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
|
<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Postfix TLS Support </title>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
</head>
<body>
<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix TLS Support
</h1>
<hr>
<h2> What Postfix TLS support does for you </h2>
<p> Transport Layer Security (TLS, formerly called SSL) provides
certificate-based authentication and encrypted sessions. An
encrypted session protects the information that is transmitted with
SMTP mail or with SASL authentication. </p>
<p> NOTE: By turning on TLS support in Postfix, you not only get
the ability to encrypt mail and to authenticate remote SMTP clients
or servers. You also turn on hundreds of thousands of lines of
OpenSSL library code. Assuming that OpenSSL is written as carefully
as Wietse's own code, every 1000 lines introduce one additional bug
into Postfix. </p>
<p> Topics covered in this document: </p>
<ul>
<li><a href="#how">How Postfix TLS support works</a>
<li><a href="#server_tls">SMTP Server specific settings</a>
<li> <a href="#client_tls">SMTP Client specific settings</a>
<li><a href="#tlsmgr_controls"> TLS manager specific settings </a>
<li><a href="#build_tls">Building Postfix with TLS support</a>
<li><a href="#problems"> Reporting problems </a>
<li><a href="#credits"> Credits </a>
</ul>
<p> And last but not least, for the impatient: </p>
<ul>
<li><a href="#quick-start">Getting started, quick and dirty</a>
</ul>
<h2><a name="how">How Postfix TLS support works</a></h2>
<p> The diagram below shows the main elements of the Postfix TLS
architecture and their relationships. Colored boxes with numbered
names represent Postfix daemon programs. Other colored boxes
represent storage elements. </p>
<ul>
<li> <p> The <a href="smtpd.8.html">smtpd(8)</a> server implements the SMTP over TLS server
side. </p>
<li> <p> The <a href="smtp.8.html">smtp(8)</a> client implements the SMTP (and LMTP) over TLS
client side. </p>
<li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server maintains the pseudo-random number
generator (PRNG) that seeds the TLS engines in the <a href="smtpd.8.html">smtpd(8)</a> server
and <a href="smtp.8.html">smtp(8)</a> client processes, and maintains the TLS session key
cache files. </p>
</ul>
<p> Not shown in the figure are the <a href="tlsproxy.8.html">tlsproxy(8)</a> server and the
<a href="postscreen.8.html">postscreen(8)</a> server. These use TLS in the same manner as <a href="smtpd.8.html">smtpd(8)</a>.
</p>
<table>
<tr> <td>Network<tt>-> </tt> </td> <td align="center"
bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a> <br> </td> <td colspan="2">
<tt> <---seed----<br><br><-key/cert-> </tt> </td> <td
align="center" bgcolor="#f0f0ff"> <br> <a href="tlsmgr.8.html">tlsmgr(8)</a> <br> </td>
<td colspan="3"> <tt> ----seed---><br> <br><-key/cert->
</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtp.8.html">smtp(8)</a> <br>
</td> <td> <tt> -></tt>Network </td> </tr>
<tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td>
</td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table>
</td> <td align="center"> |<br> |</td> <td align="left"> <table>
<tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td>
</tr> </table> </td> <td colspan="3"> </td> </tr>
<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
smtpd<br> session<br> key cache </td> <td> </td> <td align="center"
bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td
align="center" bgcolor="#f0f0ff"> smtp<br> session<br> key cache
</td>
<td colspan="2"> </td> </tr>
</table>
<h2><a name="server_tls">SMTP Server specific settings</a></h2>
<p> Topics covered in this section: </p>
<ul>
<li><a href="#server_cert_key">Server-side certificate and private
key configuration </a>
<li><a href="#server_pfs">Server-side forward-secrecy configuration </a>
<li><a href="#server_logging"> Server-side TLS activity logging
</a>
<li><a href="#server_enable">Enabling TLS in the Postfix SMTP server </a>
<li><a href="#server_vrfy_client">Client certificate verification</a>
<li><a href="#server_tls_auth">Supporting AUTH over TLS only</a>
<li><a href="#server_tls_cache">Server-side TLS session cache</a>
<li><a href="#server_access">Server access control</a>
<li><a href="#server_cipher">Server-side cipher controls</a>
<li><a href="#server_misc"> Miscellaneous server controls</a>
</ul>
<h3><a name="server_cert_key">Server-side certificate and private
key configuration </a> </h3>
<p> In order to use TLS, the Postfix SMTP server generally needs
a certificate and a private key. Both must be in "PEM" format. The
private key must not be encrypted, meaning: the key must be accessible
without a password. The certificate and private key may be in the same
file, in which case the certificate file should be owned by "root" and
not be readable by any other user. If the key is stored separately,
this access restriction applies to the key file only, and the
certificate file may be "world-readable". </p>
<p> Public Internet MX hosts without certificates signed by a
well-known public CA must still generate, and be prepared to present
to most clients, a self-signed or private-CA signed certificate.
The remote SMTP client will generally not be able to verify the
self-signed certificate, but unless the client is running Postfix
or similar software, it will only negotiate TLS ciphersuites that
require a server certificate. </p>
<p> For servers that are <b>not</b> public Internet MX hosts, Postfix
supports configurations with no certificates. This entails the use of
just the anonymous TLS ciphers, which are not supported by typical SMTP
clients. Since some clients may not fall back to plain text after a TLS
handshake failure, a certificate-less Postfix SMTP server will be unable
to receive email from some TLS-enabled clients. To avoid accidental
configurations with no certificates, Postfix enables certificate-less
operation only when the administrator explicitly sets
"<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none". This ensures that new Postfix SMTP server
configurations will not accidentally enable TLS without certificates. </p>
<p> Note that server certificates are <b>not</b> optional in TLS 1.3. To
run without certificates you'd have to disable the TLS 1.3 protocol by
including '!TLSv1.3' in "<a href="postconf.5.html#smtpd_tls_protocols">smtpd_tls_protocols</a>" and perhaps also
"<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a>". It is simpler instead to just
configure a certificate chain. Certificate-less operation is not
recommended. <p>
<p> RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported.
Most sites only have RSA certificates. You can configure all three
at the same time, in which case the ciphersuite negotiated with the
remote SMTP client determines which certificate is used. If your
DNS zone is signed, and you want to publish DANE TLSA (<a href="http://tools.ietf.org/html/rfc6698">RFC 6698</a>,
<a href="http://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>) records, these must match all of the configured
certificate chains. Since the best practice is to publish "3 1 1"
certificate associations, create a separate TLSA record to match
each public-key certificate digest. </p>
<h4> Creating the server certificate file </h4>
<p> To verify the Postfix SMTP server certificate, the remote SMTP
client must receive the issuing CA certificates via the TLS handshake
or via public-key infrastructure. This means that the Postfix server
public-key certificate file must include the server certificate
first, then the issuing CA(s) (bottom-up order). The Postfix SMTP
server certificate must be usable as SSL server certificate and
hence pass the "<tt>openssl verify -purpose sslserver ...</tt>" test.
</p>
<p> The examples that follow show how to create a server certificate
file. We assume that the certificate for "server.example.com" was
issued by "intermediate CA" which itself has a certificate issued
by "root CA". </p>
<ul>
<li> <p> With legacy public CA trust verification, you can omit the
root certificate from the "server.pem" certificate file. If the
client trusts the root CA, it will already have a local copy of the
root CA certificate. Omitting the root CA certificate reduces the
size of the server TLS handshake. </p>
<blockquote>
<pre>
% <b>cat server_cert.pem intermediate_CA.pem > server.pem</b>
</pre>
</blockquote>
<li> <p> If you publish DANE TLSA (<a href="http://tools.ietf.org/html/rfc6698">RFC 6698</a>, <a href="http://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>)
"2 0 1" or "2 1 1" records to specify root CA certificate digests,
you must include the corresponding root CA certificates in the
"server.pem" certificate file. </p>
<blockquote>
<pre>
% <b>cat server_cert.pem intermediate_CA.pem root.pem > server.pem</b>
</pre>
</blockquote>
<p> Remote SMTP clients will be able to use the TLSA record you
publish (which only contains the certificate digest) only if they
have access to the corresponding certificate. Failure to verify
certificates per the server's published TLSA records will typically
cause the SMTP client to defer mail delivery. The foregoing also
applies to "2 0 2" and "2 1 2" TLSA records or any other digest of
a CA certificate, but it is expected that SHA256 will be by far the
most common digest for TLSA. </p>
<p> As a best practice, publish "3 1 1" TLSA associations that specify
the SHA256 digest of the server's public key. These continue to work
unmodified when a certificate is renewed with the same public/private
key pair. </p>
</ul>
<p> For instructions on how to compute the digest of a certificate
or its public key for use in TLSA records, see the documentation of
the <a href="postconf.5.html#smtpd_tls_fingerprint_digest">smtpd_tls_fingerprint_digest</a> <a href="postconf.5.html">main.cf</a> parameter. </p>
<p> When a new key or certificate is generated, an additional TLSA
record with the new digest must be published in advance of the
actual deployment of the new key or certificate on the server. You
must allow sufficient time for any TLSA RRsets with only the old
digest to expire from DNS caches. The safest practice is to wait
until the DNSSEC signature on the previous TLSA RRset expires, and
only then switch the server to use new keys published in the updated
TLSA RRset. Once the new certificate trust chain and private key
are in effect, the DNS should be updated once again to remove the
old digest from the TLSA RRset. </p>
<p> If you want the Postfix SMTP server to accept remote SMTP client
certificates issued by one or more root CAs, append the root
certificate to $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>
directory. </p>
<h4> Configuring the server certificate and key files </h4>
<p> Example: Postfix ≥ 3.4 all-in-one chain file(s). One or more
chain files that start with a key that is immediately followed by the
corresponding certificate and any additional issuer certificates. A
single file can hold multiple <i>(key, cert, [chain])</i> sequences, one
per algorithm. It is typically simpler to keep the chain for each
algorithm in its own file. Most users are likely to deploy just a
single RSA chain, but with OpenSSL 1.1.1, it is possible to deploy up to
five chains, one each for RSA, ECDSA, ED25519, ED448 and even the
obsolete DSA. </p>
<blockquote>
<pre>
# Postfix ≥ 3.4. Preferred configuration interface. Each file
# starts with the private key, followed by the corresponding
# certificate, and any intermediate issuer certificates. The root CA
# cert may also be needed when published as a DANE trust anchor.
#
<a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> =
/etc/postfix/rsa.pem,
/etc/postfix/ecdsa.pem,
/etc/postfix/ed25519.pem,
/etc/postfix/ed448.pem
</pre>
</blockquote>
<p> You can also store the keys separately from their certificates, again
provided each is listed before the corresponding certificate chain. Storing a
key and its associated certificate chain in separate files is not recommended,
because this is prone to race conditions during key rollover, as there is no
way to update multiple files atomically. </p>
<blockquote>
<pre>
# Postfix ≥ 3.4.
# Storing keys separately from the associated certificates is not
# recommended.
<a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> =
/etc/postfix/rsakey.pem,
/etc/postfix/rsacerts.pem,
/etc/postfix/ecdsakey.pem,
/etc/postfix/ecdsacerts.pem
</pre>
</blockquote>
<p> The below examples show the legacy algorithm-specific configurations
for Postfix 3.3 and older. With Postfix ≤ 3.3, even if the key is
stored in the same file as the certificate, the file is read twice and a
(brief) race condition still exists during key rollover. While Postfix
≥ 3.4 avoids the race when the key and certificate are in the same
file, you should use the new "<a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a>" interface shown
above. <p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/server.pem
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = $<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a>
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> = /etc/postfix/server-dsa.pem
<a href="postconf.5.html#smtpd_tls_dkey_file">smtpd_tls_dkey_file</a> = $<a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a>
</pre>
</blockquote>
<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
# Some clients will not be ECDSA capable, so you will likely still need
# an RSA certificate and private key.
#
<a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a> = /etc/postfix/server-ecdsa.pem
<a href="postconf.5.html#smtpd_tls_eckey_file">smtpd_tls_eckey_file</a> = $<a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a>
</pre>
</blockquote>
<p> TLS without certificates for servers serving exclusively
anonymous-cipher capable clients: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
# Not recommended: breaks TLS 1.3 and clients that don't support
# anonymous cipher suites.
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none
</pre>
</blockquote>
<p> To verify a remote SMTP client certificate, the Postfix SMTP
server needs to trust the certificates of the issuing Certification
Authorities. These certificates in "PEM" format can be stored in a
single $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> or in multiple files, one CA per file in
the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> contains the CA certificates of one or
more trusted CAs. The file is opened (with root privileges) before
Postfix enters the optional chroot jail and so need not be accessible
from inside the chroot jail. </p>
<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>
directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a>
privileges) from the files in the directory when the information
is needed. Thus, the $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> directory needs to be
accessible inside the optional chroot jail. </p>
<p> When you configure the Postfix SMTP server to request <a
href="#server_vrfy_client">client certificates</a>, the DNs of Certification
Authorities in $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> are sent to the client, in order to allow
it to choose an identity signed by a CA you trust. If no $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>
is specified, no preferred CA list is sent, and the client is free to
choose an identity signed by any CA. Many clients use a fixed identity
regardless of the preferred CA list and you may be able to reduce TLS
negotiation overhead by installing client CA certificates mostly or
only in $<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a>. In the latter case you need not specify a
$<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>. </p>
<p> Note, that unless client certificates are used to allow greater
access to TLS authenticated clients, it is best to not ask for
client certificates at all, as in addition to increased overhead
some clients (notably in some cases qmail) are unable to complete
the TLS handshake when client certificates are requested. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/CAcert.pem
<a href="postconf.5.html#smtpd_tls_CApath">smtpd_tls_CApath</a> = /etc/postfix/certs
</pre>
</blockquote>
<h3><a name="server_pfs"> Server-side forward-secrecy configuration </a> </h3>
<p> If you want to take maximal advantage of ciphers that offer <a
href="FORWARD_SECRECY_README.html#dfn_fs">forward secrecy</a> see
the <a href="FORWARD_SECRECY_README.html#quick-start">Getting
started</a> section of <a
href="FORWARD_SECRECY_README.html">FORWARD_SECRECY_README</a>. The
full document conveniently presents all information about Postfix
forward secrecy support in one place: what forward secrecy is, how
to tweak settings, and what you can expect to see when Postfix uses
ciphers with forward secrecy. </p>
<h3><a name="server_logging"> Server-side TLS activity logging </a> </h3>
<p> To get additional information about Postfix SMTP server TLS
activity you can increase the log level from 0..4. Each logging
level also includes the information that is logged at a lower
logging level. </p>
<blockquote>
<table border="1">
<tr> <th> Level </th> <th> Postfix 2.9 and later</th> <th> Earlier
releases. </th> </tr>
<tr> <td valign="top"> 0 </td> <td valign="top" colspan="2"> Disable
logging of TLS activity. </td> </tr>
<tr> <td valign="top"> 1 </td> <td valign="top"> Log only a summary
message on TLS handshake completion — no logging of client
certificate trust-chain verification errors if client certificate
verification is not required. </td> <td valign="top"> Log the summary
message, peer certificate summary information and unconditionally log
trust-chain verification errors. </td> </tr>
<tr> <td valign="top"> 2 </td> <td valign="top" colspan="2"> Also
log levels during TLS negotiation. </td> </tr>
<tr> <td valign="top"> 3 </td> <td valign="top" colspan="2"> Also
log hexadecimal and ASCII dump of TLS negotiation process. </td>
</tr>
<tr> <td valign="top"> 4 </td> <td valign="top" colspan="2"> Also
log hexadecimal and ASCII dump of complete transmission after
STARTTLS. </td></tr>
</table>
</blockquote>
<p> Use log level 3 only in case of problems. Use of log level 4 is
strongly discouraged. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 0
</pre>
</blockquote>
<p> To include information about the protocol and cipher used as
well as the client and issuer CommonName into the "Received:"
message header, set the <a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> variable to true.
The default is no, as the information is not necessarily authentic.
Only information recorded at the final destination is reliable,
since the headers may be changed by intermediate servers. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes
</pre>
</blockquote>
<h3><a name="server_enable">Enabling TLS in the Postfix SMTP server </a> </h3>
<p> By default, TLS is disabled in the Postfix SMTP server, so no
difference to plain Postfix is visible. Explicitly switch it on
with "<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may
</pre>
</blockquote>
<p> With this, the Postfix SMTP server announces STARTTLS support to
remote SMTP clients, but does not require that clients use TLS encryption.
</p>
<p> Note: when an unprivileged user invokes "sendmail -bs", STARTTLS
is never offered due to insufficient privileges to access the Postfix
SMTP server
private key. This is intended behavior. </p>
<p> <a name="server_enforce">You can ENFORCE the use of TLS</a>,
so that the Postfix SMTP server announces STARTTLS and accepts no
mail without TLS encryption, by setting
"<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt". According to <a href="http://tools.ietf.org/html/rfc2487">RFC 2487</a> this
MUST NOT be applied in case
of a publicly-referenced Postfix SMTP server. This option is off
by default and should only seldom be used. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt
</pre>
</blockquote>
<p> TLS is sometimes used in the non-standard "wrapper" mode where
a server always uses TLS, instead of announcing STARTTLS support
and waiting for remote SMTP clients to request TLS service. Some
clients, namely
Outlook [Express] prefer the "wrapper" mode. This is true for OE
(Win32 < 5.0 and Win32 >=5.0 when run on a port<>25
and OE (5.01 Mac on all ports). </p>
<p> It is strictly discouraged to use this mode from <a href="postconf.5.html">main.cf</a>. If
you want to support this service, enable a special port in <a href="master.5.html">master.cf</a>
and specify "-o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a>=yes" (note: no space around
the "=") as an <a href="smtpd.8.html">smtpd(8)</a> command line option. Port 465 (smtps) was
once chosen for this feature.
</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="master.5.html">master.cf</a>:
smtps inet n - n - - smtpd
-o <a href="postconf.5.html#smtpd_tls_wrappermode">smtpd_tls_wrappermode</a>=yes -o <a href="postconf.5.html#smtpd_sasl_auth_enable">smtpd_sasl_auth_enable</a>=yes
</pre>
</blockquote>
<h3><a name="server_vrfy_client">Client certificate verification</a> </h3>
<p> To receive a remote SMTP client certificate, the Postfix SMTP
server must explicitly ask for one (any contents of $<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a>
are also sent to the client as a hint for choosing a certificate from
a suitable CA). Unfortunately, Netscape clients will either complain
if no matching client certificate is available or will offer the user
client a list of certificates to choose from. Additionally some MTAs
(notably some versions of qmail) are unable to complete TLS negotiation
when client certificates are requested, and abort the SMTP session. So
this option is "off" by default. You will however need the certificate
if you want to use certificate based relaying with, for example, the
<a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> feature. A server that wants client certificates
must first present its own certificate. While Postfix by default
offers anonymous ciphers to remote SMTP clients, these are automatically
suppressed
when the Postfix SMTP server is configured to ask for client
certificates. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may
</pre>
</blockquote>
<p> When TLS is <a href="#server_enforce">enforced</a> you may also decide
to REQUIRE a remote SMTP client certificate for all TLS connections,
by setting "<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes". This feature implies
"<a href="postconf.5.html#smtpd_tls_ask_ccert">smtpd_tls_ask_ccert</a> = yes". When TLS is not enforced,
"<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes" is ignored and a warning is
logged. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_req_ccert">smtpd_tls_req_ccert</a> = yes
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt
</pre>
</blockquote>
<p> The client certificate verification depth is specified with the
<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where <a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> was ignored.
When you configure trust in a
root CA, it is not necessary to explicitly trust intermediary CAs signed
by the root CA, unless $<a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> is less than the
number of CAs in the certificate chain for the clients of interest. With
a verify depth of 1 you can only verify certificates directly signed
by a trusted CA, and all trusted intermediary CAs need to be configured
explicitly. With a verify depth of 2 you can verify clients signed by a
root CA or a direct intermediary CA (so long as the client is correctly
configured to supply its intermediate CA certificate). </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_ccert_verifydepth">smtpd_tls_ccert_verifydepth</a> = 2
</pre>
</blockquote>
<h3><a name="server_tls_auth">Supporting AUTH over TLS only</a></h3>
<p> Sending AUTH data over an unencrypted channel poses a security
risk. When TLS layer encryption is required
("<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt"), the Postfix SMTP server will
announce and accept AUTH only after the TLS layer has been activated
with STARTTLS. When TLS layer encryption is optional
("<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may"), it may however still be useful
to only offer AUTH when TLS is active. To maintain compatibility
with non-TLS clients, the default is to accept AUTH without encryption.
In order to change this behavior, set
"<a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = yes". </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_auth_only">smtpd_tls_auth_only</a> = no
</pre>
</blockquote>
<h3><a name="server_tls_cache">Server-side TLS session cache</a> </h3>
<p> The Postfix SMTP server and the remote SMTP client negotiate a
session, which takes some computer time and network bandwidth. SSL
protocol versions other than SSLv2 support resumption of cached
sessions. Not only is this more CPU and bandwidth efficient, it
also reduces latency as only one network round-trip is used to
resume a session while it takes two round-trips to create a session
from scratch. </p>
<p> Since Postfix uses multiple <a href="smtpd.8.html">smtpd(8)</a> service processes, an
in-memory cache is not sufficient for session re-use. Clients store
at most one cached session per server and are very unlikely to
repeatedly connect to the same server process. Thus session caching
in the Postfix SMTP server generally requires a shared cache (an
alternative available with Postfix ≥ 2.11 is described below).
</p>
<p> To share the session information between multiple
<a href="smtpd.8.html">smtpd(8)</a> processes, a session cache database is used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high.</p>
<p> Starting with Postfix 2.11, linked with a compatible OpenSSL
library (at least 0.9.8h, preferably 1.0.0 or later) the Postfix
SMTP server supports <a href="http://tools.ietf.org/html/rfc5077">RFC 5077</a> TLS session resumption without
server-side state when the remote SMTP client also supports <a href="http://tools.ietf.org/html/rfc5077">RFC</a>
<a href="http://tools.ietf.org/html/rfc5077">5077</a>. The session is encrypted by the server in a <i>session
ticket</i> returned to client for storage. When a client sends a
valid session ticket, the server decrypts it and resumes the session,
provided neither the ticket nor the session have expired. This
makes it possible to resume cached sessions without allocating space
for a shared database on the server. Consequently, for Postfix
≥ 2.11 the <a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> parameter should
generally be left empty. Session caching can be disabled by setting
the session cache timeout to zero, otherwise the timeout must be
at least 2 minutes and at most 100 days. </p>
<p> Note, session tickets can only be negotiated if the client
disables SSLv2 and does not use the legacy SSLv2 compatible HELLO
message. This is true by default with the Postfix ≥ 2.6 SMTP
client. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtpd_scache
</pre>
</blockquote>
<p> Note: as of version 2.5, Postfix no longer uses root privileges
when opening this file. The file should now be stored under the
Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p>
<p> Cached Postfix SMTP server session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600sec (=1 hour). <a href="http://tools.ietf.org/html/rfc2246">RFC 2246</a>
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_session_cache_timeout">smtpd_tls_session_cache_timeout</a> = 3600s
</pre>
</blockquote>
<p> As of Postfix 2.11 this setting cannot exceed 100 days. If set
≤ 0, session caching is disabled. If set to a positive value
less than 2 minutes, the minimum value of 2 minutes is used instead. </p>
<p> When the Postfix SMTP server does not save TLS sessions to an
external cache database, client-side session caching is unlikely
to be useful. To reduce waste of client resources, the Postfix SMTP server can
be configured to not issue TLS session ids. By default the Postfix
SMTP server always issues TLS session ids. This works around known
interoperability issues with some MUAs, and prevents possible
interoperability issues with other MTAs. </p>
<p> Example: </p>
<blockquote>
<pre>
<a href="postconf.5.html#smtpd_tls_always_issue_session_ids">smtpd_tls_always_issue_session_ids</a> = no
</pre>
</blockquote>
<h3><a name="server_access">Server access control</a> </h3>
<p> Postfix TLS support introduces three additional features for
Postfix SMTP server access control: </p>
<blockquote>
<dl>
<dt> <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a> </dt> <dd> <p> Allow the remote SMTP
client request if the client certificate fingerprint or certificate
public key fingerprint (Postfix 2.9 and later) is listed in the
client certificate table (see <a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> discussion below).
</p> </dd>
<dt> <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> </dt> <dd> <p> Allow the remote SMTP
client request if the client certificate passes trust chain verification.
Useful with private-label CAs that only issue certificates to trusted
clients (and not otherwise). </p> </dd>
<dt> <a href="postconf.5.html#check_ccert_access">check_ccert_access</a> <a href="DATABASE_README.html">type:table</a></dt> <dd> <p> Use the remote
SMTP client certificate fingerprint or public key fingerprint
(Postfix 2.9 and later) as the lookup key for the specified <a href="access.5.html">access(5)</a>
table. </p> </dd>
</dl>
</blockquote>
<p> The digest algorithm used to compute the client certificate
fingerprints is specified with the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtpd_tls_fingerprint_digest">smtpd_tls_fingerprint_digest</a>
parameter. The default is "md5", for compatibility with Postfix
versions < 2.5. </p>
<p> The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature must be used with caution,
because it can result in too many access permissions. Use this
feature only if a special CA issues the client certificates, and
only if this CA is listed as trusted CA. If other CAs are trusted,
any owner of a valid client certificate would be authorized.
The <a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> feature can be practical for a
specially created email relay server. </p>
<p> It is however recommended to stay with the <a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
feature and list all certificates via $<a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a>, as
<a href="postconf.5.html#permit_tls_all_clientcerts">permit_tls_all_clientcerts</a> does not permit any control when a
certificate must no longer be used (e.g. an employee leaving). </p>
<p> Example: </p>
<blockquote>
<pre>
# With Postfix 2.10 and later, the mail relay policy is
# preferably specified under <a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a>.
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a> =
<a href="postconf.5.html#permit_mynetworks">permit_mynetworks</a>
<a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
<a href="postconf.5.html#reject_unauth_destination">reject_unauth_destination</a>
</pre>
<pre>
# Older configurations combine relay control and spam control under
# <a href="postconf.5.html#smtpd_recipient_restrictions">smtpd_recipient_restrictions</a>. To use this example with Postfix ≥
# 2.10 specify "<a href="postconf.5.html#smtpd_relay_restrictions">smtpd_relay_restrictions</a>=".
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_recipient_restrictions">smtpd_recipient_restrictions</a> =
<a href="postconf.5.html#permit_mynetworks">permit_mynetworks</a>
<a href="postconf.5.html#permit_tls_clientcerts">permit_tls_clientcerts</a>
<a href="postconf.5.html#reject_unauth_destination">reject_unauth_destination</a>
...other rules...
</pre>
</blockquote>
<p> Example: Postfix lookup tables are in the form of (key, value)
pairs. Since we only need the key, the value can be chosen freely, e.g.
the name of the user or host:</p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#relay_clientcerts">relay_clientcerts</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/relay_clientcerts
/etc/postfix/relay_clientcerts:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home
</pre>
</blockquote>
<p> To extract the public key fingerprint from an X.509 certificate,
you need to extract the public key from the certificate and compute
the appropriate digest of its DER (ASN.1) encoding. With OpenSSL
the "-pubkey" option of the "x509" command extracts the public
key always in "PEM" format. We pipe the result to another OpenSSL
command that converts the key to DER and then to the "dgst" command
to compute the fingerprint. </p>
<p> The actual command to transform the key to DER format depends
on the version of OpenSSL used. With OpenSSL 1.0.0 and later, the
"pkey" command supports all key types. With OpenSSL 0.9.8 and
earlier, the key type is always RSA (nobody uses DSA, and EC
keys are not fully supported by 0.9.8), so the "rsa" command is
used. </p>
<blockquote>
<pre>
# OpenSSL 1.0 with all certificates and SHA-1 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha1 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58
# OpenSSL 0.9.8 with RSA certificates and MD5 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl rsa -pubin -outform DER |
openssl dgst -md5 -c
(stdin)= f4:62:60:f6:12:8f:d5:8d:28:4d:13:a7:db:b2:ff:50
</pre>
</blockquote>
<p> Note: Postfix 2.9.0–2.9.5 computed the public key
fingerprint incorrectly. To use public-key fingerprints, upgrade
to Postfix 2.9.6 or later. </p>
<h3><a name="server_cipher">Server-side cipher controls</a> </h3>
<p> The Postfix SMTP server supports 5 distinct cipher grades as
specified by the <a href="postconf.5.html#smtpd_tls_mandatory_ciphers">smtpd_tls_mandatory_ciphers</a> configuration parameter,
which determines the minimum cipher grade with mandatory TLS
encryption. The default minimum cipher grade for mandatory TLS is
"medium" which is essentially 128-bit encryption or better. The
<a href="postconf.5.html#smtpd_tls_ciphers">smtpd_tls_ciphers</a> parameter (Postfix ≥ 2.6) controls the minimum
cipher grade used with opportunistic TLS. Here, the default minimum
cipher grade is "medium" for Postfix releases after the middle of
2015, "export" for older Postfix releases. With Postfix < 2.6,
the minimum opportunistic TLS cipher grade is always "export". </p>
<p> By default anonymous ciphers are enabled. They are automatically
disabled when remote SMTP client certificates are requested. If
clients are expected to always verify the Postfix SMTP
server certificate you may want to disable anonymous ciphers
by setting "<a href="postconf.5.html#smtpd_tls_mandatory_exclude_ciphers">smtpd_tls_mandatory_exclude_ciphers</a> = aNULL" or
"<a href="postconf.5.html#smtpd_tls_exclude_ciphers">smtpd_tls_exclude_ciphers</a> = aNULL", as appropriate. One can't force
a remote SMTP client to check the server certificate, so excluding
anonymous ciphers is generally unnecessary. </p>
<p> With mandatory and opportunistic TLS encryption, the Postfix
SMTP server by default disables SSLv2 and SSLv3 with Postfix releases
after the middle of 2015; older releases only disable SSLv2 for
mandatory TLS. The mandatory TLS protocol list is specified via the
<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> configuration parameter. The
<a href="postconf.5.html#smtpd_tls_protocols">smtpd_tls_protocols</a> parameter (Postfix ≥ 2.6)
controls the SSL/TLS protocols used with opportunistic TLS. </p>
<p> Note that the OpenSSL library only supports protocol exclusion
(not inclusion). For this reason, Postfix can exclude only protocols
that are known at the time the Postfix software is written. If new
protocols are added to the OpenSSL library, they cannot be excluded
without corresponding changes to the Postfix source code. </p>
<p> For a server that is not a public Internet MX host, Postfix
supports configurations with no <a href="#server_cert_key">server
certificates</a> that use <b>only</b> the anonymous ciphers. This is
enabled by explicitly setting "<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = none"
and not specifying an <a href="postconf.5.html#smtpd_tls_dcert_file">smtpd_tls_dcert_file</a> or <a href="postconf.5.html#smtpd_tls_eccert_file">smtpd_tls_eccert_file</a>.
Such configurations may not interoperate with some clients, and require
that TLSv1.3 be explicitly disabled. Therefore, they are not
recommended, it is better and simpler to just configure a suitable
certificate. </p>
<p> Example, MSA that requires TLSv1 or higher, not SSLv2 or SSLv3,
with high grade ciphers: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/cert.pem
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = /etc/postfix/key.pem
<a href="postconf.5.html#smtpd_tls_mandatory_ciphers">smtpd_tls_mandatory_ciphers</a> = high
<a href="postconf.5.html#smtpd_tls_mandatory_exclude_ciphers">smtpd_tls_mandatory_exclude_ciphers</a> = aNULL, MD5
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = encrypt
# Preferred syntax with Postfix ≥ 2.5:
<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> = !SSLv2, !SSLv3
# Legacy syntax:
<a href="postconf.5.html#smtpd_tls_mandatory_protocols">smtpd_tls_mandatory_protocols</a> = TLSv1
</pre>
</blockquote>
<p> With Postfix ≥ 3.4, specify instead a single file that holds the
key followed by the corresponding certificate and any associated issuing
certificates, leaving the "<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a>" and "<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a>"
and related DSA and ECDSA parameters empty. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_tls_chain_files">smtpd_tls_chain_files</a> = /etc/postfix/rsachain.pem
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> =
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> =
...
</pre>
</blockquote>
<p> If you want to take maximal advantage of ciphers that offer <a
href="FORWARD_SECRECY_README.html#dfn_fs">forward secrecy</a> see
the <a href="FORWARD_SECRECY_README.html#quick-start">Getting
started</a> section of <a
href="FORWARD_SECRECY_README.html">FORWARD_SECRECY_README</a>. The
full document conveniently presents all information about Postfix
forward secrecy support in one place: what forward secrecy is, how
to tweak settings, and what you can expect to see when Postfix uses
ciphers with forward secrecy. </p>
<p> Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and later
allows TLS servers to preempt the TLS client's cipher-suite preference list.
This is possible only with SSLv3 and later, as in SSLv2 the client
chooses the cipher-suite from a list supplied by the server. </p>
<p> By default, the OpenSSL server selects the client's most preferred
cipher-suite that the server supports. With SSLv3 and later, the server
may choose its own most preferred cipher-suite that is supported (offered)
by the client. Setting "<a href="postconf.5.html#tls_preempt_cipherlist">tls_preempt_cipherlist</a> = yes" enables server
cipher-suite preferences. The default OpenSSL behavior applies with
"<a href="postconf.5.html#tls_preempt_cipherlist">tls_preempt_cipherlist</a> = no". </p>
<p> While server cipher-suite selection may in some cases lead to
a more secure or performant cipher-suite choice, there is some risk
of interoperability issues. In the past, some SSL clients have
listed lower priority ciphers that they did not implement correctly.
If the server chooses a cipher that the client prefers less, it may
select a cipher whose client implementation is flawed. Most notably
Windows 2003 Microsoft Exchange servers have flawed implementations
of DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA.
Enabling server cipher-suite selection may create interoperability
issues with Windows 2003 Microsoft Exchange clients. </p>
<h3><a name="server_misc"> Miscellaneous server controls</a> </h3>
<p> The <a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> parameter limits the time of Postfix
SMTP server write and read operations during TLS startup and shutdown
handshake procedures. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtpd_starttls_timeout">smtpd_starttls_timeout</a> = 300s
</pre>
</blockquote>
<p> With Postfix 2.8 and later, the <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter
specifies a list or bit-mask of default-enabled OpenSSL bug
work-arounds to disable. This may be necessary if one of the
work-arounds enabled by default in OpenSSL proves to pose a security
risk, or introduces an unexpected interoperability issue. The list
of enabled bug work-arounds is OpenSSL-release-specific. See the
<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter documentation for the list of
supported values.</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = 0xFFFFFFFF
<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = CVE-2010-4180
</pre>
</blockquote>
<p> With Postfix ≥ 2.11, the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter specifies
a list or bit-mask of OpenSSL options to enable. Specify one or
more of the named options below, or a hexadecimal bitmask of options
found in the ssl.h file corresponding to the run-time OpenSSL
library. While it may be reasonable to turn off all bug workarounds
(see above), it is not a good idea to attempt to turn on all features.
See the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter documentation for the list of
supported values. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> = no_ticket, no_compression
</pre>
</blockquote>
<p> You should only enable features via the hexadecimal mask when
the need to control the feature is critical (to deal with a new
vulnerability or a serious interoperability problem). Postfix DOES
NOT promise backwards compatible behavior with respect to the mask
bits. A feature enabled via the mask in one release may be enabled
by other means in a later release, and the mask bit will then be
ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better
solution. </p>
<h2> <a name="client_tls">SMTP Client specific settings</a> </h2>
<p> Topics covered in this section: </p>
<ul>
<li><a href="#client_tls_levels"> Configuring TLS in the SMTP/LMTP client </a>
<li><a href="#client_logging"> Client-side TLS activity logging </a>
<li><a href="#client_cert_key">Client-side certificate and private
key configuration </a>
<li><a href="#client_tls_reuse">Client-side TLS connection reuse</a>
<li><a href="#client_tls_cache">Client-side TLS session cache</a>
<li><a href="#client_tls_limits"> Client TLS limitations </a>
<li><a href="#client_tls_policy"> Per-destination TLS policy </a>
<li><a href="#client_tls_discover"> Discovering servers that support TLS </a>
<li><a href="#client_vrfy_server">Server certificate verification depth</a>
<li> <a href="#client_cipher">Client-side cipher controls </a>
<li> <a href="#client_smtps">Client-side SMTPS support </a>
<li> <a href="#client_misc"> Miscellaneous client controls </a>
</ul>
<h3><a name="client_tls_levels"> Configuring TLS in the SMTP/LMTP client </a>
</h3>
<p> Similar to the Postfix SMTP server, the Postfix SMTP/LMTP client
implements multiple TLS security levels. These levels are described
in more detail in the sections that follow.</p>
<dl>
<dt><b>none</b></dt>
<dd><a href="#client_tls_none">No TLS.</a></dd>
<dt><b>may</b></dt>
<dd><a href="#client_tls_may">Opportunistic TLS.</a></dd>
<dt><b>encrypt</b></dt>
<dd><a href="#client_tls_encrypt">Mandatory TLS encryption.</a>
<dt><b>dane</b></dt>
<dd><a href="#client_tls_dane">Opportunistic DANE TLS.</a>
<dt><b>dane-only</b></dt>
<dd><a href="#client_tls_dane">Mandatory DANE TLS.</a>
<dt><b>fingerprint</b></dt>
<dd><a href="#client_tls_fprint">Certificate fingerprint verification.</a>
<dt><b>verify</b></dt>
<dd><a href="#client_tls_verify">Mandatory server certificate verification.</a>
<dt><b>secure</b></dt>
<dd><a href="#client_tls_secure">Secure-channel TLS.</a>
</dl>
<h4><a name="client_lmtp_tls"> TLS support in the LMTP delivery agent </a> </h4>
<p> The <a href="smtp.8.html">smtp(8)</a> and <a href="lmtp.8.html">lmtp(8)</a> delivery agents are implemented by a
single dual-purpose program. Specifically, all the TLS features
described below apply
equally to SMTP and LMTP, after replacing the "smtp_" prefix of the each
parameter name with "lmtp_".
<p> The Postfix LMTP delivery agent can communicate with LMTP servers
listening
on UNIX-domain sockets. When server certificate verification is enabled
and the server is listening on a UNIX-domain socket, the $<a href="postconf.5.html#myhostname">myhostname</a>
parameter is used to set the TLS verification <i>nexthop</i> and
<i>hostname</i>. </p>
<p> NOTE: Opportunistic encryption of LMTP traffic over UNIX-domain
sockets or loopback TCP connections is futile. TLS is only useful
in this context when
it is mandatory, typically to allow at least one of the server or the
client to authenticate the other. The "null" cipher grade may be
appropriate in this context, when available on both client and server.
The "null" ciphers provide authentication without encryption. </p>
<h4><a name="client_tls_none"> No TLS encryption </a> </h4>
<p> At the "none" TLS security level, TLS encryption is
disabled. This is the default security level, and
can be configured explicitly by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = none".
For LMTP, use the corresponding "lmtp_" parameter. </p>
<p> Per-destination settings may override this default setting, in which case
TLS is used selectively, only with destinations explicitly configured
for TLS. </p>
<p> You can disable TLS for a subset of destinations, while leaving
it enabled for the rest. With the Postfix TLS <a
href="#client_tls_policy">policy table</a>, specify the "none"
security level.
<h4><a name="client_tls_may"> Opportunistic TLS </a> </h4>
<p> At the "may" TLS security level, TLS encryption is <i>opportunistic</i>.
The SMTP transaction is encrypted if the STARTTLS ESMTP feature
is supported by the server. Otherwise, messages are sent in the clear.
Opportunistic TLS can be configured by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may".
For LMTP, use the corresponding "lmtp_" parameter. </p>
<p> The "<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>" and "<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a>" configuration
parameters (Postfix ≥ 2.6) provide control over the cipher grade
and protocols used with opportunistic TLS. With earlier Postfix
releases, opportunistic TLS always uses the cipher grade "export"
and enables all protocols. </p>
<p> With opportunistic TLS, mail delivery continues even if the
server certificate is untrusted or bears the wrong name.
When the TLS handshake fails for an opportunistic
TLS session, rather than give up on mail delivery, the Postfix SMTP
client retries the transaction
with TLS disabled. Trying an unencrypted connection makes
it possible to deliver mail to sites with non-interoperable server
TLS implementations. </p>
<p> Opportunistic encryption is never used for LMTP over UNIX-domain
sockets. The communications channel is already confidential without
TLS, so the only potential benefit of TLS is authentication. Do not
configure opportunistic TLS for LMTP deliveries over UNIX-domain sockets.
Only configure TLS for LMTP over UNIX-domain sockets at the
<a href="#client_tls_encrypt">encrypt</a> security level or higher.
Attempts to configure opportunistic encryption of LMTP sessions will
be ignored with a warning written to the mail logs. </p>
<p> You can enable opportunistic TLS just for selected destinations. With
the Postfix TLS <a href="#client_tls_policy">policy table</a>,
specify the "may" security level. </p>
<p> This is the most common security level for TLS protected SMTP
sessions, stronger security is not generally available and, if needed,
is typically only configured on a per-destination basis. See the section
on TLS <a href="#client_tls_limits">limitations</a> above. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may
</pre>
</blockquote>
<h4><a name="client_tls_encrypt"> Mandatory TLS encryption </a> </h4>
<p> At the "encrypt" TLS security level, messages are sent only
over TLS encrypted sessions. The SMTP transaction is aborted unless
the STARTTLS ESMTP feature is supported by the remote SMTP server.
If no suitable
servers are found, the message will be deferred.
Mandatory TLS encryption can be configured by setting
"<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = encrypt". Even though TLS
encryption is always used, mail delivery continues even if the server
certificate is untrusted or bears the wrong name.
For LMTP, use the corresponding "lmtp_" parameter. </p>
<p> At this security level and higher, the <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>
and <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> configuration parameters determine
the list of sufficiently secure SSL protocol versions and the minimum
cipher strength. If the protocol or cipher requirements are not
met, the mail transaction is aborted. The documentation for these
parameters includes useful interoperability and security guidelines.
</p>
<p> Despite the potential for eliminating passive eavesdropping attacks,
mandatory TLS encryption is not viable as a default security level for
mail delivery to the public Internet. Some MX hosts do not support TLS at
all, and some of those that do have broken implementations. On a host
that delivers mail to the Internet, you should not configure mandatory
TLS encryption as the default security level. </p>
<p> You can enable mandatory TLS encryption just for specific destinations.
With the Postfix TLS <a href="#client_tls_policy">policy
table</a>, specify the "encrypt" security level.
</p>
<p> Examples: </p>
<p> In the example below, traffic to <i>example.com</i> and its sub-domains
via the corresponding MX hosts always uses TLS. The SSLv2 protocol
will be disabled (the default setting of <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>
excludes SSLv2+3). Only high- or medium-strength (i.e. 128 bit or
better) ciphers will be used by default for all "encrypt" security
level sessions. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
/etc/postfix/tls_policy:
example.com encrypt
.example.com encrypt
</pre>
</blockquote>
<p> In the next example, secure message submission is configured
via the MSA "<tt>[example.net]:587</tt>". TLS sessions are encrypted
without authentication, because this MSA does not possess an acceptable
certificate. This MSA is known to be capable of "TLSv1" and "high" grade
ciphers, so these are selected via the <a href="#client_tls_policy">policy
table</a>. </p>
<p><b>Note:</b> the policy table lookup key is the verbatim next-hop
specification from the recipient domain, <a href="transport.5.html">transport(5)</a> table or <a href="postconf.5.html#relayhost">relayhost</a>
parameter, with any enclosing square brackets and optional port. Take
care to be consistent: the suffixes ":smtp" or ":25" or no port suffix
result in different policy table lookup keys, even though they are
functionally equivalent nexthop specifications. Use at most one of these
forms for all destinations. Below, the policy table has multiple keys,
just in case the transport table entries are not specified consistently. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
/etc/services:
submission 587/tcp msa # mail message submission
/etc/postfix/tls_policy:
[example.net]:587 encrypt protocols=TLSv1 ciphers=high
[example.net]:msa encrypt protocols=TLSv1 ciphers=high
[example.net]:submission encrypt protocols=TLSv1 ciphers=high
</pre>
</blockquote>
<h4><a name="client_tls_dane">DANE TLS authentication.</a> </h4>
<p> The Postfix SMTP client supports two TLS security levels based
on DANE TLSA (<a href="http://tools.ietf.org/html/rfc6698">RFC 6698</a>, <a href="http://tools.ietf.org/html/rfc7671">RFC 7671</a>, <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>) records. The opportunistic
"dane" level and the mandatory "dane-only" level. </p>
<p> The "dane" level is a stronger form of <a
href="#client_tls_may">opportunistic</a> TLS that is resistant to
man in the middle and downgrade attacks when the destination domain
uses DNSSEC to publish DANE TLSA records for its MX hosts. If a
remote SMTP server has "usable" (see section 3 of <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>) DANE
TLSA records, the server connection will be authenticated. When
DANE authentication fails, there is no fallback to unauthenticated
or plaintext delivery. </p>
<p> If TLSA records are published for a given remote SMTP server
(implying TLS support), but are all "unusable" due to unsupported
parameters or malformed data, the Postfix SMTP client will use <a
href="#client_tls_encrypt">mandatory</a> unauthenticated TLS.
Otherwise, when no TLSA records are published, the Postfix SMTP
client behavior is the same as with <a href="#client_tls_may">may</a>. </p>
<p> TLSA records must be published in DNSSEC validated DNS zones.
Any TLSA records in DNS zones not protected via DNSSEC are ignored.
The Postfix SMTP client will not look for TLSA records associated
with MX hosts whose "A" or "AAAA" records lie in an "insecure" DNS
zone. Such lookups have been observed to cause interoperability
issues with poorly implemented DNS servers, and are in any case not
expected to ever yield "secure" results, since that would require
a very unlikely DLV DNS trust anchor configured between the host
record and the associated "_25._tcp" child TLSA record. </p>
<p> The "dane-only" level is a form of <a
href="#client_tls_secure">secure-channel</a> TLS based on the DANE PKI.
If "usable" TLSA records are present these are used to authenticate the
remote SMTP server. Otherwise, or when server certificate verification
fails, delivery via the server in question tempfails. </p>
<p> At both security levels, the TLS policy for the destination is
obtained via TLSA records validated with DNSSEC. For TLSA policy
to be in effect, the destination domain's containing DNS zone must
be signed and the Postfix SMTP client's operating system must be
configured to send its DNS queries to a recursive DNS nameserver
that is able to validate the signed records. Each MX host's DNS
zone needs to also be signed, and needs to publish DANE TLSA (see
section 3 of <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>) records that specify how that MX host's TLS
certificate is to be verified. </p>
<p> TLSA records do not preempt the normal SMTP MX host
selection algorithm, if some MX hosts support TLSA and others do
not, TLS security will vary from delivery to delivery. It is up
to the domain owner to configure their MX hosts and their DNS
sensibly. To configure the Postfix SMTP client for DNSSEC lookups
see the documentation for the <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> <a href="postconf.5.html">main.cf</a>
parameter. The <a href="postconf.5.html#tls_dane_digests">tls_dane_digests</a> parameter controls the list of
supported digests. </p>
<p> As explained in section 3 of <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a>, certificate usages "0"
and "1", which are intended to "constrain" existing Web-PKI trust,
are not supported with MTA-to-MTA SMTP. Rather, TLSA records with
usages "0" and "1" are treated as "unusable". </p>
<p> The Postfix SMTP client supports only certificate usages "2"
and "3". Experimental support for silently mapping certificate
usage "1" to "3" has been withdrawn starting with Postfix 3.2. </p>
<p> When usable TLSA records are obtained for the remote SMTP server
the Postfix SMTP client sends the SNI TLS extension in its SSL
client hello message. This may help the remote SMTP server live
up to its promise to provide a certificate that matches its TLSA
records. </p>
<p> For purposes of protocol and cipher selection, the "dane"
security level is treated like a "mandatory" TLS security level,
and weak ciphers and protocols are disabled. Since DANE authenticates
server certificates the "aNULL" cipher-suites are transparently
excluded at this level, no need to configure this manually. <a href="http://tools.ietf.org/html/rfc7672">RFC</a>
<a href="http://tools.ietf.org/html/rfc7672">7672</a> (DANE) TLS authentication is available with Postfix 2.11 and
later. </p>
<p> When a DANE TLSA record specifies a trust-anchor (TA) certificate
(that is an issuing CA), the strategy used to verify the peername
of the server certificate is unconditionally "nexthop, hostname".
Both the nexthop domain and the hostname obtained from the
DNSSEC-validated MX lookup are safe from forgery and the server
certificate must contain at least one of these names. </p>
<p> When a DANE TLSA record specifies an end-entity (EE) certificate,
(that is the actual server certificate), as with the fingerprint
security level below, no name checks or certificate expiration checks
are applied. The server certificate (or its public key) either matches
the DANE record or not. Server administrators should publish such
EE records in preference to all other types. </p>
<p> The pre-requisites for DANE support in the Postfix SMTP client are: </p>
<ul>
<li> A <i>compile-time</i> OpenSSL library that supports the TLS SNI
extension and "SHA-2" message digests.
<li> A <i>compile-time</i> DNS resolver library that supports DNSSEC.
Postfix binaries built on an older system will not support DNSSEC even
if deployed on a system with an updated resolver library.
<li> The "<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>" must be set to "dnssec".
<li> The "<a href="postconf.5.html#smtp_host_lookup">smtp_host_lookup</a>" parameter must include "dns".
<li> A DNSSEC-validating recursive resolver (see note below).
</ul>
<p> The above client pre-requisites do not apply to the Postfix SMTP server.
It will support DANE provided it supports TLSv1 and its TLSA records are
published in a DNSSEC signed zone. To receive DANE secured mail for multiple
domains, use the same hostname to add the server to each domain's MX
records. There are no plans to implement SNI in the Postfix SMTP server. </p>
<p> Note: The Postfix SMTP client's internal stub DNS resolver is
DNSSEC-aware, but it does not itself validate DNSSEC records, rather
it delegates DNSSEC validation to the operating system's configured
recursive DNS nameserver. The Postfix DNS client relies on a secure
channel to the resolver's cache for DNSSEC integrity, but does not
support TSIG to protect the transmission channel between itself and
the nameserver. Therefore, it is strongly recommended (DANE security
guarantee void otherwise) that each MTA run a local DNSSEC-validating
recursive resolver ("unbound" from nlnetlabs.nl is a reasonable
choice) listening on the loopback interface, and that the system
be configured to use <i>only</i> this local nameserver. The local
nameserver may forward queries to an upstream recursive resolver
on another host if desired. </p>
<p> Note: When the operating system's recursive nameserver is not
local, enabling EDNS0 expanded DNS packet sizes and turning on the
DNSSEC "DO" bit in the DNS request and/or the new DNSSEC-specific
records returned in the nameserver's replies may cause problems
with older or buggy firewall and DNS server implementations.
Therefore, Postfix does not enable DNSSEC by default. Since MX
lookups happen before the security level is determined, DANE support
is disabled for all destinations unless you set "<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>
= dnssec". To enable DNSSEC lookups selectively, define a new
dedicated transport with a "-o <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>=dnssec"
override in <a href="master.5.html">master.cf</a> and route selected domains to that transport.
If DNSSEC proves to be sufficiently reliable for these domains, you
can enable it for all destinations by changing the global
<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> in <a href="postconf.5.html">main.cf</a>. </p>
<p><b>Example</b>: "dane" security for selected destinations, with
opportunistic TLS by default. This is the recommended configuration
for early adopters. <p>
<ul>
<li> <p> The "example.com" destination uses DANE, but if TLSA records
are not present or are unusable, mail is deferred. </p>
<li> <p> The "example.org" destination uses DANE if possible, but if no TLSA
records are found opportunistic TLS is used. </p>
</ul>
<blockquote>
<pre>
<a href="postconf.5.html">main.cf</a>:
indexed = ${<a href="postconf.5.html#default_database_type">default_database_type</a>}:${<a href="postconf.5.html#config_directory">config_directory</a>}/
#
# default: Opportunistic TLS with no DNSSEC lookups.
#
<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may
<a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a> = enabled
#
# Per-destination TLS policy
#
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = ${indexed}tls_policy
#
# <a href="postconf.5.html#default_transport">default_transport</a> = smtp, but some destinations are special:
#
<a href="postconf.5.html#transport_maps">transport_maps</a> = ${indexed}transport
</pre>
</blockquote>
<blockquote>
<pre>
transport:
example.com dane
example.org dane
</pre>
</blockquote>
<blockquote>
<pre>
tls_policy:
example.com dane-only
</pre>
</blockquote>
<blockquote>
<pre>
<a href="master.5.html">master.cf</a>:
dane unix - - n - - smtp
-o <a href="postconf.5.html#smtp_dns_support_level">smtp_dns_support_level</a>=dnssec
-o <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a>=dane
</pre>
</blockquote>
<h4><a name="client_tls_fprint"> Certificate fingerprint verification </a> </h4>
<p> At the <i>fingerprint</i> security level, no trusted Certification
Authorities are used or required. The certificate trust chain,
expiration date, etc., are not checked. Instead, the
<a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a> parameter or the "match" attribute
in the <a href="#client_tls_policy">policy</a> table lists the
remote SMTP server certificate fingerprint or public key fingerprint.
Certificate fingerprint verification is available with Postfix 2.5
and later, public-key fingerprint support is available with Postfix
2.9 and later. </p>
<p> If certificate fingerprints are exchanged securely, this is the
strongest, and least scalable security level. The administrator needs
to securely collect the fingerprints of the X.509 certificates of each
peer server, store them into a local file, and update this local file
whenever the peer server's public certificate changes. If public key
fingerprints are used in place of fingerprints of the entire certificate,
the fingerprints remain valid even after the certificate is renewed,
<b>provided</b> that the same public/private keys are used to obtain
the new certificate. </p>
<p> Fingerprint verification may be feasible for an SMTP "VPN" connecting
a small number of branch offices over the Internet, or for secure
connections to a central mail hub. It works poorly if the remote SMTP
server is managed by a third party, and its public certificate changes
periodically without prior coordination with the verifying site. </p>
<p> The digest algorithm used to calculate the fingerprint is
selected by the <b><a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a></b> parameter. In the <a
href="#client_tls_policy">policy</a> table multiple fingerprints can be
combined with a "|" delimiter in a single match attribute, or multiple
match attributes can be employed. The ":" character is not used as a
delimiter as it occurs between each pair of fingerprint (hexadecimal)
digits. </p>
<p> Example: fingerprint TLS security with an internal mailhub.
Two matching fingerprints are listed. The <a href="postconf.5.html#relayhost">relayhost</a> may be multiple
physical hosts behind a load-balancer, each with its own private/public
key and self-signed certificate. Alternatively, a single <a href="postconf.5.html#relayhost">relayhost</a> may
be in the process of switching from one set of private/public keys to
another, and both keys are trusted just prior to the transition. </p>
<blockquote>
<pre>
<a href="postconf.5.html#relayhost">relayhost</a> = [mailhub.example.com]
<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = fingerprint
<a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = md5
<a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a> =
3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
</pre>
</blockquote>
<p> Example: Certificate fingerprint verification with selected destinations.
As in the example above, we show two matching fingerprints: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
<a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = md5
</pre>
</blockquote>
<blockquote>
<pre>
/etc/postfix/tls_policy:
example.com fingerprint
match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
</pre>
</blockquote>
<p> To extract the public key fingerprint from an X.509 certificate,
you need to extract the public key from the certificate and compute
the appropriate digest of its DER (ASN.1) encoding. With OpenSSL
the "-pubkey" option of the "x509" command extracts the public
key always in "PEM" format. We pipe the result to another OpenSSL
command that converts the key to DER and then to the "dgst" command
to compute the fingerprint. </p>
<p> The actual command to transform the key to DER format depends
on the version of OpenSSL used. With OpenSSL 1.0.0 and later, the
"pkey" command supports all key types. With OpenSSL 0.9.8 and
earlier, the key type is always RSA (nobody uses DSA, and EC
keys are not fully supported by 0.9.8), so the "rsa" command is
used. </p>
<blockquote>
<pre>
# OpenSSL 1.0 with all certificates and SHA-1 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha1 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58
# OpenSSL 0.9.8 with RSA certificates and MD5 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl rsa -pubin -outform DER |
openssl dgst -md5 -c
(stdin)= f4:62:60:f6:12:8f:d5:8d:28:4d:13:a7:db:b2:ff:50
</pre>
</blockquote>
<p> Note: Postfix 2.9.0–2.9.5 computed the public key
fingerprint incorrectly. To use public-key fingerprints, upgrade
to Postfix 2.9.6 or later. </p>
<h4><a name="client_tls_verify"> Mandatory server certificate verification </a> </h4>
<p> At the <i>verify</i> TLS security level, messages are sent only over
TLS encrypted sessions if the remote SMTP server certificate is
valid (not
expired or revoked, and signed by a trusted Certification Authority)
and where the server certificate name matches a known pattern.
Mandatory
server certificate verification can be configured by setting
"<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = verify". The
<a href="postconf.5.html#smtp_tls_verify_cert_match">smtp_tls_verify_cert_match</a> parameter can override the default
"hostname" certificate name matching strategy. Fine-tuning the
matching strategy is generally only appropriate for <a
href="#client_tls_secure">secure-channel</a> destinations.
For LMTP use the corresponding "lmtp_" parameters. </p>
<p> If the server certificate chain is trusted (see <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a>
and <a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>), any DNS names in the SubjectAlternativeName
certificate extension are used to verify the remote SMTP server name.
If no
DNS names are specified, the certificate CommonName is checked.
If you want mandatory encryption without server certificate
verification, see <a href="#client_tls_encrypt">above</a>. </p>
<p> With Postfix ≥ 2.11 the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter
or more typically the corresponding per-destination "tafile" attribute
optionally modifies trust chain verification. If the parameter is
not empty the root CAs in CAfile and CApath are no longer trusted.
Rather, the Postfix SMTP client will only trust certificate-chains
signed by one of the trust-anchors contained in the chosen files.
The specified trust-anchor certificates and public keys are not
subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these
certificates also may be found "in the middle" of the trust chain
presented by the remote SMTP server, and any untrusted issuing
parent certificates will be ignored. </p>
<p> Despite the potential for eliminating "man-in-the-middle" and other
attacks, mandatory certificate trust chain and subject name verification
is not viable as a default Internet mail delivery policy. Some MX hosts
do not support TLS at all, and a significant portion of TLS-enabled
MTAs use self-signed certificates, or certificates that are signed by
a private Certification Authority. On a machine that delivers mail to
the Internet, you should not configure mandatory server certificate
verification as a default policy. </p>
<p> Mandatory server certificate verification as a default security
level may be appropriate if you know that you will only connect to
servers that support <a href="http://tools.ietf.org/html/rfc2487">RFC 2487</a> <i>and</i> that present verifiable
server certificates. An example would be a client that sends all
email to a central mailhub that offers the necessary STARTTLS
support. In such cases, you can often use a <a
href="#client_tls_secure">secure-channel</a> configuration instead.
</p>
<p> You can enable mandatory server certificate verification just
for specific destinations. With the Postfix TLS <a
href="#client_tls_policy">policy table</a>, specify the "verify"
security level. </p>
<p> Example: </p>
<p> In this example, the Postfix SMTP client encrypts all traffic to the
<i>example.com</i> domain. The peer hostname is verified, but
verification is vulnerable to DNS response forgery. Mail transmission
to <i>example.com</i> recipients uses "high" grade ciphers. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
indexed = ${<a href="postconf.5.html#default_database_type">default_database_type</a>}:${<a href="postconf.5.html#config_directory">config_directory</a>}/
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = ${<a href="postconf.5.html#config_directory">config_directory</a>}/CAfile.pem
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = ${indexed}tls_policy
/etc/postfix/tls_policy:
example.com verify ciphers=high
</pre>
</blockquote>
<h4><a name="client_tls_secure"> Secure server certificate verification </a> </h4>
<p> At the <i>secure</i> TLS security level, messages are sent only over
<i>secure-channel</i> TLS sessions where DNS forgery resistant server
certificate verification succeeds. If no suitable servers are found, the
message will be deferred. Postfix secure-channels
can be configured by setting "<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = secure".
The <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter can override the default
"nexthop, dot-nexthop" certificate match strategy.
For LMTP, use the corresponding "lmtp_" parameters. </p>
<p> If the server certificate chain is trusted (see <a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> and
<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>), any DNS names in the SubjectAlternativeName certificate
extension are used to verify the remote SMTP server name. If no DNS names
are
specified, the CommonName is checked. If you want mandatory encryption
without server certificate verification, see <a
href="#client_tls_encrypt">above</a>. </p>
<p> With Postfix ≥ 2.11 the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter
or more typically the corresponding per-destination "tafile" attribute
optionally modifies trust chain verification. If the parameter is
not empty the root CAs in CAfile and CApath are no longer trusted.
Rather, the Postfix SMTP client will only trust certificate-chains
signed by one of the trust-anchors contained in the chosen files.
The specified trust-anchor certificates and public keys are not
subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these
certificates also may be found "in the middle" of the trust chain
presented by the remote SMTP server, and any untrusted issuing
parent certificates will be ignored. </p>
<p> Despite the potential for eliminating "man-in-the-middle" and other
attacks, mandatory secure server certificate verification is not
viable as a default Internet mail delivery policy. Some MX hosts
do not support TLS at all, and a significant portion of TLS-enabled
MTAs use self-signed certificates, or certificates that are signed
by a private Certification Authority. On a machine that delivers mail
to the Internet, you should not configure secure TLS verification
as a default policy. </p>
<p> Mandatory secure server certificate verification as a default
security level may be appropriate if you know that you will only
connect to servers that support <a href="http://tools.ietf.org/html/rfc2487">RFC 2487</a> <i>and</i> that present
verifiable server certificates. An example would be a client that
sends all email to a central mailhub that offers the necessary
STARTTLS support. </p>
<p> You can enable secure TLS verification just for specific destinations.
With the Postfix TLS <a href="#client_tls_policy">policy table</a>,
specify the "secure" security level. </p>
<p> Examples: </p>
<ul>
<li> <p> Secure-channel TLS without <a href="transport.5.html">transport(5)</a> table overrides: </p>
<p> The Postfix SMTP client will encrypt all traffic and verify the
destination name
immune from forged DNS responses. MX lookups are still used to find
the hostnames of the SMTP servers for <i>example.com</i>, but these
hostnames are not used when
checking the names in the server certificate(s). Rather, the requirement
is that the MX hosts for <i>example.com</i> have trusted certificates
with a subject name of <i>example.com</i> or a sub-domain, see the
documentation for the <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter. </p>
<p> The related domains <i>example.co.uk</i> and <i>example.co.jp</i> are
hosted on the same MX hosts as the primary <i>example.com</i> domain, and
traffic to these is secured by verifying the primary <i>example.com</i>
domain in the server certificates. This frees the server administrator
from needing the CA to sign certificates that list all the secondary
domains. The downside is that clients that want secure channels to the
secondary domains need explicit TLS <a href="#client_tls_policy">policy
table</a> entries. </p>
<p> Note, there are two ways to handle related domains. The first is to
use the default routing for each domain, but add policy table entries
to override the expected certificate subject name. The second is to
override the next-hop in the transport table, and use a single policy
table entry for the common nexthop. We choose the first approach,
because it works better when domain ownership changes. With the second
approach we securely deliver mail to the wrong destination, with the
first approach, authentication fails and mail stays in the local queue,
the first approach is more appropriate in most cases. <p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAfile.pem
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
/etc/postfix/transport:
/etc/postfix/tls_policy:
example.com secure
example.co.uk secure match=example.com:.example.com
example.co.jp secure match=example.com:.example.com
</pre>
</blockquote>
<li> <p> Secure-channel TLS with <a href="transport.5.html">transport(5)</a> table overrides: <p>
<p> In this case traffic to <i>example.com</i> and its related domains
is sent to a single logical gateway (to avoid a single point of failure,
its name may resolve to one or more load-balancer addresses, or to the
combined addresses of multiple physical hosts). All the physical hosts
reachable via the gateway's IP addresses have the logical gateway name
listed in their certificates. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAfile.pem
<a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
/etc/postfix/transport:
example.com <a href="smtp.8.html">smtp</a>:[tls.example.com]
example.co.uk <a href="smtp.8.html">smtp</a>:[tls.example.com]
example.co.jp <a href="smtp.8.html">smtp</a>:[tls.example.com]
/etc/postfix/tls_policy:
[tls.example.com] secure match=tls.example.com
</pre>
</blockquote>
</ul>
<h3><a name="client_logging"> Client-side TLS activity logging </a> </h3>
<p> To get additional information about Postfix SMTP client TLS
activity you can increase the loglevel from 0..4. Each logging
level also includes the information that is logged at a lower
logging level. </p>
<blockquote>
<table border="1">
<tr> <th> Level </th> <th> Postfix 2.9 and later</th> <th> Earlier
releases. </th> </tr>
<tr> <td valign="top"> 0 </td> <td valign="top" colspan="2"> Disable
logging of TLS activity. </td> </tr>
<tr> <td valign="top"> 1 </td> <td valign="top"> Log only a summary
message on TLS handshake completion — no logging of remote SMTP
server certificate trust-chain verification errors if server certificate
verification is not required. </td> <td valign="top"> Log the summary
message and unconditionally log trust-chain verification errors.
</td> </tr>
<tr> <td valign="top"> 2 </td> <td valign="top" colspan="2"> Also
log levels during TLS negotiation. </td> </tr>
<tr> <td valign="top"> 3 </td> <td valign="top" colspan="2"> Also
log hexadecimal and ASCII dump of TLS negotiation process. </td>
</tr>
<tr> <td valign="top"> 4 </td> <td valign="top" colspan="2"> Also
log hexadecimal and ASCII dump of complete transmission after
STARTTLS. </td> </tr>
</table>
</blockquote>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 0
</pre>
</blockquote>
<h3><a name="client_cert_key">Client-side certificate and private
key configuration </a> </h3>
<p> Do not configure Postfix SMTP client certificates unless you <b>must</b>
present
client TLS certificates to one or more servers. Client certificates are
not usually needed, and can cause problems in configurations that work
well without them. The recommended setting is to let the defaults stand: </p>
<blockquote>
<pre>
<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> =
<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> =
<a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> =
<a href="postconf.5.html#smtp_tls_dkey_file">smtp_tls_dkey_file</a> =
# Postfix ≥ 2.6
<a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a> =
<a href="postconf.5.html#smtp_tls_eckey_file">smtp_tls_eckey_file</a> =
# Postfix ≥ 3.4
<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> =
</pre>
</blockquote>
<p> The best way to use the default settings is to comment out the above
parameters in <a href="postconf.5.html">main.cf</a> if present. </p>
<p> During TLS startup negotiation the Postfix SMTP client may present a
certificate to the remote SMTP server. Browsers typically let the user
select among the certificates that match the CA names indicated by the
remote SMTP server. The Postfix SMTP client does not yet have a mechanism
to select from multiple candidate certificates on the fly, and supports a
single set of certificates (at most one per public key algorithm). </p>
<p> RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported.
You can configure all three at the same time, in which case the
cipher used determines which certificate is presented. </p>
<p> It is possible for the Postfix SMTP client to use the same
key/certificate pair as the Postfix SMTP server. If a certificate
is to be presented, it must be in "PEM" format. The private key
must not be encrypted, meaning: it must be accessible without
password. Both parts (certificate and private key) may be in the
same file. </p>
<p> With OpenSSL 1.1.1 and Postfix ≥ 3.4 it is also possible to
configure Ed25519 and Ed448 certificates. Rather than add two more
pairs of key and certificate parameters, Postfix 3.4 introduces a new
"<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a>" parameter which specifies all the configured
certificates at once, and handles files that hold both the key and the
associated certificates in one pass, thereby avoiding potential race
conditions during key rollover. </p>
<p> To enable remote SMTP servers to verify the Postfix SMTP client
certificate, the issuing CA certificates must be made available to the
server. You should include the required certificates in the client
certificate file, the client certificate first, then the issuing
CA(s) (bottom-up order). </p>
<p> Example: the certificate for "client.example.com" was issued by
"intermediate CA" which itself has a certificate issued by "root CA".
As the "root" super-user create the client.pem file with: </p>
<blockquote>
<pre>
# <b>umask 077</b>
# <b>cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem </b>
</pre>
</blockquote>
<p> A Postfix SMTP client certificate supplied here must be usable
as SSL client certificate and hence pass the "openssl verify -purpose
sslclient ..." test. </p>
<p> A server that trusts the root CA has a local copy of the root
CA certificate, so it is not necessary to include the root CA
certificate here. Leaving it out of the "chain.pem" file reduces
the overhead of the TLS exchange. </p>
<p> If you want the Postfix SMTP client to accept remote SMTP server
certificates issued by these CAs, append the root certificate to
$<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or install it in the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. </p>
<p> Example: Postfix ≥ 3.4 all-in-one chain file(s). One or more
chain files that start with a key that is immediately followed by the
corresponding certificate and any additional issuer certificates. A
single file can hold multiple <i>(key, cert, [chain])</i> sequences, one
per algorithm. It is typically simpler to keep the chain for each
algorithm in its own file. Most users are likely to deploy at most a
single RSA chain, but with OpenSSL 1.1.1, it is possible to deploy up
five chains, one each for RSA, ECDSA, ED25519, ED448 and even the
obsolete DSA. </p>
<blockquote>
<pre>
# Postfix ≥ 3.4. Preferred configuration interface. Each file
# starts with the private key, followed by the corresponding
# certificate, and any intermediate issuer certificates.
#
<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> =
/etc/postfix/rsa.pem,
/etc/postfix/ecdsa.pem,
/etc/postfix/ed25519.pem,
/etc/postfix/ed448.pem
</pre>
</blockquote>
<p> You can also store the keys separately from their certificates, again
provided each is listed before the corresponding certificate chain. Storing a
key and its associated certificate chain in separate files is not recommended,
because this is prone to race conditions during key rollover, as there is no
way to update multiple files atomically. </p>
<blockquote>
<pre>
# Postfix ≥ 3.4.
# Storing keys separately from the associated certificates is not
# recommended.
<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a> =
/etc/postfix/rsakey.pem,
/etc/postfix/rsacerts.pem,
/etc/postfix/ecdsakey.pem,
/etc/postfix/ecdsacerts.pem
</pre>
</blockquote>
<p> The below examples show the legacy algorithm-specific configurations
for Postfix 3.3 and older. With Postfix ≤ 3.3, even if the key is
stored in the same file as the certificate, the file is read twice and a
(brief) race condition still exists during key rollover. While Postfix
≥ 3.4 avoids the race when the key and certificate are in the same
file, you should use the new "<a href="postconf.5.html#smtp_tls_chain_files">smtp_tls_chain_files</a>" interface shown
above. <p>
<p> RSA key and certificate examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a> = /etc/postfix/client.pem
<a href="postconf.5.html#smtp_tls_key_file">smtp_tls_key_file</a> = $<a href="postconf.5.html#smtp_tls_cert_file">smtp_tls_cert_file</a>
</pre>
</blockquote>
<p> Their DSA counterparts: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a> = /etc/postfix/client-dsa.pem
<a href="postconf.5.html#smtp_tls_dkey_file">smtp_tls_dkey_file</a> = $<a href="postconf.5.html#smtp_tls_dcert_file">smtp_tls_dcert_file</a>
</pre>
</blockquote>
<p> Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 1.0.0): </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a> = /etc/postfix/client-ecdsa.pem
<a href="postconf.5.html#smtp_tls_eckey_file">smtp_tls_eckey_file</a> = $<a href="postconf.5.html#smtp_tls_eccert_file">smtp_tls_eccert_file</a>
</pre>
</blockquote>
<p> To verify a remote SMTP server certificate, the Postfix SMTP
client needs to trust the certificates of the issuing Certification
Authorities. These certificates in "pem" format can be stored in a
single $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> or in multiple files, one CA per file in
the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory. If you use a directory, don't forget
to create the necessary "hash" links with: </p>
<blockquote>
<pre>
# <b>$OPENSSL_HOME/bin/c_rehash <i>/path/to/directory</i> </b>
</pre>
</blockquote>
<p> The $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> contains the CA certificates of one or more
trusted CAs. The file is opened (with root privileges) before Postfix
enters the optional chroot jail and so need not be accessible from inside the
chroot jail. </p>
<p> Additional trusted CAs can be specified via the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a>
directory, in which case the certificates are read (with $<a href="postconf.5.html#mail_owner">mail_owner</a>
privileges) from the files in the directory when the information
is needed. Thus, the $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> directory needs to be accessible
inside the optional chroot jail. </p>
<p> The choice between $<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> and $<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> is
a space/time tradeoff. If there are many trusted CAs, the cost of
preloading them all into memory may not pay off in reduced access time
when the certificate is needed. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/CAcert.pem
<a href="postconf.5.html#smtp_tls_CApath">smtp_tls_CApath</a> = /etc/postfix/certs
</pre>
</blockquote>
<h3><a name="client_tls_reuse">Client-side TLS connection reuse</a> </h3>
<p> Historically, the Postfix SMTP client has supported multiple
deliveries per plaintext connection. Postfix 3.4 introduces support
for multiple deliveries per TLS-encrypted connection. Multiple
deliveries per connection improve mail delivery performance,
especially for destinations that throttle clients that don't combine
deliveries. </p>
<p> To enable multiple deliveries per TLS connection, specify:</p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_connection_reuse">smtp_tls_connection_reuse</a> = yes
</pre>
</blockquote>
<p> Alternatively, specify the attribute "connection_reuse=yes" in
an <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> entry. </p>
<p> The implementation of TLS connection reuse relies on the same
<a href="scache.8.html">scache(8)</a> service as used for delivering plaintext SMTP mail, the
same <a href="tlsproxy.8.html">tlsproxy(8)</a> daemon as used by the <a href="postscreen.8.html">postscreen(8)</a> service, and
relies on the same hints from the <a href="qmgr.8.html">qmgr(8)</a> daemon.
See "<a href="CONNECTION_CACHE_README.html">Postfix Connection
Cache</a>" for a description of the underlying connection reuse
infrastructure. </p>
<p> Initial SMTP handshake:</p>
<pre> <a href="smtp.8.html">smtp(8)</a> -> remote SMTP server</pre>
<p> Reused SMTP/TLS connection, or new SMTP/TLS connection: </p>
<pre> <a href="smtp.8.html">smtp(8)</a> -> <a href="tlsproxy.8.html">tlsproxy(8)</a> -> remote SMTP server </pre>
<p> Cached SMTP/TLS connection:</p>
<pre> <a href="scache.8.html">scache(8)</a> -> <a href="tlsproxy.8.html">tlsproxy(8)</a> -> remote SMTP server</pre>
<p> As of Postfix 3.4, TLS connection reuse is disabled by default.
This may change once the impact on over-all performance is understood.
</p>
<h3><a name="client_tls_cache">Client-side TLS session cache</a> </h3>
<p> The remote SMTP server and the Postfix SMTP client negotiate a
session, which takes some computer time and network bandwidth. By
default, this session information is cached only in the <a href="smtp.8.html">smtp(8)</a>
process actually using this session and is lost when the process
terminates. To share the session information between multiple
<a href="smtp.8.html">smtp(8)</a> processes, a persistent session cache can be used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the <a href="tlsmgr.8.html">tlsmgr(8)</a> process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high. Future
Postfix SMTP servers may limit the number of sessions that a client
is allowed to negotiate per unit time.</p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtp_scache
</pre>
</blockquote>
<p> Note: as of version 2.5, Postfix no longer uses root privileges
when opening this file. The file should now be stored under the
Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned <a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. </p>
<p> Cached Postfix SMTP client session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600s (=1 hour). <a href="http://tools.ietf.org/html/rfc2246">RFC 2246</a>
recommends a maximum of 24 hours. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_session_cache_timeout">smtp_tls_session_cache_timeout</a> = 3600s
</pre>
</blockquote>
<p> As of Postfix 2.11 this setting cannot exceed 100 days. If set
≤ 0, session caching is disabled. If set to a positive value
less than 2 minutes, the minimum value of 2 minutes is used instead. </p>
<h3><a name="client_tls_limits"> Client TLS limitations </a>
</h3>
<p> The security properties of TLS communication channels are
application specific. While the TLS protocol can provide a confidential,
tamper-resistant, mutually authenticated channel between client
and server, not all of these security features are applicable to every
communication. </p>
<p> For example, while mutual TLS authentication between browsers and web
servers is possible, it is not practical, or even useful, for web-servers
that serve the public to verify the identity of every potential user. In
practice, most HTTPS transactions are asymmetric: the browser verifies
the HTTPS server's identity, but the user remains anonymous. Much of
the security policy is up to the client. If the client chooses to not
verify the server's name, the server is not aware of this. There are many
interesting browser security topics, but we shall not dwell
on them here. Rather, our goal is to understand the security features
of TLS in conjunction with SMTP. </p>
<p> An important SMTP-specific observation is that a public MX host is
even more at the mercy of the SMTP client than is an HTTPS server. Not only
can it not enforce due care in the client's use of TLS, but it cannot even
enforce the use of TLS, because TLS support in SMTP clients is still the
exception rather than the rule. One cannot, in practice, limit access to
one's MX hosts to just TLS-enabled clients. Such a policy would result
in a vast reduction in one's ability to communicate by email with the
world at large. </p>
<p> One may be tempted to try enforcing TLS for mail from specific
sending organizations, but this, too, runs into obstacles. One such
obstacle is that we don't know who is (allegedly) sending mail until
we see the "MAIL FROM:" SMTP command, and at that point, if TLS
is not already in use, a potentially sensitive sender address (and
with SMTP PIPELINING one or more of the recipients) has (have) already been
leaked in the clear. Another obstacle is that mail from the sender to
the recipient may be forwarded, and the forwarding organization may not
have any security arrangements with the final destination. Bounces also
need to be protected. These can only be identified by the IP address and
HELO name of the connecting client, and it is difficult to keep track
of all the potential IP addresses or HELO names of the outbound email
servers of the sending organization. </p>
<p> Consequently, TLS security for mail delivery to public MX hosts is
almost entirely the client's responsibility. The server is largely a
passive enabler of TLS security, the rest is up to the client. While the
server has a greater opportunity to mandate client security policy when
it is a dedicated MSA that only handles outbound mail from trusted clients,
below we focus on the client security policy. </p>
<p> On the SMTP client, there are further complications. When
delivering mail to a given domain, in contrast to HTTPS, one rarely
uses the domain name directly as the target host of the SMTP session.
More typically, one uses MX lookups — these are usually
unauthenticated — to obtain the domain's SMTP server hostname(s).
When, as is current practice, the client verifies the insecurely
obtained MX hostname, it is subject to a DNS man-in-the-middle
attack. </p>
<p> Adoption of DNSSEC and <a href="http://tools.ietf.org/html/rfc6698">RFC6698</a> (DANE) may gradually (as domains
implement DNSSEC and publish TLSA records for their MX hosts) address
the DNS man-in-the-middle risk and provide scalable key management
for SMTP with TLS. Postfix ≥ 2.11 supports the new <a
href="#client_tls_dane">dane</a> and <a href="#client_tls_dane">dane-only</a>
security levels that take advantage of these standards. </p>
<p> If clients instead attempted to verify the recipient domain name,
an SMTP server for multiple domains would need to
list all its email domain names in its certificate, and generate a
new certificate each time a new domain were added. At least some CAs set
fairly low limits (20 for one prominent CA) on the number of names that
server certificates can contain. This approach is not consistent with
current practice and does not scale. </p>
<p> It is regrettably the case that TLS <i>secure-channels</i>
(fully authenticated and immune to man-in-the-middle attacks) impose
constraints on the sending and receiving sites that preclude ubiquitous
deployment. One needs to manually configure this type of security for
each destination domain, and in many cases implement non-default TLS
<a href="#client_tls_policy">policy table</a> entries for additional
domains hosted at a common secured destination. For these reasons
secure-channel configurations
will never be the norm. For the generic domain with which you
have made no specific security arrangements, this security level is not
a good fit. </p>
<p> Given that strong authentication is not generally possible, and that
verifiable certificates cost time and money, many servers that implement
TLS use self-signed certificates or private CAs. This further limits
the applicability of verified TLS on the public Internet. </p>
<p> Historical note: while the documentation of these issues and many of the
related features were new with Postfix 2.3, the issue was well
understood before Postfix 1.0, when Lutz Jänicke was designing
the first unofficial Postfix TLS patch. See his original post <a
href="http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html">http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html</a>
and the first response <a
href="http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html">http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html</a>.
The problem is not even unique to SMTP or even TLS, similar issues exist
for secure connections via aliases for HTTPS and Kerberos. SMTP merely
uses indirect naming (via MX records) more frequently. </p>
<h3> <a name="client_tls_policy"> TLS policy table </a>
</h3>
<p> A small fraction of servers offer STARTTLS but the negotiation
consistently fails. As long as encryption is not mandatory, the
Postfix SMTP client retries the delivery immediately with TLS
disabled, without any need to explicitly disable TLS for the problem
destinations. </p>
<p> The policy table is specified via the <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a>
parameter. This lists optional lookup tables with the Postfix SMTP client
TLS security policy by next-hop destination. </p>
<p> The TLS policy table is indexed by the full next-hop destination,
which is either the recipient domain, or the verbatim next-hop
specified in the transport table, $<a href="postconf.5.html#local_transport">local_transport</a>, $<a href="postconf.5.html#virtual_transport">virtual_transport</a>,
$<a href="postconf.5.html#relay_transport">relay_transport</a> or $<a href="postconf.5.html#default_transport">default_transport</a>. This includes any enclosing
square brackets and any non-default destination server port suffix. The
<a href="#client_lmtp_tls">LMTP</a> socket type prefix (inet: or unix:)
is not included in the lookup key. </p>
<p> Only the next-hop domain, or $<a href="postconf.5.html#myhostname">myhostname</a> with LMTP over UNIX-domain
sockets, is used as the nexthop name for certificate verification. The
port and any enclosing square brackets are used in the table lookup key,
but are not used for server name verification. </p>
<p> When the lookup key is a domain name without enclosing square brackets
or any <i>:port</i> suffix (typically the recipient domain), and the full
domain is not found in the table, just as with the <a href="transport.5.html">transport(5)</a> table,
the parent domain starting with a leading "." is matched recursively. This
allows one to specify a security policy for a recipient domain and all
its sub-domains. </p>
<p> The lookup result is a security level, followed by an optional
list of whitespace and/or comma separated name=value attributes
that override related <a href="postconf.5.html">main.cf</a> settings. The TLS security <a
href="#client_tls_levels">levels</a> are described above. Below, we
describe the corresponding table syntax: </p>
<dl>
<dt><b>none</b></dt> <dd><a href="#client_tls_none">No TLS</a>. No
additional attributes are supported at this level. </dd>
<dt><b>may</b></dt> <dd><a href="#client_tls_may">Opportunistic TLS</a>.
The optional "ciphers", "exclude" and "protocols" attributes
(available for opportunistic TLS with Postfix ≥ 2.6) override the
"<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>", "<a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a>" and "<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a>"
configuration parameters. At this level and higher, the optional
"servername" attribute (available with Postfix ≥ 3.4) overrides the
global "<a href="postconf.5.html#smtp_tls_servername">smtp_tls_servername</a>" parameter, enabling per-destination
configuration of the SNI extension sent to the remote SMTP server. </dd>
<dt><b>encrypt</b></dt> <dd><a href="#client_tls_encrypt"> Mandatory encryption</a>.
Mail is delivered only if the remote SMTP server offers STARTTLS
and the TLS handshake succeeds. At this level and higher, the optional
"protocols" attribute overrides the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>
parameter, the optional "ciphers" attribute overrides the
<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> parameter, and the optional
"exclude" attribute (Postfix ≥ 2.6) overrides the <a href="postconf.5.html">main.cf</a>
<a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> parameter. </dd>
<dt><b>dane</b></dt> <dd><a href="#client_tls_dane">Opportunistic DANE TLS</a>.
The TLS policy for the destination is obtained via TLSA records in
DNSSEC. If no TLSA records are found, the effective security level
used is <a href="#client_tls_may">may</a>. If TLSA records are
found, but none are usable, the effective security level is <a
href="#client_tls_encrypt">encrypt</a>. When usable TLSA records
are obtained for the remote SMTP server, SSLv2+3 are automatically
disabled (see <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>), and the server certificate
must match the TLSA records. <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a> (DANE) TLS authentication
and DNSSEC support is available with Postfix 2.11 and later. </dd>
<dt><b>dane-only</b></dt> <dd><a href="#client_tls_dane">Mandatory DANE TLS</a>.
The TLS policy for the destination is obtained via TLSA records in
DNSSEC. If no TLSA records are found, or none are usable, no
connection is made to the server. When usable TLSA records are
obtained for the remote SMTP server, SSLv2+3 are automatically disabled
(see <a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a>), and the server certificate must
match the TLSA records. <a href="http://tools.ietf.org/html/rfc7672">RFC 7672</a> (DANE) TLS authentication and
DNSSEC support is available with Postfix 2.11 and later. </dd>
<dt><b>fingerprint</b></dt> <dd><a href="#client_tls_fprint">Certificate
fingerprint verification.</a> Available with Postfix 2.5 and
later. At this security level, there are no trusted Certification
Authorities. The certificate trust chain, expiration date, ... are
not checked. Instead, the optional <b>match</b> attribute, or else
the <a href="postconf.5.html">main.cf</a> <b><a href="postconf.5.html#smtp_tls_fingerprint_cert_match">smtp_tls_fingerprint_cert_match</a></b> parameter, lists
the server certificate fingerprints or public key fingerprints
(Postfix 2.9 and later). The
digest algorithm used to calculate fingerprints is selected by the
<b><a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a></b> parameter. Multiple fingerprints can
be combined with a "|" delimiter in a single match attribute, or multiple
match attributes can be employed. The ":" character is not used as a
delimiter as it occurs between each pair of fingerprint (hexadecimal)
digits. </dd>
<dt><b>verify</b></dt> <dd><a href="#client_tls_verify">Mandatory
server certificate verification</a>. Mail is delivered only if the
TLS handshake succeeds, if the remote SMTP server certificate can
be validated (not expired or revoked, and signed by a trusted
Certification Authority), and if the server certificate name matches
the optional "match" attribute (or the <a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_verify_cert_match">smtp_tls_verify_cert_match</a>
parameter value when no optional "match" attribute is specified).
With Postfix ≥ 2.11 the "tafile" attribute optionally modifies
trust chain verification in the same manner as the
"<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter. The "tafile" attribute
may be specified multiple times to load multiple trust-anchor
files. </dd>
<dt><b>secure</b></dt> <dd><a href="#client_tls_secure">Secure certificate
verification.</a> Mail is delivered only if the TLS handshake succeeds,
if the remote SMTP server certificate can be validated (not expired
or revoked, and signed by a trusted Certification Authority), and if the
server certificate name matches the optional "match" attribute (or the
<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> parameter value when no optional
"match" attribute is specified). With Postfix ≥ 2.11 the "tafile"
attribute optionally modifies trust chain verification in the same manner
as the "<a href="postconf.5.html#smtp_tls_trust_anchor_file">smtp_tls_trust_anchor_file</a>" parameter. The "tafile" attribute
may be specified multiple times to load multiple trust-anchor
files. </dd>
</dl>
<p> Notes: </p>
<ul>
<li> <p> The "match" attribute is especially useful to verify TLS
certificates for domains that are hosted on a shared server. In
that case, specify "match" rules for the shared server's name.
While secure verification can also be achieved with manual routing
overrides in Postfix <a href="transport.5.html">transport(5)</a> tables, that approach can deliver
mail to the wrong host when domains are assigned to new gateway
hosts. The "match" attribute approach avoids the problems of manual
routing overrides; mail is deferred if verification of a new MX
host fails. </p>
<li> <p> When a policy table entry specifies multiple match patterns,
multiple match strategies, or multiple protocols, these must be
separated by colons. </p>
<li> <p> The "exclude" attribute (Postfix ≥ 2.6) is used to disable
ciphers that cause handshake failures with a specific mandatory TLS
destination, without disabling the ciphers for all mandatory destinations.
Alternatively, you can exclude ciphers that cause issues with multiple
remote servers in <a href="postconf.5.html">main.cf</a>, and selectively enable them on a per-destination
basis in the policy table by setting a shorter or empty exclusion list. The
per-destination "exclude" list preempts both the opportunistic and
mandatory security level exclusions, so that all excluded ciphers
can be enabled for known-good destinations. For non-mandatory TLS
destinations that exhibit cipher-specific problems, Postfix will fall
back to plain-text delivery. If plain-text is not acceptable make TLS
mandatory and exclude the problem ciphers. </p>
</ul>
<p>
Example:
</p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/tls_policy
# Postfix 2.5 and later
<a href="postconf.5.html#smtp_tls_fingerprint_digest">smtp_tls_fingerprint_digest</a> = md5
/etc/postfix/tls_policy:
example.edu none
example.mil may
example.gov encrypt ciphers=high
example.com verify match=hostname:dot-nexthop ciphers=high
example.net secure
.example.net secure match=.example.net:example.net
[mail.example.org]:587 secure match=nexthop
# Postfix 2.5 and later
[thumb.example.org] fingerprint
match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
# Postfix 2.6 and later
example.info may protocols=!SSLv2 ciphers=medium exclude=3DES
</pre>
</blockquote>
<p> <b>Note:</b> The "hostname" strategy if listed in a non-default setting
of <a href="postconf.5.html#smtp_tls_secure_cert_match">smtp_tls_secure_cert_match</a> or in the "match" attribute in the policy
table can render the "secure" level vulnerable to DNS forgery. Do not use
the "hostname" strategy for <a href="#client_tls_secure">secure-channel</a>
configurations in environments where DNS security is not assured. </p>
<h3> <a name="client_tls_discover"> Discovering servers that support
TLS </a> </h3>
<p> As we decide on a "per site" basis whether or not to use TLS,
it would be good to have a list of sites that offered "STARTTLS".
We can collect it ourselves with this option. </p>
<p> If the <a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> feature is enabled and a
server offers STARTTLS while TLS is not already enabled for that
server, the Postfix SMTP client logs a line as follows: </p>
<blockquote>
<pre>
postfix/smtp[pid]: Host offered STARTTLS: [hostname.example.com]
</pre>
</blockquote>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_note_starttls_offer">smtp_tls_note_starttls_offer</a> = yes
</pre>
</blockquote>
<h3><a name="client_vrfy_server">Server certificate verification depth</a> </h3>
<p> The server certificate verification depth is specified with the
<a href="postconf.5.html">main.cf</a> <a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where <a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> was ignored.
When you configure trust
in a root CA, it is not necessary to explicitly trust intermediary CAs
signed by the root CA, unless $<a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> is less than the
number of CAs in the certificate chain for the servers of interest. With
a verify depth of 1 you can only verify certificates directly signed
by a trusted CA, and all trusted intermediary CAs need to be configured
explicitly. With a verify depth of 2 you can verify servers signed by a
root CA or a direct intermediary CA (so long as the server is correctly
configured to supply its intermediate CA certificate). </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_scert_verifydepth">smtp_tls_scert_verifydepth</a> = 2
</pre>
</blockquote>
<h3> <a name="client_cipher">Client-side cipher controls </a> </h3>
<p> The Postfix SMTP client supports 5 distinct cipher grades
as specified by the <a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> configuration
parameter. This setting controls the minimum acceptable SMTP client
TLS cipher grade for use with mandatory TLS encryption. The default
value "medium" is suitable for most destinations with which you may
want to enforce TLS, and is beyond the reach of today's cryptanalytic
methods. See <a href="postconf.5.html#smtp_tls_policy_maps">smtp_tls_policy_maps</a> for information on how to configure
ciphers on a per-destination basis. </p>
<p> By default anonymous ciphers are allowed, and automatically
disabled when remote SMTP server certificates are verified. If you
want to
disable anonymous ciphers even at the "encrypt" security level, set
"<a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> = aNULL"; and to
disable anonymous ciphers even with opportunistic TLS, set
"<a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a> = aNULL". There is generally
no need to take these measures. Anonymous ciphers save bandwidth
and TLS session cache space, if certificates are ignored, there is
little point in requesting them. </p>
<p> The "<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a>" configuration parameter (Postfix ≥ 2.6)
provides control over the minimum cipher grade for opportunistic TLS.
The default minimum cipher grade for opportunistic TLS is "medium"
for Postfix releases after the middle of 2015, and "export" for
older releases. With Postfix < 2.6, the minimum opportunistic
TLS cipher grade is always "export". </p>
<p> With mandatory and opportunistic TLS encryption, the Postfix
SMTP client will by default disable SSLv2 and SSLv3. The mandatory
TLS protocol list is specified via the
<a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> configuration parameter. The corresponding
<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a> parameter (Postfix ≥ 2.6) controls
the SSL/TLS protocols used with opportunistic TLS. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_mandatory_ciphers">smtp_tls_mandatory_ciphers</a> = medium
<a href="postconf.5.html#smtp_tls_mandatory_exclude_ciphers">smtp_tls_mandatory_exclude_ciphers</a> = RC4, MD5
<a href="postconf.5.html#smtp_tls_exclude_ciphers">smtp_tls_exclude_ciphers</a> = aNULL
# Preferred form with Postfix ≥ 2.5:
<a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> = !SSLv2
# Legacy form for Postfix < 2.5:
<a href="postconf.5.html#smtp_tls_mandatory_protocols">smtp_tls_mandatory_protocols</a> = SSLv3, TLSv1
# Also available with Postfix ≥ 2.6:
<a href="postconf.5.html#smtp_tls_ciphers">smtp_tls_ciphers</a> = medium
<a href="postconf.5.html#smtp_tls_protocols">smtp_tls_protocols</a> = !SSLv2
</pre>
</blockquote>
<h3> <a name="client_smtps">Client-side SMTPS support </a> </h3>
<p> These sections show how to send mail to a server that does not
support STARTTLS, but that provides the deprecated SMTPS service
on TCP port 465. Depending on the Postfix version, some additional
tooling may be required. </p>
<h4> Postfix ≥ 3.0 </h4>
<p> The Postfix SMTP client has SMTPS support built-in as of version
3.0. Use one of the following examples, to send all remote mail,
or to send only some remote mail, to an SMTPS server. </p>
<h5> Postfix ≥ 3.0: Sending all remote mail to an SMTPS server </h5>
<p> The first example will send all remote mail over SMTPS through
a provider's server called "mail.example.com": </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
# Client-side SMTPS requires "encrypt" or stronger.
<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = encrypt
<a href="postconf.5.html#smtp_tls_wrappermode">smtp_tls_wrappermode</a> = yes
# The [] suppress MX lookups.
<a href="postconf.5.html#relayhost">relayhost</a> = [mail.example.com]:465
</pre>
</blockquote>
<p> Use "postfix reload" to make the change effective. </p>
<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL authentication.
</p>
<h5> Postfix ≥ 3.0: Sending only mail for a specific destination
via SMTPS </h5>
<p> The second example will send only mail for "example.com" via
SMTPS. This time, Postfix uses a transport map to deliver only
mail for "example.com" via SMTPS: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport
/etc/postfix/transport:
example.com relay-smtps:example.com:465
/etc/postfix/<a href="master.5.html">master.cf</a>:
relay-smtps unix - - n - - smtp
# Client-side SMTPS requires "encrypt" or stronger.
-o <a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a>=encrypt
-o <a href="postconf.5.html#smtp_tls_wrappermode">smtp_tls_wrappermode</a>=yes
</pre>
</blockquote>
<p> Use "postmap <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport" and "postfix reload"
to make the change effective. </p>
<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL
authentication. </p>
<h4> Postfix < 3.0 </h4>
<p> Although older Postfix SMTP client versions do not support TLS
wrapper mode, it is relatively easy to forward a connection through
the stunnel program if Postfix needs to deliver mail to some legacy
system that doesn't support STARTTLS. </p>
<h5> Postfix < 3.0: Sending all remote mail to an SMTPS server </h5>
<p> The first example uses SMTPS to send all remote mail to a
provider's mail server called "mail.example.com". </p>
<p> A minimal stunnel.conf file is sufficient to set up a tunnel
from local port 11125 to the remote destination "mail.example.com"
and port "smtps". Postfix will later use this tunnel to connect to
the remote server. </p>
<blockquote>
<pre>
/path/to/stunnel.conf:
[smtp-tls-wrapper]
accept = 11125
client = yes
connect = mail.example.com:smtps
</pre>
</blockquote>
<p> To test this tunnel, use: </p>
<blockquote>
<pre>
$ telnet localhost 11125
</pre>
</blockquote>
<p> This should produce the greeting from the remote SMTP server
at mail.example.com. </p>
<p> On the Postfix side, the <a href="postconf.5.html#relayhost">relayhost</a> feature sends all remote
mail through the local stunnel listener on port 11125: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#relayhost">relayhost</a> = [127.0.0.1]:11125
</pre>
</blockquote>
<p> Use "postfix reload" to make the change effective. </p>
<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL
authentication. </p>
<h4> Postfix < 3.0: Sending only mail for a specific destination via SMTPS </h4>
<p> The second example will use SMTPS to send only mail for
"example.com" via SMTPS. It uses the same stunnel configuration
file as the first example, so it won't be repeated here. </p>
<p> This time, the Postfix side uses a transport map to direct only
mail for "example.com" through the tunnel: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#transport_maps">transport_maps</a> = <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport
/etc/postfix/transport:
example.com relay:[127.0.0.1]:11125
</pre>
</blockquote>
<p> Use "postmap <a href="DATABASE_README.html#types">hash</a>:/etc/postfix/transport" and "postfix reload"
to make the change effective. </p>
<p> See <a href="SOHO_README.html">SOHO_README</a> for additional information about SASL authentication.
</p>
<h3> <a name="client_misc"> Miscellaneous client controls </a> </h3>
<p> The <a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> parameter limits the time of Postfix
SMTP client write and read operations during TLS startup and shutdown
handshake procedures. In case of problems the Postfix SMTP client
tries the next network address on the mail exchanger list, and
defers delivery if no alternative server is available. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_starttls_timeout">smtp_starttls_timeout</a> = 300s
</pre>
</blockquote>
<p> With Postfix 2.8 and later, the <a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> parameter
specifies a list or bit-mask of OpenSSL bug work-arounds to disable. This
may be necessary if one of the work-arounds enabled by default in
OpenSSL proves to pose a security risk, or introduces an unexpected
interoperability issue. Some bug work-arounds known to be problematic
are disabled in the default value of the parameter when linked with
an OpenSSL library that could be vulnerable. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = 0xFFFFFFFF
<a href="postconf.5.html#tls_disable_workarounds">tls_disable_workarounds</a> = CVE-2010-4180, LEGACY_SERVER_CONNECT
</pre>
</blockquote>
<p> Note: Disabling LEGACY_SERVER_CONNECT is not wise at this
time, lots of servers are still unpatched and Postfix is <a
href="http://www.postfix.org/wip.html#tls-renegotiation">not
significantly vulnerable</a> to the renegotiation issue in the TLS
protocol. </p>
<p> With Postfix ≥ 2.11, the <a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> parameter specifies
a list or bit-mask of OpenSSL options to enable. Specify one or
more of the named options below, or a hexadecimal bitmask of options
found in the ssl.h file corresponding to the run-time OpenSSL
library. While it may be reasonable to turn off all bug workarounds
(see above), it is not a good idea to attempt to turn on all features.
</p>
<p> A future version of OpenSSL may by default no longer allow
connections to servers that don't support secure renegotiation.
Since the exposure for SMTP is minimal, and some SMTP servers may
remain unpatched, you can add LEGACY_SERVER_CONNECT to the
options to restore the more permissive default of current OpenSSL
releases. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_ssl_options">tls_ssl_options</a> = NO_TICKET, NO_COMPRESSION, LEGACY_SERVER_CONNECT
</pre>
</blockquote>
<p> You should only enable features via the hexadecimal mask when
the need to control the feature is critical (to deal with a new
vulnerability or a serious interoperability problem). Postfix DOES
NOT promise backwards compatible behavior with respect to the mask
bits. A feature enabled via the mask in one release may be enabled
by other means in a later release, and the mask bit will then be
ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better
solution. </p>
<h2><a name="tlsmgr_controls"> TLS manager specific settings </a> </h2>
<p> The security of cryptographic software such as TLS depends
critically on the ability to generate unpredictable numbers for
keys and other information. To this end, the <a href="tlsmgr.8.html">tlsmgr(8)</a> process
maintains a Pseudo Random Number Generator (PRNG) pool. This is
queried by the <a href="smtp.8.html">smtp(8)</a> and <a href="smtpd.8.html">smtpd(8)</a> processes when they initialize.
By default, these daemons request 32 bytes, the equivalent to 256
bits. This is more than sufficient to generate a 128bit (or 168bit)
session key. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_daemon_random_bytes">tls_daemon_random_bytes</a> = 32
</pre>
</blockquote>
<p> In order to feed its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a> reads
entropy from an external source, both at startup and during run-time.
Specify a good entropy source, like EGD or /dev/urandom; be sure
to only use non-blocking sources (on OpenBSD, use /dev/arandom
when <a href="tlsmgr.8.html">tlsmgr(8)</a> complains about /dev/urandom timeout errors).
If the entropy source is not a
regular file, you must prepend the source type to the source name:
"dev:" for a device special file, or "egd:" for a source with EGD
compatible socket interface. </p>
<p> Examples (specify only one in <a href="postconf.5.html">main.cf</a>): </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = egd:/var/run/egd-pool
</pre>
</blockquote>
<p> By default, <a href="tlsmgr.8.html">tlsmgr(8)</a> reads 32 bytes from the external entropy
source at each seeding event. This amount (256bits) is more than
sufficient for generating a 128bit symmetric key. With EGD and
device entropy sources, the <a href="tlsmgr.8.html">tlsmgr(8)</a> limits the amount of data
read at each step to 255 bytes. If you specify a regular file as
entropy source, a larger amount of data can be read. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_bytes">tls_random_bytes</a> = 32
</pre>
</blockquote>
<p> In order to update its in-memory PRNG pool, the <a href="tlsmgr.8.html">tlsmgr(8)</a>
queries the external entropy source again after a pseudo-random
amount of time. The time is calculated using the PRNG, and is
between 0 and the maximal time specified with <a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a>.
The default maximal time interval is 1 hour. </p>
<p> Example: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_reseed_period">tls_random_reseed_period</a> = 3600s
</pre>
</blockquote>
<p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> process saves the PRNG state to a persistent
exchange file at regular times and when the process terminates, so
that it can recover the PRNG state the next time it starts up.
This file is created when it does not exist. </p>
<p> Examples: </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#tls_random_exchange_name">tls_random_exchange_name</a> = /var/lib/postfix/prng_exch
<a href="postconf.5.html#tls_random_prng_update_period">tls_random_prng_update_period</a> = 3600s
</pre>
</blockquote>
<p> As of version 2.5, Postfix no longer uses root privileges when
opening this file. The file should now be stored under the Postfix-owned
<a href="postconf.5.html#data_directory">data_directory</a>. As a migration aid, an attempt to open the file
under a non-Postfix directory is redirected to the Postfix-owned
<a href="postconf.5.html#data_directory">data_directory</a>, and a warning is logged. If you wish to continue
using a pre-existing PRNG state file, move it to the <a href="postconf.5.html#data_directory">data_directory</a>
and change the ownership to the account specified with the <a href="postconf.5.html#mail_owner">mail_owner</a>
parameter. </p>
<p> With earlier Postfix versions the default file location
is under the Postfix configuration directory, which is not the
proper place for information that is modified by Postfix. </p>
<h2><a name="quick-start">Getting started, quick and dirty</a></h2>
<p> The following steps will get you started quickly. Because you
sign your own Postfix public key certificate, you get TLS encryption
but no TLS authentication. This is sufficient for testing, and
for exchanging email with sites that you have no trust relationship
with. For real authentication you need also enable DNSSEC record
signing for your domain and publish TLSA records and/or your Postfix
public key certificate needs to be signed by a recognized Certification
Authority. To authenticate the certificates of remote host you
need a DNSSEC-validating local resolver and to enable <a
href="#client_tls_dane">DANE</a> authentication and/or configure
the Postfix SMTP client with a list of public key certificates of
Certification Authorities, but make sure to read about the <a
href="#client_tls_limits">limitations</a> of the latter approach.
</p>
<p> In the examples below, user input is shown in <b><tt>bold</tt></b>
font, and a "<tt>#</tt>" prompt indicates a super-user shell. </p>
<ul>
<li> <p> <a href="#built-in">Quick-start TLS with Postfix ≥ 3.1</a>.</p>
<li> <p> <a href="#self-signed">Self-signed server certificate</a>.</p>
<li> <p> <a href="#private-ca">Private Certification Authority</a>. </p>
</ul>
<h3><a name="built-in">Quick-start TLS with Postfix ≥ 3.1</a></h3>
<p> Postfix 3.1 provides built-in support for enabling TLS in the
SMTP client and server and for ongoing certificate and DANE TLSA
record management.
<ul>
<li> <p> <a href="#quick-client">Quick-start TLS in the Postfix ≥ 3.1 SMTP client</a>. </p>
<li> <p> <a href="#quick-server">Quick-start TLS in the Postfix ≥ 3.1 SMTP server</a>. </p>
</ul>
<h4> <a name="quick-client">Quick-start TLS in the Postfix ≥ 3.1 SMTP client</a>. </h4>
<p> If you are using Postfix 3.1 or later, and your SMTP client TLS
settings are in their default state, you can enable <a
href="#client_tls_may">opportunistic</a> TLS in the SMTP client as
follows: </p>
<blockquote>
<pre>
# <a href="postfix-tls.1.html">postfix tls</a> enable-client
# postfix reload
</pre>
</blockquote>
<p> If some of the Postfix SMTP client TLS settings are not in their
default state, this will not make any changes, but will instead
suggest the minimal required settings for SMTP client TLS. The
"postfix reload" command is optional, it is only needed if you want
the settings to take effect right away. Note, this does not enable
trust in any public certification authorities, and does not configure
client TLS certificates as these are largely pointless with <a
href="#client_tls_may">opportunistic</a> TLS. </p>
<p> There is not yet a turn-key command for enabling <a
href="#client_tls_dane">DANE</a> authentication. This is because
DANE requires changes to your <b>resolv.conf</b> file and a
corresponding DNSSEC-validating resolver local to the Postfix host,
these changes are difficult to automate in a portable way. </p>
<p> If you're willing to revert your settings to the defaults and
switch to a "stock" opportunistic TLS configuration, then you can:
erase all the SMTP client TLS settings and then enable client TLS: </p>
<blockquote>
<pre>
# postconf -X `postconf -nH | egrep '^smtp(_|_enforce_|_use_)tls'`
# <a href="postfix-tls.1.html">postfix tls</a> enable-client
# postfix reload
</pre>
</blockquote>
<h4><a name="quick-server">Quick-start TLS in the Postfix ≥ 3.1 SMTP server</a>.</h4>
<p> If you are using Postfix 3.1 or later, and your SMTP server TLS
settings are in their default state, you can enable
opportunistic TLS in the SMTP server as follows: </p>
<blockquote>
<pre>
# <a href="postfix-tls.1.html">postfix tls</a> enable-server
# postfix reload
</pre>
</blockquote>
<p> If some of the Postfix SMTP client TLS settings are not in their
default state, this will not make any changes, but will instead
suggest the minimal required settings for SMTP client TLS. The
"postfix reload" command is optional, it is only needed if you want
the settings to take effect right away. This will generate a
self-signed private key and certificate and enable TLS in the Postfix
SMTP server. </p>
<p> If you're willing to revert your settings to the defaults and
switch to a "stock" server TLS configuration, then you can: erase
all the SMTP server TLS settings and then enable server TLS: </p>
<blockquote>
<pre>
# postconf -X `postconf -nH | egrep '^smtpd(_|_enforce_|_use_)tls'`
# <a href="postfix-tls.1.html">postfix tls</a> enable-server
# postfix reload
</pre>
</blockquote>
<p> Postfix ≥ 3.1 provides additional built-in support for ongoing
management of TLS in the SMTP server, via additional "<a href="postfix-tls.1.html">postfix tls</a>"
sub-commands. These make it easy to generate certificate signing
requests, create and deploy new keys and certificates, and generate
DANE TLSA records. See the <a href="postfix-tls.1.html">postfix-tls(1)</a> documentation for details.
</p>
<h3><a name="self-signed">Self-signed server certificate</a></h3>
<p> The following commands (credits: Viktor Dukhovni) generate and
install a 2048-bit RSA private key and 10-year self-signed certificate
for the local Postfix system. This requires super-user privileges.
(By using date-specific filenames for the certificate and key files,
and updating <a href="postconf.5.html">main.cf</a> with new filenames, a potential race condition
in which the key and certificate might not match is avoided).
</p>
<blockquote>
<pre>
# dir="$(postconf -h <a href="postconf.5.html#config_directory">config_directory</a>)"
# fqdn=$(postconf -h <a href="postconf.5.html#myhostname">myhostname</a>)
# case $fqdn in /*) fqdn=$(cat "$fqdn");; esac
# ymd=$(date +%Y-%m-%d)
# key="${dir}/key-${ymd}.pem"; rm -f "${key}"
# cert="${dir}/cert-${ymd}.pem"; rm -f "${cert}"
# (umask 077; openssl genrsa -out "${key}" 2048) &&
openssl req -new -key "${key}" \
-x509 -subj "/CN=${fqdn}" -days 3650 -out "${cert}" &&
postconf -e \
"<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = ${cert}" \
"<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = ${key}" \
'<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may' \
'<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes' \
'<a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 1' \
'<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may' \
'<a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 1' \
'<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> = <a href="DATABASE_README.html#types">btree</a>:${<a href="postconf.5.html#data_directory">data_directory</a>}/smtp_scache' \
'<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom'
</pre>
</blockquote>
<p> Note: the last command requires both single (') and double (")
quotes. </p>
<p> The <a href="postconf.1.html">postconf(1)</a> command above enables opportunistic TLS for
receiving and sending mail. It also enables logging of TLS connections
and recording of TLS use in the "Received" header. TLS session
caching is also enabled in the Postfix SMTP client. With Postfix
≥ 2.10, the SMTP server does not need an explicit session cache
since session reuse is better handled via <a href="http://tools.ietf.org/html/rfc5077">RFC 5077</a> TLS session
tickets. </p>
<h3><a name="private-ca">Private Certification Authority</a></h3>
<ul>
<li> <p> Become your own Certification Authority, so that you can
sign your own certificates, and so that your own systems can
authenticate certificates from your own CA. This example uses the
CA.pl script that ships with OpenSSL. On some systems, OpenSSL
installs this as <tt>/usr/local/openssl/misc/CA.pl</tt>. Some systems
install this as
part of a package named <tt>openssl-perl</tt> or something similar.
The script creates a private key in <tt>./demoCA/private/cakey.pem</tt>
and a public key in <tt>./demoCA/cacert.pem</tt>.</p>
<blockquote>
<pre>
% <b>/usr/local/ssl/misc/CA.pl -newca</b>
CA certificate filename (or enter to create)
Making CA certificate ...
Using configuration from /etc/ssl/openssl.cnf
Generating a 1024 bit RSA private key
....................++++++
.....++++++
writing new private key to './demoCA/private/cakey.pem'
Enter PEM pass phrase:<b>whatever</b>
</pre>
</blockquote>
<li> <p> Create an unpassworded private key for host foo.porcupine.org and create
an unsigned public key certificate. </p>
<blockquote>
<pre>
% <b>(umask 077; openssl req -new -newkey rsa:2048 -nodes -keyout foo-key.pem -out foo-req.pem)</b>
Using configuration from /etc/ssl/openssl.cnf
Generating a 2048 bit RSA private key
........................................++++++
....++++++
writing new private key to 'foo-key.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]:<b>US</b>
State or Province Name (full name) [Some-State]:<b>New York</b>
Locality Name (eg, city) []:<b>Westchester</b>
Organization Name (eg, company) [Internet Widgits Pty Ltd]:<b>Porcupine</b>
Organizational Unit Name (eg, section) []:
Common Name (eg, YOUR name) []:<b>foo.porcupine.org</b>
Email Address []:<b>wietse@porcupine.org</b>
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:<b>whatever</b>
An optional company name []:
</pre>
</blockquote>
<li> <p> Sign the public key certificate for host foo.porcupine.org with the
Certification Authority private key that we created a few
steps ago. </p>
<blockquote>
<pre>
% <b>openssl ca -out foo-cert.pem -days 365 -infiles foo-req.pem</b>
Using configuration from /etc/ssl/openssl.cnf
Enter PEM pass phrase:<b>whatever</b>
Check that the request matches the signature
Signature ok
The Subjects Distinguished Name is as follows
countryName :PRINTABLE:'US'
stateOrProvinceName :PRINTABLE:'New York'
localityName :PRINTABLE:'Westchester'
organizationName :PRINTABLE:'Porcupine'
commonName :PRINTABLE:'foo.porcupine.org'
emailAddress :IA5STRING:'wietse@porcupine.org'
Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365 days)
Sign the certificate? [y/n]:<b>y</b>
1 out of 1 certificate requests certified, commit? [y/n]<b>y</b>
Write out database with 1 new entries
Data Base Updated
</pre>
</blockquote>
<li> <p> Install the host private key, the host public key certificate,
and the Certification Authority certificate files. This requires
super-user privileges. </p>
<p> The following commands assume that the key and certificate will
be installed for the local Postfix MTA. You will need to adjust the
commands if the Postfix MTA is on a different host. </p>
<blockquote>
<pre>
# <b>cp demoCA/cacert.pem foo-key.pem foo-cert.pem /etc/postfix</b>
# <b>chmod 644 /etc/postfix/foo-cert.pem /etc/postfix/cacert.pem</b>
# <b>chmod 400 /etc/postfix/foo-key.pem</b>
</pre>
</blockquote>
<li> <p> Configure Postfix, by adding the following to
<tt>/etc/postfix/<a href="postconf.5.html">main.cf</a> </tt>. It is generally best to not configure
client certificates, unless there are servers which authenticate your mail
submission via client certificates. Often servers that perform TLS client
authentication will issue the required certificates signed by their own
CA. If you configure the client certificate and key incorrectly, you
will be unable to send mail to sites that request client certificate,
but don't require them from all clients. </p>
<blockquote>
<pre>
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#smtp_tls_CAfile">smtp_tls_CAfile</a> = /etc/postfix/cacert.pem
<a href="postconf.5.html#smtp_tls_session_cache_database">smtp_tls_session_cache_database</a> =
<a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtp_tls_session_cache
<a href="postconf.5.html#smtp_tls_security_level">smtp_tls_security_level</a> = may
<a href="postconf.5.html#smtp_tls_loglevel">smtp_tls_loglevel</a> = 1
<a href="postconf.5.html#smtpd_tls_CAfile">smtpd_tls_CAfile</a> = /etc/postfix/cacert.pem
<a href="postconf.5.html#smtpd_tls_cert_file">smtpd_tls_cert_file</a> = /etc/postfix/foo-cert.pem
<a href="postconf.5.html#smtpd_tls_key_file">smtpd_tls_key_file</a> = /etc/postfix/foo-key.pem
<a href="postconf.5.html#smtpd_tls_received_header">smtpd_tls_received_header</a> = yes
<a href="postconf.5.html#smtpd_tls_session_cache_database">smtpd_tls_session_cache_database</a> =
<a href="DATABASE_README.html#types">btree</a>:/var/lib/postfix/smtpd_tls_session_cache
<a href="postconf.5.html#tls_random_source">tls_random_source</a> = dev:/dev/urandom
<a href="postconf.5.html#smtpd_tls_security_level">smtpd_tls_security_level</a> = may
<a href="postconf.5.html#smtpd_tls_loglevel">smtpd_tls_loglevel</a> = 1
</pre>
</blockquote>
</ul>
<h2><a name="build_tls">Building Postfix with TLS support</a></h2>
<p> These instructions assume that you build Postfix from source
code as described in the <a href="INSTALL.html">INSTALL</a> document. Some modification may
be required if you build Postfix from a vendor-specific source
package. </p>
<p> To build Postfix with TLS support, first we need to generate
the <tt>make(1)</tt> files with the necessary definitions. This is
done by invoking the command "<tt>make makefiles</tt>" in the Postfix
top-level directory and with arguments as shown next. </p>
<p> <b> NOTE: Do not use Gnu TLS. It will spontaneously terminate
a Postfix daemon process with exit status code 2, instead of allowing
Postfix to 1) report the error to the maillog file, and to 2) provide
plaintext service where this is appropriate. </b> </p>
<ul>
<li> <p> If the OpenSSL include files (such as <tt>ssl.h</tt>) are
in directory <tt>/usr/include/openssl</tt>, and the OpenSSL libraries
(such as <tt>libssl.so</tt> and <tt>libcrypto.so</tt>) are in
directory <tt>/usr/lib</tt>: </p>
<blockquote>
<pre>
% <b>make tidy</b> # if you have left-over files from a previous build
% <b>make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"</b>
</pre>
</blockquote>
<li> <p> If the OpenSSL include files (such as <tt>ssl.h</tt>) are
in directory <tt>/usr/local/include/openssl</tt>, and the OpenSSL
libraries (such as <tt>libssl.so</tt> and <tt>libcrypto.so</tt>)
are in directory <tt>/usr/local/lib</tt>: </p>
<blockquote>
<pre>
% <b>make tidy</b> # if you have left-over files from a previous build
% <b>make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-L/usr/local/lib -lssl -lcrypto" </b>
</pre>
</blockquote>
<p> On Solaris, specify the <tt>-R</tt> option as shown below:
<blockquote>
<pre>
% <b>make tidy</b> # if you have left-over files from a previous build
% <b>make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto" </b>
</pre>
</blockquote>
</ul>
<p> If you need to apply other customizations (such as Berkeley DB
databases, MySQL, PostgreSQL, LDAP or SASL), see the respective
Postfix README documents, and combine their "<tt>make makefiles</tt>"
instructions with the instructions above: </p>
<blockquote>
<pre>
% <b>make tidy</b> # if you have left-over files from a previous build
% <b>make makefiles CCARGS="-DUSE_TLS \
<i>(other -D or -I options)</i>" \
AUXLIBS="-lssl -lcrypto \
<i>(other -l options for libraries in /usr/lib)</i> \
<i>(-L/path/name + -l options for other libraries)</i>"</b>
</pre>
</blockquote>
<p> To complete the build process, see the Postfix <a href="INSTALL.html">INSTALL</a>
instructions. Postfix has TLS support turned off by default, so
you can start using Postfix as soon as it is installed. </p>
<h2> <a name="problems"> Reporting problems </a> </h2>
<p> Problems are preferably reported via <postfix-users@postfix.org>.
See <a href="http://www.postfix.org/lists.html">http://www.postfix.org/lists.html</a> for subscription information.
When reporting a problem, please be thorough in the report. Patches,
when possible, are greatly appreciated too. </p>
<h2><a name="credits">Credits </a> </h2>
<ul>
<li> TLS support for Postfix was originally developed by Lutz
Jänicke at Cottbus Technical University.
<li> Wietse Venema adopted the code, did some restructuring, and
compiled this part of the documentation from Lutz's documents.
<li> Victor Duchovni was instrumental with the re-implementation
of the <a href="postconf.5.html#smtp_tls_per_site">smtp_tls_per_site</a> code in terms of enforcement levels, which
simplified the implementation greatly.
<li> Victor Duchovni implemented the fingerprint security level,
added more sanity checks, and separated TLS connection management
from security policy enforcement. The latter change simplified the
code that verifies certificate signatures, certificate names, and
certificate fingerprints.
</ul>
</body>
</html>
|
