifconfig_gif0="A.B.C.D W.X.Y.Z"
ifconfig_gif0="inet 192.168.1.1 192.168.2.1 netmask 0xffffffff"
static_routes="vpn"
route_vpn="192.168.2.0 192.168.2.1 netmask 0xffffff00"
</programlisting>
</listitem>
<listitem>
<para>Edit your firewall script
(<filename>/etc/rc.firewall</filename>, or similar) on both
hosts, and add</para>
<programlisting>ipfw add 1 allow ip from any to any via gif0</programlisting>
</listitem>
<listitem>
<para>Make similar changes to
<filename>/etc/rc.conf</filename> on gateway host #2,
reversing the order of IP addresses.</para>
</listitem>
</itemizedlist>
</sect3>
<sect3>
<title>Step 2: Securing the link</title>
<para>To secure the link we will be using IPsec. IPsec provides
a mechanism for two hosts to agree on an encryption key, and to
then use this key in order to encrypt data between the two
hosts.</para>
<para>The are two areas of configuration to be considered here.</para>
<orderedlist>
<listitem>
<para>There must be a mechanism for two hosts to agree on the
encryption mechanism to use. Once two hosts have agreed on
this mechanism there is said to be a <quote>security association</quote>
between them.</para>
</listitem>
<listitem>
<para>There must be a mechanism for specifying which traffic
should be encrypted. Obviously, you do not want to encrypt
all your outgoing traffic -- you only want to encrypt the
traffic that is part of the VPN. The rules that you put in
place to determine what traffic will be encrypted are called
<quote>security policies</quote>.</para>
</listitem>
</orderedlist>
<para>Security associations and security policies are both
maintained by the kernel, and can be modified by userland
programs. However, before you can do this you must configure the
kernel to support IPsec and the Encapsulated Security Payload
(ESP) protocol. This is done by configuring a kernel with:</para>
<indexterm>
<primary>kernel options</primary>
<secondary>IPSEC</secondary>
</indexterm>
<programlisting>options IPSEC
options IPSEC_ESP
</programlisting>
<para>and recompiling, reinstalling, and rebooting. As before
you will need to do this to the kernels on both of the gateway
hosts.</para>
<indexterm>
<primary>IKE</primary>
</indexterm>
<para>You have two choices when it comes to setting up security
associations. You can configure them by hand between two hosts,
which entails choosing the encryption algorithm, encryption keys,
and so forth, or you can use daemons that implement the Internet
Key Exchange protocol (IKE) to do this for you.</para>
<para>I recommend the latter. Apart from anything else, it is
easier to set up.</para>
<indexterm>
<primary>IPsec</primary>
<secondary>security policies</secondary>
</indexterm>
<indexterm>
<primary><command>setkey</command></primary>
</indexterm>
<para>Editing and displaying security policies is carried out
using &man.setkey.8;. By analogy, <command>setkey</command> is
to the kernel's security policy tables as &man.route.8; is to
the kernel's routing tables. <command>setkey</command> can
also display the current security associations, and to continue
the analogy further, is akin to <command>netstat -r</command>
in that respect.</para>
<para>There are a number of choices for daemons to manage
security associations with FreeBSD. This article will describe
how to use one of these, racoon — which is available from
<filename role="package">security/ipsec-tools</filename> in the &os; Ports
collection.</para>
<indexterm>
<primary>racoon</primary>
</indexterm>
<para>The <application>racoon</application> software must be run on both gateway hosts. On each host it
is configured with the IP address of the other end of the VPN,
and a secret key (which you choose, and must be the same on both
gateways).</para>
<para>The two daemons then contact one another, confirm that they
are who they say they are (by using the secret key that you
configured). The daemons then generate a new secret key, and use
this to encrypt the traffic over the VPN. They periodically
change this secret, so that even if an attacker were to crack one
of the keys (which is as theoretically close to unfeasible as it
gets) it will not do them much good -- by the time they have cracked
the key the two daemons have chosen another one.</para>
<para>The configuration file for racoon is stored in
<filename>${PREFIX}/etc/racoon</filename>. You should find a
configuration file there, which should not need to be changed
too much. The other component of racoon's configuration,
which you will need to change, is the <quote>pre-shared
key</quote>.</para>
<para>The default racoon configuration expects to find this in
the file <filename>${PREFIX}/etc/racoon/psk.txt</filename>. It is important to note
that the pre-shared key is <emphasis>not</emphasis> the key that will be used to
encrypt your traffic across the VPN link, it is simply a token
that allows the key management daemons to trust one another.</para>
<para><filename>psk.txt</filename> contains a line for each
remote site you are dealing with. In this example, where there
are two sites, each <filename>psk.txt</filename> file will contain one line (because
each end of the VPN is only dealing with one other end).</para>
<para>On gateway host #1 this line should look like this:</para>
<programlisting>W.X.Y.Z secret</programlisting>
<para>That is, the <emphasis>public</emphasis> IP address of the remote end,
whitespace, and a text string that provides the secret.
Obviously, you should not use <quote>secret</quote> as your key -- the normal
rules for choosing a password apply.</para>
<para>On gateway host #2 the line would look like this</para>
<programlisting>A.B.C.D secret</programlisting>
<para>That is, the public IP address of the remote end, and the
same secret key. <filename>psk.txt</filename> must be mode
<literal>0600</literal> (i.e., only read/write to
<username>root</username>) before racoon will run.</para>
<para>You must run racoon on both gateway machines. You will
also need to add some firewall rules to allow the IKE traffic,
which is carried over UDP to the ISAKMP (Internet Security Association
Key Management Protocol) port. Again, this should be fairly early in
your firewall ruleset.</para>
<programlisting>ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp
ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp
</programlisting>
<para>Once racoon is running you can try pinging one gateway host
from the other. The connection is still not encrypted, but
racoon will then set up the security associations between the two
hosts -- this might take a moment, and you may see this as a
short delay before the ping commands start responding.</para>
<para>Once the security association has been set up you can
view it using &man.setkey.8;. Run</para>
<programlisting>setkey -D</programlisting>
<para>on either host to view the security association information.</para>
<para>That's one half of the problem. The other half is setting
your security policies.</para>
<para>To create a sensible security policy, let's review what's
been set up so far. This discussions hold for both ends of the
link.</para>
<para>Each IP packet that you send out has a header that contains
data about the packet. The header includes the IP addresses of
both the source and destination. As we already know, private IP
addresses, such as the <hostid role="ipaddr">192.168.x.y</hostid>
range are not supposed to appear on the public Internet.
Instead, they must first be encapsulated inside another packet.
This packet must have the public source and destination IP
addresses substituted for the private addresses.</para>
<para>So if your outgoing packet started looking like this:</para>
<mediaobject>
<imageobject>
<imagedata fileref="security/ipsec-out-pkt" align="center"/>
</imageobject>
<textobject>
<literallayout class="monospaced">
.----------------------.
| Src: 192.168.1.1 |
| Dst: 192.168.2.1 |
| <other header info> |
+----------------------+
| <packet data> |
`----------------------'</literallayout>
</textobject>
</mediaobject>
<para>Then it will be encapsulated inside another packet, looking
something like this:</para>
<mediaobject>
<imageobject>
<imagedata fileref="security/ipsec-encap-pkt" align="center"/>
</imageobject>
<textobject>
<literallayout class="monospaced">
.--------------------------.
| Src: A.B.C.D |
| Dst: W.X.Y.Z |
| <other header info> |
+--------------------------+
| .----------------------. |
| | Src: 192.168.1.1 | |
| | Dst: 192.168.2.1 | |
| | <other header info> | |
| +----------------------+ |
| | <packet data> | |
| `----------------------' |
`--------------------------'</literallayout>
</textobject>
</mediaobject>
<para>This encapsulation is carried out by the
<devicename>gif</devicename> device. As
you can see, the packet now has real IP addresses on the outside,
and our original packet has been wrapped up as data inside the
packet that will be put out on the Internet.</para>
<para>Obviously, we want all traffic between the VPNs to be
encrypted. You might try putting this in to words, as:</para>
<para><quote>If a packet leaves from <hostid
role="ipaddr">A.B.C.D</hostid>, and it is destined for <hostid
role="ipaddr">W.X.Y.Z</hostid>, then encrypt it, using the
necessary security associations.</quote></para>
<para><quote>If a packet arrives from <hostid
role="ipaddr">W.X.Y.Z</hostid>, and it is destined for <hostid
role="ipaddr">A.B.C.D</hostid>, then decrypt it, using the
necessary security associations.</quote></para>
<para>That's close, but not quite right. If you did this, all
traffic to and from <hostid role="ipaddr">W.X.Y.Z</hostid>, even
traffic that was not part of the VPN, would be encrypted. That's
not quite what you want. The correct policy is as follows</para>
<para><quote>If a packet leaves from <hostid
role="ipaddr">A.B.C.D</hostid>, and that packet is encapsulating
another packet, and it is destined for <hostid
role="ipaddr">W.X.Y.Z</hostid>, then encrypt it, using the
necessary security associations.</quote></para>
<para><quote>If a packet arrives from <hostid
role="ipaddr">W.X.Y.Z</hostid>, and that packet is encapsulating
another packet, and it is destined for <hostid
role="ipaddr">A.B.C.D</hostid>, then decrypt it, using the
necessary security associations.</quote></para>
<para>A subtle change, but a necessary one.</para>
<para>Security policies are also set using &man.setkey.8;.
&man.setkey.8; features a configuration language for defining the
policy. You can either enter configuration instructions via
stdin, or you can use the <option>-f</option> option to specify a
filename that contains configuration instructions.</para>
<para>The configuration on gateway host #1 (which has the public
IP address <hostid role="ipaddr">A.B.C.D</hostid>) to force all
outbound traffic to <hostid role="ipaddr">W.X.Y.Z</hostid> to be
encrypted is:</para>
<programlisting>
spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require;
</programlisting>
<para>Put these commands in a file (e.g.
<filename>/etc/ipsec.conf</filename>) and then run</para>
<screen>&prompt.root; <userinput>setkey -f /etc/ipsec.conf</userinput></screen>
<para><option>spdadd</option> tells &man.setkey.8; that we want
to add a rule to the secure policy database. The rest of this
line specifies which packets will match this policy. <hostid
role="ipaddr">A.B.C.D/32</hostid> and <hostid
role="ipaddr">W.X.Y.Z/32</hostid> are the IP addresses and
netmasks that identify the network or hosts that this policy will
apply to. In this case, we want it to apply to traffic between
these two hosts. <option>ipencap</option> tells the kernel that
this policy should only apply to packets that encapsulate other
packets. <option>-P out</option> says that this policy applies
to outgoing packets, and <option>ipsec</option> says that the
packet will be secured.</para>
<para>The second line specifies how this packet will be
encrypted. <option>esp</option> is the protocol that will be
used, while <option>tunnel</option> indicates that the packet
will be further encapsulated in an IPsec packet. The repeated
use of <hostid role="ipaddr">A.B.C.D</hostid> and <hostid
role="ipaddr">W.X.Y.Z</hostid> is used to select the security
association to use, and the final <option>require</option>
mandates that packets must be encrypted if they match this
rule.</para>
<para>This rule only matches outgoing packets. You will need a
similar rule to match incoming packets.</para>
<programlisting>spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require;</programlisting>
<para>Note the <option>in</option> instead of
<option>out</option> in this case, and the necessary reversal of
the IP addresses.</para>
<para>The other gateway host (which has the public IP address
<hostid role="ipaddr">W.X.Y.Z</hostid>) will need similar rules.</para>
<programlisting>spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require;
spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require;</programlisting>
<para>Finally, you need to add firewall rules to allow ESP and
IPENCAP packets back and forth. These rules will need to be
added to both hosts.</para>
<programlisting>ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z
ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D
ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z
ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D
</programlisting>
<para>Because the rules are symmetric you can use the same rules
on each gateway host.</para>
<para>Outgoing packets will now look something like this:</para>
<mediaobject>
<imageobject>
<imagedata fileref="security/ipsec-crypt-pkt" align="center"/>
</imageobject>
<textobject>
<literallayout class="monospaced">
.------------------------------. --------------------------.
| Src: A.B.C.D | |
| Dst: W.X.Y.Z | |
| <other header info> | | Encrypted
+------------------------------+ | packet.
| .--------------------------. | -------------. | contents
| | Src: A.B.C.D | | | | are
| | Dst: W.X.Y.Z | | | | completely
| | <other header info> | | | |- secure
| +--------------------------+ | | Encap'd | from third
| | .----------------------. | | -. | packet | party
| | | Src: 192.168.1.1 | | | | Original |- with real | snooping
| | | Dst: 192.168.2.1 | | | | packet, | IP addr |
| | | <other header info> | | | |- private | |
| | +----------------------+ | | | IP addr | |
| | | <packet data> | | | | | |
| | `----------------------' | | -' | |
| `--------------------------' | -------------' |
`------------------------------' --------------------------'
</literallayout>
</textobject>
</mediaobject>
<para>When they are received by the far end of the VPN they will
first be decrypted (using the security associations that have
been negotiated by racoon). Then they will enter the
<devicename>gif</devicename> interface, which will unwrap
the second layer, until you are left with the innermost
packet, which can then travel in to the inner network.</para>
<para>You can check the security using the same &man.ping.8; test from
earlier. First, log in to the
<hostid role="ipaddr">A.B.C.D</hostid> gateway machine, and
run:</para>
<programlisting>tcpdump dst host 192.168.2.1</programlisting>
<para>In another log in session on the same host run</para>
<programlisting>ping 192.168.2.1</programlisting>
<para>This time you should see output like the following:</para>
<programlisting>XXX tcpdump output</programlisting>
<para>Now, as you can see, &man.tcpdump.1; shows the ESP packets. If
you try to examine them with the <option>-s</option> option you will see
(apparently) gibberish, because of the encryption.</para>
<para>Congratulations. You have just set up a VPN between two
remote sites.</para>
<itemizedlist>
<title>Summary</title>
<listitem>
<para>Configure both kernels with:</para>
<programlisting>options IPSEC
options IPSEC_ESP
</programlisting>
</listitem>
<listitem>
<para>Install <filename
role="package">security/ipsec-tools</filename>. Edit
<filename>${PREFIX}/etc/racoon/psk.txt</filename> on both
gateway hosts, adding an entry for the remote host's IP
address and a secret key that they both know. Make sure
this file is mode 0600.</para>
</listitem>
<listitem>
<para>Add the following lines to
<filename>/etc/rc.conf</filename> on each host:</para>
<programlisting>ipsec_enable="YES"
ipsec_file="/etc/ipsec.conf"
</programlisting>
</listitem>
<listitem>
<para>Create an <filename>/etc/ipsec.conf</filename> on each
host that contains the necessary spdadd lines. On gateway
host #1 this would be:</para>
<programlisting>
spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec
esp/tunnel/A.B.C.D-W.X.Y.Z/require;
spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec
esp/tunnel/W.X.Y.Z-A.B.C.D/require;
</programlisting>
<para>On gateway host #2 this would be:</para>
<programlisting>
spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec
esp/tunnel/W.X.Y.Z-A.B.C.D/require;
spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec
esp/tunnel/A.B.C.D-W.X.Y.Z/require;
</programlisting>
</listitem>
<listitem>
<para>Add firewall rules to allow IKE, ESP, and IPENCAP
traffic to both hosts:</para>
<programlisting>
ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp
ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp
ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z
ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D
ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z
ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D
</programlisting>
</listitem>
</itemizedlist>
<para>The previous two steps should suffice to get the VPN up and
running. Machines on each network will be able to refer to one
another using IP addresses, and all traffic across the link will
be automatically and securely encrypted.</para>
</sect3>
</sect2>
</sect1>
<sect1 id="openssh">
<sect1info>
<authorgroup>
<author>
<firstname>Chern</firstname>
<surname>Lee</surname>
<contrib>Contributed by </contrib>
</author>
<!-- 21 April 2001 -->
</authorgroup>
</sect1info>
<title>OpenSSH</title>
<indexterm><primary>OpenSSH</primary></indexterm>
<indexterm>
<primary>security</primary>
<secondary>OpenSSH</secondary>
</indexterm>
<para><application>OpenSSH</application> is a set of network connectivity tools used to
access remote machines securely. It can be used as a direct
replacement for <command>rlogin</command>,
<command>rsh</command>, <command>rcp</command>, and
<command>telnet</command>. Additionally, TCP/IP
connections can be tunneled/forwarded securely through SSH.
<application>OpenSSH</application> encrypts all traffic to effectively eliminate eavesdropping,
connection hijacking, and other network-level attacks.</para>
<para><application>OpenSSH</application> is maintained by the OpenBSD project, and is based
upon SSH v1.2.12 with all the recent bug fixes and updates. It
is compatible with both SSH protocols 1 and 2.</para>
<sect2>
<title>Advantages of Using OpenSSH</title>
<para>Normally, when using &man.telnet.1; or &man.rlogin.1;,
data is sent over the network in an clear, un-encrypted form.
Network sniffers anywhere in between the client and server can
steal your user/password information or data transferred in
your session. <application>OpenSSH</application> offers a variety of authentication and
encryption methods to prevent this from happening.</para>
</sect2>
<sect2>
<title>Enabling sshd</title>
<indexterm>
<primary>OpenSSH</primary>
<secondary>enabling</secondary>
</indexterm>
<para>The
<application>sshd</application> is an option presented during
a <literal>Standard</literal> install of &os;. To see if
<application>sshd</application> is enabled, check the
<filename>rc.conf</filename> file for:</para>
<screen>sshd_enable="YES"</screen>
<para>This will load &man.sshd.8;, the daemon program for <application>OpenSSH</application>,
the next time your system initializes. Alternatively, it is
possible to use <filename>/etc/rc.d/sshd</filename> &man.rc.8;
script to start <application>OpenSSH</application>:</para>
<programlisting>/etc/rc.d/sshd start</programlisting>
</sect2>
<sect2>
<title>SSH Client</title>
<indexterm>
<primary>OpenSSH</primary>
<secondary>client</secondary>
</indexterm>
<para>The &man.ssh.1; utility works similarly to
&man.rlogin.1;.</para>
<screen>&prompt.root; <userinput>ssh <replaceable>user@example.com</replaceable></userinput>
Host key not found from the list of known hosts.
Are you sure you want to continue connecting (yes/no)? <userinput>yes</userinput>
Host 'example.com' added to the list of known hosts.
user@example.com's password: <userinput>*******</userinput></screen>
<para>The login will continue just as it would have if a session was
created using <command>rlogin</command> or
<command>telnet</command>. SSH utilizes a key fingerprint
system for verifying the authenticity of the server when the
client connects. The user is prompted to enter
<literal>yes</literal> only when
connecting for the first time. Future attempts to login are all
verified against the saved fingerprint key. The SSH client
will alert you if the saved fingerprint differs from the
received fingerprint on future login attempts. The fingerprints
are saved in <filename>~/.ssh/known_hosts</filename>, or
<filename>~/.ssh/known_hosts2</filename> for SSH v2
fingerprints.</para>
<para>By default, recent versions of the
<application>OpenSSH</application> servers only accept SSH v2
connections. The client will use version 2 if possible and
will fall back to version 1. The client can also be forced to
use one or the other by passing it the <option>-1</option> or
<option>-2</option> for version 1 or version 2, respectively.
The version 1 compatibility is maintained in the client for
backwards compatibility with older versions.</para>
</sect2>
<sect2>
<title>Secure Copy</title>
<indexterm>
<primary>OpenSSH</primary>
<secondary>secure copy</secondary>
</indexterm>
<indexterm><primary><command>scp</command></primary></indexterm>
<para>The &man.scp.1; command works similarly to
&man.rcp.1;; it copies a file to or from a remote machine,
except in a secure fashion.</para>
<screen>&prompt.root; <userinput> scp <replaceable>user@example.com:/COPYRIGHT COPYRIGHT</replaceable></userinput>
user@example.com's password: <userinput>*******</userinput>
COPYRIGHT 100% |*****************************| 4735
00:00
&prompt.root;</screen>
<para>Since the fingerprint was already saved for this host in the
previous example, it is verified when using &man.scp.1;
here.</para>
<para>The arguments passed to &man.scp.1; are similar
to &man.cp.1;, with the file or files in the first
argument, and the destination in the second. Since the file is
fetched over the network, through SSH, one or more of the file
arguments takes on the form
<option>user@host:<path_to_remote_file></option>.</para>
</sect2>
<sect2>
<title>Configuration</title>
<indexterm>
<primary>OpenSSH</primary>
<secondary>configuration</secondary>
</indexterm>
<para>The system-wide configuration files for both the
<application>OpenSSH</application> daemon and client reside
within the <filename>/etc/ssh</filename> directory.</para>
<para><filename>ssh_config</filename> configures the client
settings, while <filename>sshd_config</filename> configures the
daemon.</para>
<para>Additionally, the <option>sshd_program</option>
(<filename>/usr/sbin/sshd</filename> by default), and
<option>sshd_flags</option> <filename>rc.conf</filename>
options can provide more levels of configuration.</para>
</sect2>
<sect2 id="security-ssh-keygen">
<title>ssh-keygen</title>
<para>Instead of using passwords, &man.ssh-keygen.1; can
be used to generate DSA or RSA keys to authenticate a user:</para>
<screen>&prompt.user; <userinput>ssh-keygen -t <replaceable>dsa</replaceable></userinput>
Generating public/private dsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_dsa):
Created directory '/home/user/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_dsa.
Your public key has been saved in /home/user/.ssh/id_dsa.pub.
The key fingerprint is:
bb:48:db:f2:93:57:80:b6:aa:bc:f5:d5:ba:8f:79:17 user@host.example.com
</screen>
<para>&man.ssh-keygen.1; will create a public and private
key pair for use in authentication. The private key is stored in
<filename>~/.ssh/id_dsa</filename> or
<filename>~/.ssh/id_rsa</filename>, whereas the public key is
stored in <filename>~/.ssh/id_dsa.pub</filename> or
<filename>~/.ssh/id_rsa.pub</filename>, respectively for DSA and
RSA key types. The public key must be placed in
<filename>~/.ssh/authorized_keys</filename> of the remote
machine in order for the setup to work. Similarly, RSA version
1 public keys should be placed in
<filename>~/.ssh/authorized_keys</filename>.</para>
<para>This will allow connection to the remote machine based upon
SSH keys instead of passwords.</para>
<para>If a passphrase is used in &man.ssh-keygen.1;, the user
will be prompted for a password each time in order to use the
private key. &man.ssh-agent.1; can alleviate the strain of
repeatedly entering long passphrases, and is explored in the
<xref linkend="security-ssh-agent"/> section below.</para>
<warning><para>The various options and files can be different
according to the <application>OpenSSH</application> version
you have on your system; to avoid problems you should consult
the &man.ssh-keygen.1; manual page.</para></warning>
</sect2>
<sect2 id="security-ssh-agent">
<title>ssh-agent and ssh-add</title>
<para>The &man.ssh-agent.1; and &man.ssh-add.1; utilities provide
methods for <application>SSH</application> keys to be loaded
into memory for use, without needing to type the passphrase
each time.</para>
<para>The &man.ssh-agent.1; utility will handle the authentication
using the private key(s) that are loaded into it.
&man.ssh-agent.1; should be used to launch another application.
At the most basic level, it could spawn a shell or at a more
advanced level, a window manager.</para>
<para>To use &man.ssh-agent.1; in a shell, first it will need to
be spawned with a shell as an argument. Secondly, the
identity needs to be added by running &man.ssh-add.1; and
providing it the passphrase for the private key. Once these
steps have been completed the user will be able to &man.ssh.1;
to any host that has the corresponding public key installed.
For example:</para>
<screen>&prompt.user; ssh-agent <replaceable>csh</replaceable>
&prompt.user; ssh-add
Enter passphrase for /home/user/.ssh/id_dsa:
Identity added: /home/user/.ssh/id_dsa (/home/user/.ssh/id_dsa)
&prompt.user;</screen>
<para>To use &man.ssh-agent.1; in X11, a call to
&man.ssh-agent.1; will need to be placed in
<filename>~/.xinitrc</filename>. This will provide the
&man.ssh-agent.1; services to all programs launched in X11.
An example <filename>~/.xinitrc</filename> file might look
like this:</para>
<programlisting>exec ssh-agent <replaceable>startxfce4</replaceable></programlisting>
<para>This would launch &man.ssh-agent.1;, which would in turn
launch <application>XFCE</application>, every time X11 starts.
Then once that is done and X11 has been restarted so that the
changes can take effect, simply run &man.ssh-add.1; to load
all of your SSH keys.</para>
</sect2>
<sect2 id="security-ssh-tunneling">
<title>SSH Tunneling</title>
<indexterm>
<primary>OpenSSH</primary>
<secondary>tunneling</secondary>
</indexterm>
<para><application>OpenSSH</application> has the ability to create a tunnel to encapsulate
another protocol in an encrypted session.</para>
<para>The following command tells &man.ssh.1; to create a tunnel
for <application>telnet</application>:</para>
<screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>5023:localhost:23 user@foo.example.com</replaceable></userinput>
&prompt.user;</screen>
<para>The <command>ssh</command> command is used with the
following options:</para>
<variablelist>
<varlistentry>
<term><option>-2</option></term>
<listitem>
<para>Forces <command>ssh</command> to use version 2 of
the protocol. (Do not use if you are working with older
SSH servers)</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-N</option></term>
<listitem>
<para>Indicates no command, or tunnel only. If omitted,
<command>ssh</command> would initiate a normal
session.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-f</option></term>
<listitem>
<para>Forces <command>ssh</command> to run in the
background.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-L</option></term>
<listitem>
<para>Indicates a local tunnel in
<replaceable>localport:remotehost:remoteport</replaceable>
fashion.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>user@foo.example.com</option></term>
<listitem>
<para>The remote SSH server.</para>
</listitem>
</varlistentry>
</variablelist>
<para>An SSH tunnel works by creating a listen socket on
<hostid>localhost</hostid> on the specified port.
It then forwards any connection received
on the local host/port via the SSH connection to the specified
remote host and port.</para>
<para>In the example, port <replaceable>5023</replaceable> on
<hostid>localhost</hostid> is being forwarded to port
<replaceable>23</replaceable> on <hostid>localhost</hostid>
of the remote machine. Since <replaceable>23</replaceable> is <application>telnet</application>,
this would create a secure <application>telnet</application> session through an SSH tunnel.</para>
<para>This can be used to wrap any number of insecure TCP
protocols such as SMTP, POP3, FTP, etc.</para>
<example>
<title>Using SSH to Create a Secure Tunnel for SMTP</title>
<screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>5025:localhost:25 user@mailserver.example.com</replaceable></userinput>
user@mailserver.example.com's password: <userinput>*****</userinput>
&prompt.user; <userinput>telnet localhost 5025</userinput>
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP</screen>
<para>This can be used in conjunction with an
&man.ssh-keygen.1; and additional user accounts to create a
more seamless/hassle-free SSH tunneling environment. Keys
can be used in place of typing a password, and the tunnels
can be run as a separate user.</para>
</example>
<sect3>
<title>Practical SSH Tunneling Examples</title>
<sect4>
<title>Secure Access of a POP3 Server</title>
<para>At work, there is an SSH server that accepts
connections from the outside. On the same office network
resides a mail server running a POP3 server. The network,
or network path between your home and office may or may not
be completely trustable. Because of this, you need to check
your e-mail in a secure manner. The solution is to create
an SSH connection to your office's SSH server, and tunnel
through to the mail server.</para>
<screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>2110:mail.example.com:110 user@ssh-server.example.com</replaceable></userinput>
user@ssh-server.example.com's password: <userinput>******</userinput></screen>
<para>When the tunnel is up and running, you can point your
mail client to send POP3 requests to <hostid>localhost</hostid>
port 2110. A connection here will be forwarded securely across
the tunnel to <hostid>mail.example.com</hostid>.</para>
</sect4>
<sect4>
<title>Bypassing a Draconian Firewall</title>
<para>Some network administrators impose extremely draconian
firewall rules, filtering not only incoming connections,
but outgoing connections. You may be only given access
to contact remote machines on ports 22 and 80 for SSH
and web surfing.</para>
<para>You may wish to access another (perhaps non-work
related) service, such as an Ogg Vorbis server to stream
music. If this Ogg Vorbis server is streaming on some other
port than 22 or 80, you will not be able to access it.</para>
<para>The solution is to create an SSH connection to a machine
outside of your network's firewall, and use it to tunnel to
the Ogg Vorbis server.</para>
<screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>8888:music.example.com:8000 user@unfirewalled-system.example.org</replaceable></userinput>
user@unfirewalled-system.example.org's password: <userinput>*******</userinput></screen>
<para>Your streaming client can now be pointed to
<hostid>localhost</hostid> port 8888, which will be
forwarded over to <hostid>music.example.com</hostid> port
8000, successfully evading the firewall.</para>
</sect4>
</sect3>
</sect2>
<sect2>
<title>The <varname>AllowUsers</varname> Users Option</title>
<para>It is often a good idea to limit which users can log in and
from where. The <literal>AllowUsers</literal> option is a good
way to accomplish this. For example, to only allow the
<username>root</username> user to log in from
<hostid role="ipaddr">192.168.1.32</hostid>, something like this
would be appropriate in the
<filename>/etc/ssh/sshd_config</filename> file:</para>
<programlisting>AllowUsers root@192.168.1.32</programlisting>
<para>To allow the user <username>admin</username> to log in from
anywhere, just list the username by itself:</para>
<programlisting>AllowUsers admin</programlisting>
<para>Multiple users should be listed on the same line, like so:</para>
<programlisting>AllowUsers root@192.168.1.32 admin</programlisting>
<note>
<para>It is important that you list each user that needs to
log in to this machine; otherwise they will be locked out.</para>
</note>
<para>After making changes to
<filename>/etc/ssh/sshd_config</filename> you must tell
&man.sshd.8; to reload its config files, by running:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/sshd reload</userinput></screen>
</sect2>
<sect2>
<title>Further Reading</title>
<para><ulink url="http://www.openssh.com/">OpenSSH</ulink></para>
<para>&man.ssh.1; &man.scp.1; &man.ssh-keygen.1;
&man.ssh-agent.1; &man.ssh-add.1; &man.ssh.config.5;</para>
<para>&man.sshd.8; &man.sftp-server.8; &man.sshd.config.5;</para>
</sect2>
</sect1>
<sect1 id="fs-acl">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
</sect1info>
<title>File System Access Control Lists</title>
<indexterm>
<primary>ACL</primary>
</indexterm>
<para>In conjunction with file system enhancements like snapshots, FreeBSD 5.0
and later offers the security of File System Access Control Lists
(<acronym>ACL</acronym>s).</para>
<para>Access Control Lists extend the standard &unix;
permission model in a highly compatible (&posix;.1e) way. This feature
permits an administrator to make use of and take advantage of a
more sophisticated security model.</para>
<para>To enable <acronym>ACL</acronym> support for <acronym>UFS</acronym>
file systems, the following:</para>
<programlisting>options UFS_ACL</programlisting>
<para>must be compiled into the kernel. If this option has
not been compiled in, a warning message will be displayed
when attempting to mount a file system supporting <acronym>ACL</acronym>s.
This option is included in the <filename>GENERIC</filename> kernel.
<acronym>ACL</acronym>s rely on extended attributes being enabled on
the file system. Extended attributes are natively supported in the next generation
&unix; file system, <acronym>UFS2</acronym>.</para>
<note><para>A higher level of administrative overhead is required to
configure extended attributes on <acronym>UFS1</acronym> than on
<acronym>UFS2</acronym>. The performance of extended attributes
on <acronym>UFS2</acronym> is also substantially higher. As a
result, <acronym>UFS2</acronym> is generally recommended in preference
to <acronym>UFS1</acronym> for use with access control lists.</para></note>
<para><acronym>ACL</acronym>s are enabled by the mount-time administrative
flag, <option>acls</option>, which may be added to <filename>/etc/fstab</filename>.
The mount-time flag can also be automatically set in a persistent manner using
&man.tunefs.8; to modify a superblock <acronym>ACL</acronym>s flag in the
file system header. In general, it is preferred to use the superblock flag
for several reasons:</para>
<itemizedlist>
<listitem>
<para>The mount-time <acronym>ACL</acronym>s flag cannot be changed by a
remount (&man.mount.8; <option>-u</option>), only by means of a complete
&man.umount.8; and fresh &man.mount.8;. This means that
<acronym>ACL</acronym>s cannot be enabled on the root file system after boot.
It also means that you cannot change the disposition of a file system once
it is in use.</para>
</listitem>
<listitem>
<para>Setting the superblock flag will cause the file system to always be
mounted with <acronym>ACL</acronym>s enabled even if there is not an
<filename>fstab</filename> entry or if the devices re-order. This prevents
accidental mounting of the file system without <acronym>ACL</acronym>s
enabled, which can result in <acronym>ACL</acronym>s being improperly enforced,
and hence security problems.</para>
</listitem>
</itemizedlist>
<note><para>We may change the <acronym>ACL</acronym>s behavior to allow the flag to
be enabled without a complete fresh &man.mount.8;, but we consider it desirable to
discourage accidental mounting without <acronym>ACL</acronym>s enabled, because you
can shoot your feet quite nastily if you enable <acronym>ACL</acronym>s, then disable
them, then re-enable them without flushing the extended attributes. In general, once
you have enabled <acronym>ACL</acronym>s on a file system, they should not be disabled,
as the resulting file protections may not be compatible with those intended by the
users of the system, and re-enabling <acronym>ACL</acronym>s may re-attach the previous
<acronym>ACL</acronym>s to files that have since had their permissions changed,
resulting in other unpredictable behavior.</para></note>
<para>File systems with <acronym>ACL</acronym>s enabled will show a <literal>+</literal>
(plus) sign in their permission settings when viewed. For example:</para>
<programlisting>drwx------ 2 robert robert 512 Dec 27 11:54 private
drwxrwx---+ 2 robert robert 512 Dec 23 10:57 directory1
drwxrwx---+ 2 robert robert 512 Dec 22 10:20 directory2
drwxrwx---+ 2 robert robert 512 Dec 27 11:57 directory3
drwxr-xr-x 2 robert robert 512 Nov 10 11:54 public_html</programlisting>
<para>Here we see that the <filename>directory1</filename>,
<filename>directory2</filename>, and <filename>directory3</filename>
directories are all taking advantage of <acronym>ACL</acronym>s. The
<filename>public_html</filename> directory is not.</para>
<sect2>
<title>Making Use of <acronym>ACL</acronym>s</title>
<para>The file system <acronym>ACL</acronym>s can be viewed by the
&man.getfacl.1; utility. For instance, to view the
<acronym>ACL</acronym> settings on the <filename>test</filename>
file, one would use the command:</para>
<screen>&prompt.user; <userinput>getfacl <filename>test</filename></userinput>
#file:test
#owner:1001
#group:1001
user::rw-
group::r--
other::r--</screen>
<para>To change the <acronym>ACL</acronym> settings on this file,
invoke the &man.setfacl.1; utility. Observe:</para>
<screen>&prompt.user; <userinput>setfacl -k <filename>test</filename></userinput></screen>
<para>The <option>-k</option> flag will remove all of the
currently defined <acronym>ACL</acronym>s from a file or file
system. The more preferable method would be to use
<option>-b</option> as it leaves the basic fields required for
<acronym>ACL</acronym>s to work.</para>
<screen>&prompt.user; <userinput>setfacl -m u:trhodes:rwx,group:web:r--,o::--- <filename>test</filename></userinput></screen>
<para>In the aforementioned command, the <option>-m</option>
option was used to modify the default <acronym>ACL</acronym>
entries. Since there were no pre-defined entries, as they were
removed by the previous command, this will restore the default
options and assign the options listed. Take care to notice that
if you add a user or group which does not exist on the system,
an <errorname>Invalid argument</errorname> error will be printed
to <devicename>stdout</devicename>.</para>
</sect2>
</sect1>
<sect1 id="security-portaudit">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
</sect1info>
<title>Monitoring Third Party Security Issues</title>
<indexterm>
<primary>Portaudit</primary>
</indexterm>
<para>In recent years, the security world has made many improvements
to how vulnerability assessment is handled. The threat of system
intrusion increases as third party utilities are installed and
configured for virtually any operating system available
today.</para>
<para>Vulnerability assessment is a key factor in security, and
while &os; releases advisories for the base system, doing so
for every third party utility is beyond the &os; Project's
capability. There is a way to mitigate third party
vulnerabilities and warn administrators of known security
issues. A &os; add on utility known as
<application>Portaudit</application> exists solely for this
purpose.</para>
<para>The <filename role="port">ports-mgmt/portaudit</filename> port
polls a database, updated and maintained by the &os; Security
Team and ports developers, for known security issues.</para>
<para>To begin using <application>Portaudit</application>, one
must install it from the Ports Collection:</para>
<screen>&prompt.root; <userinput>cd /usr/ports/ports-mgmt/portaudit && make install clean</userinput></screen>
<para>During the install process, the configuration files for
&man.periodic.8; will be updated, permitting
<application>Portaudit</application> output in the daily security
runs. Ensure the daily security run emails, which are sent to
<username>root</username>'s email account, are being read. No
more configuration will be required here.</para>
<para>After installation, an administrator can update the database
and view known vulnerabilities in installed packages by invoking
the following command:</para>
<screen>&prompt.root; <userinput>portaudit -Fda</userinput></screen>
<note>
<para>The database will automatically be updated during the
&man.periodic.8; run; thus, the previous command is completely
optional. It is only required for the following
examples.</para>
</note>
<para>To audit the third party utilities installed as part of
the Ports Collection at anytime, an administrator need only run
the following command:</para>
<screen>&prompt.root; <userinput>portaudit -a</userinput></screen>
<para><application>Portaudit</application> will produce something
like this for vulnerable packages:</para>
<programlisting>Affected package: cups-base-1.1.22.0_1
Type of problem: cups-base -- HPGL buffer overflow vulnerability.
Reference: <http://www.FreeBSD.org/ports/portaudit/40a3bca2-6809-11d9-a9e7-0001020eed82.html>
1 problem(s) in your installed packages found.
You are advised to update or deinstall the affected package(s) immediately.</programlisting>
<para>By pointing a web browser to the <acronym>URL</acronym> shown,
an administrator may obtain more information about the
vulnerability in question. This will include versions affected,
by &os; Port version, along with other web sites which may contain
security advisories.</para>
<para>In short, <application>Portaudit</application> is a powerful
utility and extremely useful when coupled with the
<application>Portupgrade</application> port.</para>
</sect1>
<sect1 id="security-advisories">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
</sect1info>
<title>&os; Security Advisories</title>
<indexterm>
<primary>FreeBSD Security Advisories</primary>
</indexterm>
<para>Like many production quality operating systems, &os; publishes
<quote>Security Advisories</quote>. These advisories are usually
mailed to the security lists and noted in the Errata only
after the appropriate releases have been patched. This section
will work to explain what an advisory is, how to understand it,
and what measures to take in order to patch a system.</para>
<sect2>
<title>What does an advisory look like?</title>
<para>The &os; security advisories look similar to the one below,
taken from the &a.security-notifications.name; mailing list.</para>
<programlisting>=============================================================================
&os;-SA-XX:XX.UTIL Security Advisory
The &os; Project
Topic: denial of service due to some problem<co id="co-topic"/>
Category: core<co id="co-category"/>
Module: sys<co id="co-module"/>
Announced: 2003-09-23<co id="co-announce"/>
Credits: Person@EMAIL-ADDRESS<co id="co-credit"/>
Affects: All releases of &os;<co id="co-affects"/>
&os; 4-STABLE prior to the correction date
Corrected: 2003-09-23 16:42:59 UTC (RELENG_4, 4.9-PRERELEASE)
2003-09-23 20:08:42 UTC (RELENG_5_1, 5.1-RELEASE-p6)
2003-09-23 20:07:06 UTC (RELENG_5_0, 5.0-RELEASE-p15)
2003-09-23 16:44:58 UTC (RELENG_4_8, 4.8-RELEASE-p8)
2003-09-23 16:47:34 UTC (RELENG_4_7, 4.7-RELEASE-p18)
2003-09-23 16:49:46 UTC (RELENG_4_6, 4.6-RELEASE-p21)
2003-09-23 16:51:24 UTC (RELENG_4_5, 4.5-RELEASE-p33)
2003-09-23 16:52:45 UTC (RELENG_4_4, 4.4-RELEASE-p43)
2003-09-23 16:54:39 UTC (RELENG_4_3, 4.3-RELEASE-p39)<co id="co-corrected"/>
<acronym>CVE</acronym> Name: CVE-XXXX-XXXX<co id="co-cve"/>
For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
http://www.FreeBSD.org/security/.
I. Background<co id="co-backround"/>
II. Problem Description<co id="co-descript"/>
III. Impact<co id="co-impact"/>
IV. Workaround<co id="co-workaround"/>
V. Solution<co id="co-solution"/>
VI. Correction details<co id="co-details"/>
VII. References<co id="co-ref"/></programlisting>
<calloutlist>
<callout arearefs="co-topic">
<para>The <literal>Topic</literal> field indicates exactly what the problem is.
It is basically an introduction to the current security
advisory and notes the utility with the
vulnerability.</para>
</callout>
<callout arearefs="co-category">
<para>The <literal>Category</literal> refers to the affected part of the system
which may be one of <literal>core</literal>, <literal>contrib</literal>, or <literal>ports</literal>. The <literal>core</literal>
category means that the vulnerability affects a core
component of the &os; operating system. The <literal>contrib</literal>
category means that the vulnerability affects software
contributed to the &os; Project, such as
<application>sendmail</application>. Finally the <literal>ports</literal>
category indicates that the vulnerability affects add on
software available as part of the Ports Collection.</para>
</callout>
<callout arearefs="co-module">
<para>The <literal>Module</literal> field refers to the component location, for
instance <literal>sys</literal>. In this example, we see that the module,
<literal>sys</literal>, is affected; therefore, this vulnerability
affects a component used within the kernel.</para>
</callout>
<callout arearefs="co-announce">
<para>The <literal>Announced</literal> field reflects the date said security
advisory was published, or announced to the world. This
means that the security team has verified that the problem
does exist and that a patch has been committed to the &os;
source code repository.</para>
</callout>
<callout arearefs="co-credit">
<para>The <literal>Credits</literal> field gives credit to the individual or
organization who noticed the vulnerability and reported
it.</para>
</callout>
<callout arearefs="co-affects">
<para>The <literal>Affects</literal> field explains which releases of &os; are
affected by this vulnerability. For the kernel, a quick
look over the output from <command>ident</command> on the
affected files will help in determining the revision.
For ports, the version number is listed after the port name
in <filename>/var/db/pkg</filename>. If the system does not
sync with the &os; <acronym>CVS</acronym> repository and rebuild
daily, chances are that it is affected.</para>
</callout>
<callout arearefs="co-corrected">
<para>The <literal>Corrected</literal> field indicates the date, time, time
offset, and release that was corrected.</para>
</callout>
<callout arearefs="co-cve">
<para>Reserved for the identification information used to look up
vulnerabilities in the Common Vulnerabilities Database system.</para>
</callout>
<callout arearefs="co-backround">
<para>The <literal>Background</literal> field gives information on exactly what
the affected utility is. Most of the time this is why
the utility exists in &os;, what it is used for, and a bit
of information on how the utility came to be.</para>
</callout>
<callout arearefs="co-descript">
<para>The <literal>Problem Description</literal> field explains the security hole
in depth. This can include information on flawed code, or
even how the utility could be maliciously used to open
a security hole.</para>
</callout>
<callout arearefs="co-impact">
<para>The <literal>Impact</literal> field describes what type of impact the
problem could have on a system. For example, this could
be anything from a denial of service attack, to extra
privileges available to users, or even giving the attacker
superuser access.</para>
</callout>
<callout arearefs="co-workaround">
<para>The <literal>Workaround</literal> field offers a feasible workaround to
system administrators who may be incapable of upgrading
the system. This may be due to time constraints, network
availability, or a slew of other reasons. Regardless,
security should not be taken lightly, and an affected system
should either be patched or the security hole workaround
should be implemented.</para>
</callout>
<callout arearefs="co-solution">
<para>The <literal>Solution</literal> field offers instructions on patching the
affected system. This is a step by step tested and verified
method for getting a system patched and working
securely.</para>
</callout>
<callout arearefs="co-details">
<para>The <literal>Correction Details</literal> field displays the
<acronym>CVS</acronym> branch or release name with the
periods changed to underscore characters. It also shows
the revision number of the affected files within each
branch.</para>
</callout>
<callout arearefs="co-ref">
<para>The <literal>References</literal> field usually offers sources of other
information. This can include web <acronym>URL</acronym>s,
books, mailing lists, and newsgroups.</para>
</callout>
</calloutlist>
</sect2>
</sect1>
<sect1 id="security-accounting">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
</sect1info>
<title>Process Accounting</title>
<indexterm>
<primary>Process Accounting</primary>
</indexterm>
<para>Process accounting is a security method in which an
administrator may keep track of system resources used,
their allocation among users, provide for system monitoring,
and minimally track a user's commands.</para>
<para>This indeed has its own positive and negative points. One of
the positives is that an intrusion may be narrowed down
to the point of entry. A negative is the amount of logs
generated by process accounting, and the disk space they may
require. This section will walk an administrator through
the basics of process accounting.</para>
<sect2>
<title>Enable and Utilizing Process Accounting</title>
<para>Before making use of process accounting, it
must be enabled. To do this, execute the following
commands:</para>
<screen>&prompt.root; <userinput>touch <filename>/var/account/acct</filename></userinput>
&prompt.root; <userinput>accton <filename>/var/account/acct</filename></userinput>
&prompt.root; <userinput>echo 'accounting_enable="YES"' >> <filename>/etc/rc.conf</filename></userinput></screen>
<para>Once enabled, accounting will begin to track
<acronym>CPU</acronym> stats, commands, etc. All accounting
logs are in a non-human readable format and may be viewed
using the &man.sa.8; utility. If issued without any options,
<command>sa</command> will print information relating to the
number of per user calls, the total elapsed time in minutes,
total <acronym>CPU</acronym> and user time in minutes, average
number of I/O operations, etc.</para>
<para>To view information about commands being issued, one
would use the &man.lastcomm.1; utility. The
<command>lastcomm</command> may be used to print out commands
issued by users on specific &man.ttys.5;, for example:</para>
<screen>&prompt.root; <userinput>lastcomm ls
<username>trhodes</username> ttyp1</userinput></screen>
<para>Would print out all known usage of the <command>ls</command>
by <username>trhodes</username> on the ttyp1 terminal.</para>
<para>Many other useful options exist and are explained in the
&man.lastcomm.1;, &man.acct.5; and &man.sa.8; manual
pages.</para>
</sect2>
</sect1>
</chapter>
|