Linux preempt-rt

Check our new training course

Real-Time Linux with PREEMPT_RT

Check our new training course
with Creative Commons CC-BY-SA
lecture and lab materials

Bootlin logo

Elixir Cross Referencer

   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
.. SPDX-License-Identifier: GPL-2.0

=========
IP Sysctl
=========

/proc/sys/net/ipv4/* Variables
==============================

ip_forward - BOOLEAN
	- 0 - disabled (default)
	- not 0 - enabled

	Forward Packets between interfaces.

	This variable is special, its change resets all configuration
	parameters to their default state (RFC1122 for hosts, RFC1812
	for routers)

ip_default_ttl - INTEGER
	Default value of TTL field (Time To Live) for outgoing (but not
	forwarded) IP packets. Should be between 1 and 255 inclusive.
	Default: 64 (as recommended by RFC1700)

ip_no_pmtu_disc - INTEGER
	Disable Path MTU Discovery. If enabled in mode 1 and a
	fragmentation-required ICMP is received, the PMTU to this
	destination will be set to the smallest of the old MTU to
	this destination and min_pmtu (see below). You will need
	to raise min_pmtu to the smallest interface MTU on your system
	manually if you want to avoid locally generated fragments.

	In mode 2 incoming Path MTU Discovery messages will be
	discarded. Outgoing frames are handled the same as in mode 1,
	implicitly setting IP_PMTUDISC_DONT on every created socket.

	Mode 3 is a hardened pmtu discover mode. The kernel will only
	accept fragmentation-needed errors if the underlying protocol
	can verify them besides a plain socket lookup. Current
	protocols for which pmtu events will be honored are TCP, SCTP
	and DCCP as they verify e.g. the sequence number or the
	association. This mode should not be enabled globally but is
	only intended to secure e.g. name servers in namespaces where
	TCP path mtu must still work but path MTU information of other
	protocols should be discarded. If enabled globally this mode
	could break other protocols.

	Possible values: 0-3

	Default: FALSE

min_pmtu - INTEGER
	default 552 - minimum Path MTU. Unless this is changed mannually,
	each cached pmtu will never be lower than this setting.

ip_forward_use_pmtu - BOOLEAN
	By default we don't trust protocol path MTUs while forwarding
	because they could be easily forged and can lead to unwanted
	fragmentation by the router.
	You only need to enable this if you have user-space software
	which tries to discover path mtus by itself and depends on the
	kernel honoring this information. This is normally not the
	case.

	Default: 0 (disabled)

	Possible values:

	- 0 - disabled
	- 1 - enabled

fwmark_reflect - BOOLEAN
	Controls the fwmark of kernel-generated IPv4 reply packets that are not
	associated with a socket for example, TCP RSTs or ICMP echo replies).
	If unset, these packets have a fwmark of zero. If set, they have the
	fwmark of the packet they are replying to.

	Default: 0

fib_multipath_use_neigh - BOOLEAN
	Use status of existing neighbor entry when determining nexthop for
	multipath routes. If disabled, neighbor information is not used and
	packets could be directed to a failed nexthop. Only valid for kernels
	built with CONFIG_IP_ROUTE_MULTIPATH enabled.

	Default: 0 (disabled)

	Possible values:

	- 0 - disabled
	- 1 - enabled

fib_multipath_hash_policy - INTEGER
	Controls which hash policy to use for multipath routes. Only valid
	for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

	Default: 0 (Layer 3)

	Possible values:

	- 0 - Layer 3
	- 1 - Layer 4
	- 2 - Layer 3 or inner Layer 3 if present
	- 3 - Custom multipath hash. Fields used for multipath hash calculation
	  are determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER
	When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
	fields used for multipath hash calculation are determined by this
	sysctl.

	This value is a bitmask which enables various fields for multipath hash
	calculation.

	Possible fields are:

	====== ============================
	0x0001 Source IP address
	0x0002 Destination IP address
	0x0004 IP protocol
	0x0008 Unused (Flow Label)
	0x0010 Source port
	0x0020 Destination port
	0x0040 Inner source IP address
	0x0080 Inner destination IP address
	0x0100 Inner IP protocol
	0x0200 Inner Flow Label
	0x0400 Inner source port
	0x0800 Inner destination port
	====== ============================

	Default: 0x0007 (source IP, destination IP and IP protocol)

fib_sync_mem - UNSIGNED INTEGER
	Amount of dirty memory from fib entries that can be backlogged before
	synchronize_rcu is forced.

	Default: 512kB   Minimum: 64kB   Maximum: 64MB

ip_forward_update_priority - INTEGER
	Whether to update SKB priority from "TOS" field in IPv4 header after it
	is forwarded. The new SKB priority is mapped from TOS field value
	according to an rt_tos2priority table (see e.g. man tc-prio).

	Default: 1 (Update priority.)

	Possible values:

	- 0 - Do not update priority.
	- 1 - Update priority.

route/max_size - INTEGER
	Maximum number of routes allowed in the kernel.  Increase
	this when using large numbers of interfaces and/or routes.

	From linux kernel 3.6 onwards, this is deprecated for ipv4
	as route cache is no longer used.

neigh/default/gc_thresh1 - INTEGER
	Minimum number of entries to keep.  Garbage collector will not
	purge entries if there are fewer than this number.

	Default: 128

neigh/default/gc_thresh2 - INTEGER
	Threshold when garbage collector becomes more aggressive about
	purging entries. Entries older than 5 seconds will be cleared
	when over this number.

	Default: 512

neigh/default/gc_thresh3 - INTEGER
	Maximum number of non-PERMANENT neighbor entries allowed.  Increase
	this when using large numbers of interfaces and when communicating
	with large numbers of directly-connected peers.

	Default: 1024

neigh/default/unres_qlen_bytes - INTEGER
	The maximum number of bytes which may be used by packets
	queued for each	unresolved address by other network layers.
	(added in linux 3.3)

	Setting negative value is meaningless and will return error.

	Default: SK_WMEM_MAX, (same as net.core.wmem_default).

		Exact value depends on architecture and kernel options,
		but should be enough to allow queuing 256 packets
		of medium size.

neigh/default/unres_qlen - INTEGER
	The maximum number of packets which may be queued for each
	unresolved address by other network layers.

	(deprecated in linux 3.3) : use unres_qlen_bytes instead.

	Prior to linux 3.3, the default value is 3 which may cause
	unexpected packet loss. The current default value is calculated
	according to default value of unres_qlen_bytes and true size of
	packet.

	Default: 101

mtu_expires - INTEGER
	Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER
	The advertised MSS depends on the first hop route MTU, but will
	never be lower than this setting.

fib_notify_on_flag_change - INTEGER
        Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
        RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

        After installing a route to the kernel, user space receives an
        acknowledgment, which means the route was installed in the kernel,
        but not necessarily in hardware.
        It is also possible for a route already installed in hardware to change
        its action and therefore its flags. For example, a host route that is
        trapping packets can be "promoted" to perform decapsulation following
        the installation of an IPinIP/VXLAN tunnel.
        The notifications will indicate to user-space the state of the route.

        Default: 0 (Do not emit notifications.)

        Possible values:

        - 0 - Do not emit notifications.
        - 1 - Emit notifications.
        - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

IP Fragmentation:

ipfrag_high_thresh - LONG INTEGER
	Maximum memory used to reassemble IP fragments.

ipfrag_low_thresh - LONG INTEGER
	(Obsolete since linux-4.17)
	Maximum memory used to reassemble IP fragments before the kernel
	begins to remove incomplete fragment queues to free up resources.
	The kernel still accepts new fragments for defragmentation.

ipfrag_time - INTEGER
	Time in seconds to keep an IP fragment in memory.

ipfrag_max_dist - INTEGER
	ipfrag_max_dist is a non-negative integer value which defines the
	maximum "disorder" which is allowed among fragments which share a
	common IP source address. Note that reordering of packets is
	not unusual, but if a large number of fragments arrive from a source
	IP address while a particular fragment queue remains incomplete, it
	probably indicates that one or more fragments belonging to that queue
	have been lost. When ipfrag_max_dist is positive, an additional check
	is done on fragments before they are added to a reassembly queue - if
	ipfrag_max_dist (or more) fragments have arrived from a particular IP
	address between additions to any IP fragment queue using that source
	address, it's presumed that one or more fragments in the queue are
	lost. The existing fragment queue will be dropped, and a new one
	started. An ipfrag_max_dist value of zero disables this check.

	Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
	result in unnecessarily dropping fragment queues when normal
	reordering of packets occurs, which could lead to poor application
	performance. Using a very large value, e.g. 50000, increases the
	likelihood of incorrectly reassembling IP fragments that originate
	from different IP datagrams, which could result in data corruption.
	Default: 64

bc_forwarding - INTEGER
	bc_forwarding enables the feature described in rfc1812#section-5.3.5.2
	and rfc2644. It allows the router to forward directed broadcast.
	To enable this feature, the 'all' entry and the input interface entry
	should be set to 1.
	Default: 0

INET peer storage
=================

inet_peer_threshold - INTEGER
	The approximate size of the storage.  Starting from this threshold
	entries will be thrown aggressively.  This threshold also determines
	entries' time-to-live and time intervals between garbage collection
	passes.  More entries, less time-to-live, less GC interval.

inet_peer_minttl - INTEGER
	Minimum time-to-live of entries.  Should be enough to cover fragment
	time-to-live on the reassembling side.  This minimum time-to-live  is
	guaranteed if the pool size is less than inet_peer_threshold.
	Measured in seconds.

inet_peer_maxttl - INTEGER
	Maximum time-to-live of entries.  Unused entries will expire after
	this period of time if there is no memory pressure on the pool (i.e.
	when the number of entries in the pool is very small).
	Measured in seconds.

TCP variables
=============

somaxconn - INTEGER
	Limit of socket listen() backlog, known in userspace as SOMAXCONN.
	Defaults to 4096. (Was 128 before linux-5.4)
	See also tcp_max_syn_backlog for additional tuning for TCP sockets.

tcp_abort_on_overflow - BOOLEAN
	If listening service is too slow to accept new connections,
	reset them. Default state is FALSE. It means that if overflow
	occurred due to a burst, connection will recover. Enable this
	option _only_ if you are really sure that listening daemon
	cannot be tuned to accept connections faster. Enabling this
	option can harm clients of your server.

tcp_adv_win_scale - INTEGER
	Count buffering overhead as bytes/2^tcp_adv_win_scale
	(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
	if it is <= 0.

	Possible values are [-31, 31], inclusive.

	Default: 1

tcp_allowed_congestion_control - STRING
	Show/set the congestion control choices available to non-privileged
	processes. The list is a subset of those listed in
	tcp_available_congestion_control.

	Default is "reno" and the default setting (tcp_congestion_control).

tcp_app_win - INTEGER
	Reserve max(window/2^tcp_app_win, mss) of window for application
	buffer. Value 0 is special, it means that nothing is reserved.

	Default: 31

tcp_autocorking - BOOLEAN
	Enable TCP auto corking :
	When applications do consecutive small write()/sendmsg() system calls,
	we try to coalesce these small writes as much as possible, to lower
	total amount of sent packets. This is done if at least one prior
	packet for the flow is waiting in Qdisc queues or device transmit
	queue. Applications can still use TCP_CORK for optimal behavior
	when they know how/when to uncork their sockets.

	Default : 1

tcp_available_congestion_control - STRING
	Shows the available congestion control choices that are registered.
	More congestion control algorithms may be available as modules,
	but not loaded.

tcp_base_mss - INTEGER
	The initial value of search_low to be used by the packetization layer
	Path MTU discovery (MTU probing).  If MTU probing is enabled,
	this is the initial MSS used by the connection.

tcp_mtu_probe_floor - INTEGER
	If MTU probing is enabled this caps the minimum MSS used for search_low
	for the connection.

	Default : 48

tcp_min_snd_mss - INTEGER
	TCP SYN and SYNACK messages usually advertise an ADVMSS option,
	as described in RFC 1122 and RFC 6691.

	If this ADVMSS option is smaller than tcp_min_snd_mss,
	it is silently capped to tcp_min_snd_mss.

	Default : 48 (at least 8 bytes of payload per segment)

tcp_congestion_control - STRING
	Set the congestion control algorithm to be used for new
	connections. The algorithm "reno" is always available, but
	additional choices may be available based on kernel configuration.
	Default is set as part of kernel configuration.
	For passive connections, the listener congestion control choice
	is inherited.

	[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]

tcp_dsack - BOOLEAN
	Allows TCP to send "duplicate" SACKs.

tcp_early_retrans - INTEGER
	Tail loss probe (TLP) converts RTOs occurring due to tail
	losses into fast recovery (draft-ietf-tcpm-rack). Note that
	TLP requires RACK to function properly (see tcp_recovery below)

	Possible values:

		- 0 disables TLP
		- 3 or 4 enables TLP

	Default: 3

tcp_ecn - INTEGER
	Control use of Explicit Congestion Notification (ECN) by TCP.
	ECN is used only when both ends of the TCP connection indicate
	support for it.  This feature is useful in avoiding losses due
	to congestion by allowing supporting routers to signal
	congestion before having to drop packets.

	Possible values are:

		=  =====================================================
		0  Disable ECN.  Neither initiate nor accept ECN.
		1  Enable ECN when requested by incoming connections and
		   also request ECN on outgoing connection attempts.
		2  Enable ECN when requested by incoming connections
		   but do not request ECN on outgoing connections.
		=  =====================================================

	Default: 2

tcp_ecn_fallback - BOOLEAN
	If the kernel detects that ECN connection misbehaves, enable fall
	back to non-ECN. Currently, this knob implements the fallback
	from RFC3168, section 6.1.1.1., but we reserve that in future,
	additional detection mechanisms could be implemented under this
	knob. The value	is not used, if tcp_ecn or per route (or congestion
	control) ECN settings are disabled.

	Default: 1 (fallback enabled)

tcp_fack - BOOLEAN
	This is a legacy option, it has no effect anymore.

tcp_fin_timeout - INTEGER
	The length of time an orphaned (no longer referenced by any
	application) connection will remain in the FIN_WAIT_2 state
	before it is aborted at the local end.  While a perfectly
	valid "receive only" state for an un-orphaned connection, an
	orphaned connection in FIN_WAIT_2 state could otherwise wait
	forever for the remote to close its end of the connection.

	Cf. tcp_max_orphans

	Default: 60 seconds

tcp_frto - INTEGER
	Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
	F-RTO is an enhanced recovery algorithm for TCP retransmission
	timeouts.  It is particularly beneficial in networks where the
	RTT fluctuates (e.g., wireless). F-RTO is sender-side only
	modification. It does not require any support from the peer.

	By default it's enabled with a non-zero value. 0 disables F-RTO.

tcp_fwmark_accept - BOOLEAN
	If set, incoming connections to listening sockets that do not have a
	socket mark will set the mark of the accepting socket to the fwmark of
	the incoming SYN packet. This will cause all packets on that connection
	(starting from the first SYNACK) to be sent with that fwmark. The
	listening socket's mark is unchanged. Listening sockets that already
	have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
	unaffected.

	Default: 0

tcp_invalid_ratelimit - INTEGER
	Limit the maximal rate for sending duplicate acknowledgments
	in response to incoming TCP packets that are for an existing
	connection but that are invalid due to any of these reasons:

	  (a) out-of-window sequence number,
	  (b) out-of-window acknowledgment number, or
	  (c) PAWS (Protection Against Wrapped Sequence numbers) check failure

	This can help mitigate simple "ack loop" DoS attacks, wherein
	a buggy or malicious middlebox or man-in-the-middle can
	rewrite TCP header fields in manner that causes each endpoint
	to think that the other is sending invalid TCP segments, thus
	causing each side to send an unterminating stream of duplicate
	acknowledgments for invalid segments.

	Using 0 disables rate-limiting of dupacks in response to
	invalid segments; otherwise this value specifies the minimal
	space between sending such dupacks, in milliseconds.

	Default: 500 (milliseconds).

tcp_keepalive_time - INTEGER
	How often TCP sends out keepalive messages when keepalive is enabled.
	Default: 2hours.

tcp_keepalive_probes - INTEGER
	How many keepalive probes TCP sends out, until it decides that the
	connection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER
	How frequently the probes are send out. Multiplied by
	tcp_keepalive_probes it is time to kill not responding connection,
	after probes started. Default value: 75sec i.e. connection
	will be aborted after ~11 minutes of retries.

tcp_l3mdev_accept - BOOLEAN
	Enables child sockets to inherit the L3 master device index.
	Enabling this option allows a "global" listen socket to work
	across L3 master domains (e.g., VRFs) with connected sockets
	derived from the listen socket to be bound to the L3 domain in
	which the packets originated. Only valid when the kernel was
	compiled with CONFIG_NET_L3_MASTER_DEV.

	Default: 0 (disabled)

tcp_low_latency - BOOLEAN
	This is a legacy option, it has no effect anymore.

tcp_max_orphans - INTEGER
	Maximal number of TCP sockets not attached to any user file handle,
	held by system.	If this number is exceeded orphaned connections are
	reset immediately and warning is printed. This limit exists
	only to prevent simple DoS attacks, you _must_ not rely on this
	or lower the limit artificially, but rather increase it
	(probably, after increasing installed memory),
	if network conditions require more than default value,
	and tune network services to linger and kill such states
	more aggressively. Let me to remind again: each orphan eats
	up to ~64K of unswappable memory.

tcp_max_syn_backlog - INTEGER
	Maximal number of remembered connection requests (SYN_RECV),
	which have not received an acknowledgment from connecting client.

	This is a per-listener limit.

	The minimal value is 128 for low memory machines, and it will
	increase in proportion to the memory of machine.

	If server suffers from overload, try increasing this number.

	Remember to also check /proc/sys/net/core/somaxconn
	A SYN_RECV request socket consumes about 304 bytes of memory.

tcp_max_tw_buckets - INTEGER
	Maximal number of timewait sockets held by system simultaneously.
	If this number is exceeded time-wait socket is immediately destroyed
	and warning is printed. This limit exists only to prevent
	simple DoS attacks, you _must_ not lower the limit artificially,
	but rather increase it (probably, after increasing installed memory),
	if network conditions require more than default value.

tcp_mem - vector of 3 INTEGERs: min, pressure, max
	min: below this number of pages TCP is not bothered about its
	memory appetite.

	pressure: when amount of memory allocated by TCP exceeds this number
	of pages, TCP moderates its memory consumption and enters memory
	pressure mode, which is exited when memory consumption falls
	under "min".

	max: number of pages allowed for queueing by all TCP sockets.

	Defaults are calculated at boot time from amount of available
	memory.

tcp_min_rtt_wlen - INTEGER
	The window length of the windowed min filter to track the minimum RTT.
	A shorter window lets a flow more quickly pick up new (higher)
	minimum RTT when it is moved to a longer path (e.g., due to traffic
	engineering). A longer window makes the filter more resistant to RTT
	inflations such as transient congestion. The unit is seconds.

	Possible values: 0 - 86400 (1 day)

	Default: 300

tcp_moderate_rcvbuf - BOOLEAN
	If set, TCP performs receive buffer auto-tuning, attempting to
	automatically size the buffer (no greater than tcp_rmem[2]) to
	match the size required by the path for full throughput.  Enabled by
	default.

tcp_mtu_probing - INTEGER
	Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
	values:

	- 0 - Disabled
	- 1 - Disabled by default, enabled when an ICMP black hole detected
	- 2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_probe_interval - UNSIGNED INTEGER
	Controls how often to start TCP Packetization-Layer Path MTU
	Discovery reprobe. The default is reprobing every 10 minutes as
	per RFC4821.

tcp_probe_threshold - INTEGER
	Controls when TCP Packetization-Layer Path MTU Discovery probing
	will stop in respect to the width of search range in bytes. Default
	is 8 bytes.

tcp_no_metrics_save - BOOLEAN
	By default, TCP saves various connection metrics in the route cache
	when the connection closes, so that connections established in the
	near future can use these to set initial conditions.  Usually, this
	increases overall performance, but may sometimes cause performance
	degradation.  If set, TCP will not cache metrics on closing
	connections.

tcp_no_ssthresh_metrics_save - BOOLEAN
	Controls whether TCP saves ssthresh metrics in the route cache.

	Default is 1, which disables ssthresh metrics.

tcp_orphan_retries - INTEGER
	This value influences the timeout of a locally closed TCP connection,
	when RTO retransmissions remain unacknowledged.
	See tcp_retries2 for more details.

	The default value is 8.

	If your machine is a loaded WEB server,
	you should think about lowering this value, such sockets
	may consume significant resources. Cf. tcp_max_orphans.

tcp_recovery - INTEGER
	This value is a bitmap to enable various experimental loss recovery
	features.

	=========   =============================================================
	RACK: 0x1   enables the RACK loss detection for fast detection of lost
		    retransmissions and tail drops. It also subsumes and disables
		    RFC6675 recovery for SACK connections.

	RACK: 0x2   makes RACK's reordering window static (min_rtt/4).

	RACK: 0x4   disables RACK's DUPACK threshold heuristic
	=========   =============================================================

	Default: 0x1

tcp_reordering - INTEGER
	Initial reordering level of packets in a TCP stream.
	TCP stack can then dynamically adjust flow reordering level
	between this initial value and tcp_max_reordering

	Default: 3

tcp_max_reordering - INTEGER
	Maximal reordering level of packets in a TCP stream.
	300 is a fairly conservative value, but you might increase it
	if paths are using per packet load balancing (like bonding rr mode)

	Default: 300

tcp_retrans_collapse - BOOLEAN
	Bug-to-bug compatibility with some broken printers.
	On retransmit try to send bigger packets to work around bugs in
	certain TCP stacks.

tcp_retries1 - INTEGER
	This value influences the time, after which TCP decides, that
	something is wrong due to unacknowledged RTO retransmissions,
	and reports this suspicion to the network layer.
	See tcp_retries2 for more details.

	RFC 1122 recommends at least 3 retransmissions, which is the
	default.

tcp_retries2 - INTEGER
	This value influences the timeout of an alive TCP connection,
	when RTO retransmissions remain unacknowledged.
	Given a value of N, a hypothetical TCP connection following
	exponential backoff with an initial RTO of TCP_RTO_MIN would
	retransmit N times before killing the connection at the (N+1)th RTO.

	The default value of 15 yields a hypothetical timeout of 924.6
	seconds and is a lower bound for the effective timeout.
	TCP will effectively time out at the first RTO which exceeds the
	hypothetical timeout.

	RFC 1122 recommends at least 100 seconds for the timeout,
	which corresponds to a value of at least 8.

tcp_rfc1337 - BOOLEAN
	If set, the TCP stack behaves conforming to RFC1337. If unset,
	we are not conforming to RFC, but prevent TCP TIME_WAIT
	assassination.

	Default: 0

tcp_rmem - vector of 3 INTEGERs: min, default, max
	min: Minimal size of receive buffer used by TCP sockets.
	It is guaranteed to each TCP socket, even under moderate memory
	pressure.

	Default: 4K

	default: initial size of receive buffer used by TCP sockets.
	This value overrides net.core.rmem_default used by other protocols.
	Default: 131072 bytes.
	This value results in initial window of 65535.

	max: maximal size of receive buffer allowed for automatically
	selected receiver buffers for TCP socket. This value does not override
	net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
	automatic tuning of that socket's receive buffer size, in which
	case this value is ignored.
	Default: between 131072 and 6MB, depending on RAM size.

tcp_sack - BOOLEAN
	Enable select acknowledgments (SACKS).

tcp_comp_sack_delay_ns - LONG INTEGER
	TCP tries to reduce number of SACK sent, using a timer
	based on 5% of SRTT, capped by this sysctl, in nano seconds.
	The default is 1ms, based on TSO autosizing period.

	Default : 1,000,000 ns (1 ms)

tcp_comp_sack_slack_ns - LONG INTEGER
	This sysctl control the slack used when arming the
	timer used by SACK compression. This gives extra time
	for small RTT flows, and reduces system overhead by allowing
	opportunistic reduction of timer interrupts.

	Default : 100,000 ns (100 us)

tcp_comp_sack_nr - INTEGER
	Max number of SACK that can be compressed.
	Using 0 disables SACK compression.

	Default : 44

tcp_slow_start_after_idle - BOOLEAN
	If set, provide RFC2861 behavior and time out the congestion
	window after an idle period.  An idle period is defined at
	the current RTO.  If unset, the congestion window will not
	be timed out after an idle period.

	Default: 1

tcp_stdurg - BOOLEAN
	Use the Host requirements interpretation of the TCP urgent pointer field.
	Most hosts use the older BSD interpretation, so if you turn this on
	Linux might not communicate correctly with them.

	Default: FALSE

tcp_synack_retries - INTEGER
	Number of times SYNACKs for a passive TCP connection attempt will
	be retransmitted. Should not be higher than 255. Default value
	is 5, which corresponds to 31seconds till the last retransmission
	with the current initial RTO of 1second. With this the final timeout
	for a passive TCP connection will happen after 63seconds.

tcp_syncookies - INTEGER
	Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
	Send out syncookies when the syn backlog queue of a socket
	overflows. This is to prevent against the common 'SYN flood attack'
	Default: 1

	Note, that syncookies is fallback facility.
	It MUST NOT be used to help highly loaded servers to stand
	against legal connection rate. If you see SYN flood warnings
	in your logs, but investigation	shows that they occur
	because of overload with legal connections, you should tune
	another parameters until this warning disappear.
	See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

	syncookies seriously violate TCP protocol, do not allow
	to use TCP extensions, can result in serious degradation
	of some services (f.e. SMTP relaying), visible not by you,
	but your clients and relays, contacting you. While you see
	SYN flood warnings in logs not being really flooded, your server
	is seriously misconfigured.

	If you want to test which effects syncookies have to your
	network connections you can set this knob to 2 to enable
	unconditionally generation of syncookies.

tcp_migrate_req - BOOLEAN
	The incoming connection is tied to a specific listening socket when
	the initial SYN packet is received during the three-way handshake.
	When a listener is closed, in-flight request sockets during the
	handshake and established sockets in the accept queue are aborted.

	If the listener has SO_REUSEPORT enabled, other listeners on the
	same port should have been able to accept such connections. This
	option makes it possible to migrate such child sockets to another
	listener after close() or shutdown().

	The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
	usually be used to define the policy to pick an alive listener.
	Otherwise, the kernel will randomly pick an alive listener only if
	this option is enabled.

	Note that migration between listeners with different settings may
	crash applications. Let's say migration happens from listener A to
	B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
	the requests migrated from A. To avoid such a situation, cancel
	migration by returning SK_DROP in the type of eBPF program, or
	disable this option.

	Default: 0

tcp_fastopen - INTEGER
	Enable TCP Fast Open (RFC7413) to send and accept data in the opening
	SYN packet.

	The client support is enabled by flag 0x1 (on by default). The client
	then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
	rather than connect() to send data in SYN.

	The server support is enabled by flag 0x2 (off by default). Then
	either enable for all listeners with another flag (0x400) or
	enable individual listeners via TCP_FASTOPEN socket option with
	the option value being the length of the syn-data backlog.

	The values (bitmap) are

	=====  ======== ======================================================
	  0x1  (client) enables sending data in the opening SYN on the client.
	  0x2  (server) enables the server support, i.e., allowing data in
			a SYN packet to be accepted and passed to the
			application before 3-way handshake finishes.
	  0x4  (client) send data in the opening SYN regardless of cookie
			availability and without a cookie option.
	0x200  (server) accept data-in-SYN w/o any cookie option present.
	0x400  (server) enable all listeners to support Fast Open by
			default without explicit TCP_FASTOPEN socket option.
	=====  ======== ======================================================

	Default: 0x1

	Note that additional client or server features are only
	effective if the basic support (0x1 and 0x2) are enabled respectively.

tcp_fastopen_blackhole_timeout_sec - INTEGER
	Initial time period in second to disable Fastopen on active TCP sockets
	when a TFO firewall blackhole issue happens.
	This time period will grow exponentially when more blackhole issues
	get detected right after Fastopen is re-enabled and will reset to
	initial value when the blackhole issue goes away.
	0 to disable the blackhole detection.

	By default, it is set to 0 (feature is disabled).

tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
	The list consists of a primary key and an optional backup key. The
	primary key is used for both creating and validating cookies, while the
	optional backup key is only used for validating cookies. The purpose of
	the backup key is to maximize TFO validation when keys are rotated.

	A randomly chosen primary key may be configured by the kernel if
	the tcp_fastopen sysctl is set to 0x400 (see above), or if the
	TCP_FASTOPEN setsockopt() optname is set and a key has not been
	previously configured via sysctl. If keys are configured via
	setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
	per-socket keys will be used instead of any keys that are specified via
	sysctl.

	A key is specified as 4 8-digit hexadecimal integers which are separated
	by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
	omitted. A primary and a backup key may be specified by separating them
	by a comma. If only one key is specified, it becomes the primary key and
	any previously configured backup keys are removed.

tcp_syn_retries - INTEGER
	Number of times initial SYNs for an active TCP connection attempt
	will be retransmitted. Should not be higher than 127. Default value
	is 6, which corresponds to 63seconds till the last retransmission
	with the current initial RTO of 1second. With this the final timeout
	for an active TCP connection attempt will happen after 127seconds.

tcp_timestamps - INTEGER
	Enable timestamps as defined in RFC1323.

	- 0: Disabled.
	- 1: Enable timestamps as defined in RFC1323 and use random offset for
	  each connection rather than only using the current time.
	- 2: Like 1, but without random offsets.

	Default: 1

tcp_min_tso_segs - INTEGER
	Minimal number of segments per TSO frame.

	Since linux-3.12, TCP does an automatic sizing of TSO frames,
	depending on flow rate, instead of filling 64Kbytes packets.
	For specific usages, it's possible to force TCP to build big
	TSO frames. Note that TCP stack might split too big TSO packets
	if available window is too small.

	Default: 2

tcp_tso_rtt_log - INTEGER
	Adjustment of TSO packet sizes based on min_rtt

	Starting from linux-5.18, TCP autosizing can be tweaked
	for flows having small RTT.

	Old autosizing was splitting the pacing budget to send 1024 TSO
	per second.

	tso_packet_size = sk->sk_pacing_rate / 1024;

	With the new mechanism, we increase this TSO sizing using:

	distance = min_rtt_usec / (2^tcp_tso_rtt_log)
	tso_packet_size += gso_max_size >> distance;

	This means that flows between very close hosts can use bigger
	TSO packets, reducing their cpu costs.

	If you want to use the old autosizing, set this sysctl to 0.

	Default: 9  (2^9 = 512 usec)

tcp_pacing_ss_ratio - INTEGER
	sk->sk_pacing_rate is set by TCP stack using a ratio applied
	to current rate. (current_rate = cwnd * mss / srtt)
	If TCP is in slow start, tcp_pacing_ss_ratio is applied
	to let TCP probe for bigger speeds, assuming cwnd can be
	doubled every other RTT.

	Default: 200

tcp_pacing_ca_ratio - INTEGER
	sk->sk_pacing_rate is set by TCP stack using a ratio applied
	to current rate. (current_rate = cwnd * mss / srtt)
	If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
	is applied to conservatively probe for bigger throughput.

	Default: 120

tcp_tso_win_divisor - INTEGER
	This allows control over what percentage of the congestion window
	can be consumed by a single TSO frame.
	The setting of this parameter is a choice between burstiness and
	building larger TSO frames.

	Default: 3

tcp_tw_reuse - INTEGER
	Enable reuse of TIME-WAIT sockets for new connections when it is
	safe from protocol viewpoint.

	- 0 - disable
	- 1 - global enable
	- 2 - enable for loopback traffic only

	It should not be changed without advice/request of technical
	experts.

	Default: 2

tcp_window_scaling - BOOLEAN
	Enable window scaling as defined in RFC1323.

tcp_wmem - vector of 3 INTEGERs: min, default, max
	min: Amount of memory reserved for send buffers for TCP sockets.
	Each TCP socket has rights to use it due to fact of its birth.

	Default: 4K

	default: initial size of send buffer used by TCP sockets.  This
	value overrides net.core.wmem_default used by other protocols.

	It is usually lower than net.core.wmem_default.

	Default: 16K

	max: Maximal amount of memory allowed for automatically tuned
	send buffers for TCP sockets. This value does not override
	net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
	automatic tuning of that socket's send buffer size, in which case
	this value is ignored.

	Default: between 64K and 4MB, depending on RAM size.

tcp_notsent_lowat - UNSIGNED INTEGER
	A TCP socket can control the amount of unsent bytes in its write queue,
	thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
	reports POLLOUT events if the amount of unsent bytes is below a per
	socket value, and if the write queue is not full. sendmsg() will
	also not add new buffers if the limit is hit.

	This global variable controls the amount of unsent data for
	sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
	to the global variable has immediate effect.

	Default: UINT_MAX (0xFFFFFFFF)

tcp_workaround_signed_windows - BOOLEAN
	If set, assume no receipt of a window scaling option means the
	remote TCP is broken and treats the window as a signed quantity.
	If unset, assume the remote TCP is not broken even if we do
	not receive a window scaling option from them.

	Default: 0

tcp_thin_linear_timeouts - BOOLEAN
	Enable dynamic triggering of linear timeouts for thin streams.
	If set, a check is performed upon retransmission by timeout to
	determine if the stream is thin (less than 4 packets in flight).
	As long as the stream is found to be thin, up to 6 linear
	timeouts may be performed before exponential backoff mode is
	initiated. This improves retransmission latency for
	non-aggressive thin streams, often found to be time-dependent.
	For more information on thin streams, see
	Documentation/networking/tcp-thin.rst

	Default: 0

tcp_limit_output_bytes - INTEGER
	Controls TCP Small Queue limit per tcp socket.
	TCP bulk sender tends to increase packets in flight until it
	gets losses notifications. With SNDBUF autotuning, this can
	result in a large amount of packets queued on the local machine
	(e.g.: qdiscs, CPU backlog, or device) hurting latency of other
	flows, for typical pfifo_fast qdiscs.  tcp_limit_output_bytes
	limits the number of bytes on qdisc or device to reduce artificial
	RTT/cwnd and reduce bufferbloat.

	Default: 1048576 (16 * 65536)

tcp_challenge_ack_limit - INTEGER
	Limits number of Challenge ACK sent per second, as recommended
	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
	Default: 1000

UDP variables
=============

udp_l3mdev_accept - BOOLEAN
	Enabling this option allows a "global" bound socket to work
	across L3 master domains (e.g., VRFs) with packets capable of
	being received regardless of the L3 domain in which they
	originated. Only valid when the kernel was compiled with
	CONFIG_NET_L3_MASTER_DEV.

	Default: 0 (disabled)

udp_mem - vector of 3 INTEGERs: min, pressure, max
	Number of pages allowed for queueing by all UDP sockets.

	min: Number of pages allowed for queueing by all UDP sockets.

	pressure: This value was introduced to follow format of tcp_mem.

	max: This value was introduced to follow format of tcp_mem.

	Default is calculated at boot time from amount of available memory.

udp_rmem_min - INTEGER
	Minimal size of receive buffer used by UDP sockets in moderation.
	Each UDP socket is able to use the size for receiving data, even if
	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

	Default: 4K

udp_wmem_min - INTEGER
	Minimal size of send buffer used by UDP sockets in moderation.
	Each UDP socket is able to use the size for sending data, even if
	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

	Default: 4K

RAW variables
=============

raw_l3mdev_accept - BOOLEAN
	Enabling this option allows a "global" bound socket to work
	across L3 master domains (e.g., VRFs) with packets capable of
	being received regardless of the L3 domain in which they
	originated. Only valid when the kernel was compiled with
	CONFIG_NET_L3_MASTER_DEV.

	Default: 1 (enabled)

CIPSOv4 Variables
=================

cipso_cache_enable - BOOLEAN
	If set, enable additions to and lookups from the CIPSO label mapping
	cache.  If unset, additions are ignored and lookups always result in a
	miss.  However, regardless of the setting the cache is still
	invalidated when required when means you can safely toggle this on and
	off and the cache will always be "safe".

	Default: 1

cipso_cache_bucket_size - INTEGER
	The CIPSO label cache consists of a fixed size hash table with each
	hash bucket containing a number of cache entries.  This variable limits
	the number of entries in each hash bucket; the larger the value the
	more CIPSO label mappings that can be cached.  When the number of
	entries in a given hash bucket reaches this limit adding new entries
	causes the oldest entry in the bucket to be removed to make room.

	Default: 10

cipso_rbm_optfmt - BOOLEAN
	Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
	the CIPSO draft specification (see Documentation/netlabel for details).
	This means that when set the CIPSO tag will be padded with empty
	categories in order to make the packet data 32-bit aligned.

	Default: 0

cipso_rbm_structvalid - BOOLEAN
	If set, do a very strict check of the CIPSO option when
	ip_options_compile() is called.  If unset, relax the checks done during
	ip_options_compile().  Either way is "safe" as errors are caught else
	where in the CIPSO processing code but setting this to 0 (False) should
	result in less work (i.e. it should be faster) but could cause problems
	with other implementations that require strict checking.

	Default: 0

IP Variables
============

ip_local_port_range - 2 INTEGERS
	Defines the local port range that is used by TCP and UDP to
	choose the local port. The first number is the first, the
	second the last local port number.
	If possible, it is better these numbers have different parity
	(one even and one odd value).
	Must be greater than or equal to ip_unprivileged_port_start.
	The default values are 32768 and 60999 respectively.

ip_local_reserved_ports - list of comma separated ranges
	Specify the ports which are reserved for known third-party
	applications. These ports will not be used by automatic port
	assignments (e.g. when calling connect() or bind() with port
	number 0). Explicit port allocation behavior is unchanged.

	The format used for both input and output is a comma separated
	list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
	10). Writing to the file will clear all previously reserved
	ports and update the current list with the one given in the
	input.

	Note that ip_local_port_range and ip_local_reserved_ports
	settings are independent and both are considered by the kernel
	when determining which ports are available for automatic port
	assignments.

	You can reserve ports which are not in the current
	ip_local_port_range, e.g.::

	    $ cat /proc/sys/net/ipv4/ip_local_port_range
	    32000	60999
	    $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
	    8080,9148

	although this is redundant. However such a setting is useful
	if later the port range is changed to a value that will
	include the reserved ports. Also keep in mind, that overlapping
	of these ranges may affect probability of selecting ephemeral
	ports which are right after block of reserved ports.

	Default: Empty

ip_unprivileged_port_start - INTEGER
	This is a per-namespace sysctl.  It defines the first
	unprivileged port in the network namespace.  Privileged ports
	require root or CAP_NET_BIND_SERVICE in order to bind to them.
	To disable all privileged ports, set this to 0.  They must not
	overlap with the ip_local_port_range.

	Default: 1024

ip_nonlocal_bind - BOOLEAN
	If set, allows processes to bind() to non-local IP addresses,
	which can be quite useful - but may break some applications.

	Default: 0

ip_autobind_reuse - BOOLEAN
	By default, bind() does not select the ports automatically even if
	the new socket and all sockets bound to the port have SO_REUSEADDR.
	ip_autobind_reuse allows bind() to reuse the port and this is useful
	when you use bind()+connect(), but may break some applications.
	The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
	option should only be set by experts.
	Default: 0

ip_dynaddr - BOOLEAN
	If set non-zero, enables support for dynamic addresses.
	If set to a non-zero value larger than 1, a kernel log
	message will be printed when dynamic address rewriting
	occurs.

	Default: 0

ip_early_demux - BOOLEAN
	Optimize input packet processing down to one demux for
	certain kinds of local sockets.  Currently we only do this
	for established TCP and connected UDP sockets.

	It may add an additional cost for pure routing workloads that
	reduces overall throughput, in such case you should disable it.

	Default: 1

ping_group_range - 2 INTEGERS
	Restrict ICMP_PROTO datagram sockets to users in the group range.
	The default is "1 0", meaning, that nobody (not even root) may
	create ping sockets.  Setting it to "100 100" would grant permissions
	to the single group. "0 4294967295" would enable it for the world, "100
	4294967295" would enable it for the users, but not daemons.

tcp_early_demux - BOOLEAN
	Enable early demux for established TCP sockets.

	Default: 1

udp_early_demux - BOOLEAN
	Enable early demux for connected UDP sockets. Disable this if
	your system could experience more unconnected load.

	Default: 1

icmp_echo_ignore_all - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it.

	Default: 0

icmp_echo_enable_probe - BOOLEAN
        If set to one, then the kernel will respond to RFC 8335 PROBE
        requests sent to it.

        Default: 0

icmp_echo_ignore_broadcasts - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO and
	TIMESTAMP requests sent to it via broadcast/multicast.

	Default: 1

icmp_ratelimit - INTEGER
	Limit the maximal rates for sending ICMP packets whose type matches
	icmp_ratemask (see below) to specific targets.
	0 to disable any limiting,
	otherwise the minimal space between responses in milliseconds.
	Note that another sysctl, icmp_msgs_per_sec limits the number
	of ICMP packets	sent on all targets.

	Default: 1000

icmp_msgs_per_sec - INTEGER
	Limit maximal number of ICMP packets sent per second from this host.
	Only messages whose type matches icmp_ratemask (see below) are
	controlled by this limit. For security reasons, the precise count
	of messages per second is randomized.

	Default: 1000

icmp_msgs_burst - INTEGER
	icmp_msgs_per_sec controls number of ICMP packets sent per second,
	while icmp_msgs_burst controls the burst size of these packets.
	For security reasons, the precise burst size is randomized.

	Default: 50

icmp_ratemask - INTEGER
	Mask made of ICMP types for which rates are being limited.

	Significant bits: IHGFEDCBA9876543210

	Default mask:     0000001100000011000 (6168)

	Bit definitions (see include/linux/icmp.h):

		= =========================
		0 Echo Reply
		3 Destination Unreachable [1]_
		4 Source Quench [1]_
		5 Redirect
		8 Echo Request
		B Time Exceeded [1]_
		C Parameter Problem [1]_
		D Timestamp Request
		E Timestamp Reply
		F Info Request
		G Info Reply
		H Address Mask Request
		I Address Mask Reply
		= =========================

	.. [1] These are rate limited by default (see default mask above)

icmp_ignore_bogus_error_responses - BOOLEAN
	Some routers violate RFC1122 by sending bogus responses to broadcast
	frames.  Such violations are normally logged via a kernel warning.
	If this is set to TRUE, the kernel will not give such warnings, which
	will avoid log file clutter.

	Default: 1

icmp_errors_use_inbound_ifaddr - BOOLEAN

	If zero, icmp error messages are sent with the primary address of
	the exiting interface.

	If non-zero, the message will be sent with the primary address of
	the interface that received the packet that caused the icmp error.
	This is the behaviour many network administrators will expect from
	a router. And it can make debugging complicated network layouts
	much easier.

	Note that if no primary address exists for the interface selected,
	then the primary address of the first non-loopback interface that
	has one will be used regardless of this setting.

	Default: 0

igmp_max_memberships - INTEGER
	Change the maximum number of multicast groups we can subscribe to.
	Default: 20

	Theoretical maximum value is bounded by having to send a membership
	report in a single datagram (i.e. the report can't span multiple
	datagrams, or risk confusing the switch and leaving groups you don't
	intend to).

	The number of supported groups 'M' is bounded by the number of group
	report entries you can fit into a single datagram of 65535 bytes.

	M = 65536-sizeof (ip header)/(sizeof(Group record))

	Group records are variable length, with a minimum of 12 bytes.
	So net.ipv4.igmp_max_memberships should not be set higher than:

	(65536-24) / 12 = 5459

	The value 5459 assumes no IP header options, so in practice
	this number may be lower.

igmp_max_msf - INTEGER
	Maximum number of addresses allowed in the source filter list for a
	multicast group.

	Default: 10

igmp_qrv - INTEGER
	Controls the IGMP query robustness variable (see RFC2236 8.1).

	Default: 2 (as specified by RFC2236 8.1)

	Minimum: 1 (as specified by RFC6636 4.5)

force_igmp_version - INTEGER
	- 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
	  allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
	  Present timer expires.
	- 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
	  receive IGMPv2/v3 query.
	- 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
	  IGMPv1 query message. Will reply report if receive IGMPv3 query.
	- 3 - Enforce to use IGMP version 3. The same react with default 0.

	.. note::

	   this is not the same with force_mld_version because IGMPv3 RFC3376
	   Security Considerations does not have clear description that we could
	   ignore other version messages completely as MLDv2 RFC3810. So make
	   this value as default 0 is recommended.

``conf/interface/*``
	changes special settings per interface (where
	interface" is the name of your network interface)

``conf/all/*``
	  is special, changes the settings for all interfaces

log_martians - BOOLEAN
	Log packets with impossible addresses to kernel log.
	log_martians for the interface will be enabled if at least one of
	conf/{all,interface}/log_martians is set to TRUE,
	it will be disabled otherwise

accept_redirects - BOOLEAN
	Accept ICMP redirect messages.
	accept_redirects for the interface will be enabled if:

	- both conf/{all,interface}/accept_redirects are TRUE in the case
	  forwarding for the interface is enabled

	or

	- at least one of conf/{all,interface}/accept_redirects is TRUE in the
	  case forwarding for the interface is disabled

	accept_redirects for the interface will be disabled otherwise

	default:

		- TRUE (host)
		- FALSE (router)

forwarding - BOOLEAN
	Enable IP forwarding on this interface.  This controls whether packets
	received _on_ this interface can be forwarded.

mc_forwarding - BOOLEAN
	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
	and a multicast routing daemon is required.
	conf/all/mc_forwarding must also be set to TRUE to enable multicast
	routing	for the interface

medium_id - INTEGER
	Integer value used to differentiate the devices by the medium they
	are attached to. Two devices can have different id values when
	the broadcast packets are received only on one of them.
	The default value 0 means that the device is the only interface
	to its medium, value of -1 means that medium is not known.

	Currently, it is used to change the proxy_arp behavior:
	the proxy_arp feature is enabled for packets forwarded between
	two devices attached to different media.

proxy_arp - BOOLEAN
	Do proxy arp.

	proxy_arp for the interface will be enabled if at least one of
	conf/{all,interface}/proxy_arp is set to TRUE,
	it will be disabled otherwise

proxy_arp_pvlan - BOOLEAN
	Private VLAN proxy arp.

	Basically allow proxy arp replies back to the same interface
	(from which the ARP request/solicitation was received).

	This is done to support (ethernet) switch features, like RFC
	3069, where the individual ports are NOT allowed to
	communicate with each other, but they are allowed to talk to
	the upstream router.  As described in RFC 3069, it is possible
	to allow these hosts to communicate through the upstream
	router by proxy_arp'ing. Don't need to be used together with
	proxy_arp.

	This technology is known by different names:

	  In RFC 3069 it is called VLAN Aggregation.
	  Cisco and Allied Telesyn call it Private VLAN.
	  Hewlett-Packard call it Source-Port filtering or port-isolation.
	  Ericsson call it MAC-Forced Forwarding (RFC Draft).

shared_media - BOOLEAN
	Send(router) or accept(host) RFC1620 shared media redirects.
	Overrides secure_redirects.

	shared_media for the interface will be enabled if at least one of
	conf/{all,interface}/shared_media is set to TRUE,
	it will be disabled otherwise

	default TRUE

secure_redirects - BOOLEAN
	Accept ICMP redirect messages only to gateways listed in the
	interface's current gateway list. Even if disabled, RFC1122 redirect
	rules still apply.

	Overridden by shared_media.

	secure_redirects for the interface will be enabled if at least one of
	conf/{all,interface}/secure_redirects is set to TRUE,
	it will be disabled otherwise

	default TRUE

send_redirects - BOOLEAN
	Send redirects, if router.

	send_redirects for the interface will be enabled if at least one of
	conf/{all,interface}/send_redirects is set to TRUE,
	it will be disabled otherwise

	Default: TRUE

bootp_relay - BOOLEAN
	Accept packets with source address 0.b.c.d destined
	not to this host as local ones. It is supposed, that
	BOOTP relay daemon will catch and forward such packets.
	conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
	for the interface

	default FALSE

	Not Implemented Yet.

accept_source_route - BOOLEAN
	Accept packets with SRR option.
	conf/all/accept_source_route must also be set to TRUE to accept packets
	with SRR option on the interface

	default

		- TRUE (router)
		- FALSE (host)

accept_local - BOOLEAN
	Accept packets with local source addresses. In combination with
	suitable routing, this can be used to direct packets between two
	local interfaces over the wire and have them accepted properly.
	default FALSE

route_localnet - BOOLEAN
	Do not consider loopback addresses as martian source or destination
	while routing. This enables the use of 127/8 for local routing purposes.

	default FALSE

rp_filter - INTEGER
	- 0 - No source validation.
	- 1 - Strict mode as defined in RFC3704 Strict Reverse Path
	  Each incoming packet is tested against the FIB and if the interface
	  is not the best reverse path the packet check will fail.
	  By default failed packets are discarded.
	- 2 - Loose mode as defined in RFC3704 Loose Reverse Path
	  Each incoming packet's source address is also tested against the FIB
	  and if the source address is not reachable via any interface
	  the packet check will fail.

	Current recommended practice in RFC3704 is to enable strict mode
	to prevent IP spoofing from DDos attacks. If using asymmetric routing
	or other complicated routing, then loose mode is recommended.

	The max value from conf/{all,interface}/rp_filter is used
	when doing source validation on the {interface}.

	Default value is 0. Note that some distributions enable it
	in startup scripts.

src_valid_mark - BOOLEAN
	- 0 - The fwmark of the packet is not included in reverse path
	  route lookup.  This allows for asymmetric routing configurations
	  utilizing the fwmark in only one direction, e.g., transparent
	  proxying.

	- 1 - The fwmark of the packet is included in reverse path route
	  lookup.  This permits rp_filter to function when the fwmark is
	  used for routing traffic in both directions.

	This setting also affects the utilization of fmwark when
	performing source address selection for ICMP replies, or
	determining addresses stored for the IPOPT_TS_TSANDADDR and
	IPOPT_RR IP options.

	The max value from conf/{all,interface}/src_valid_mark is used.

	Default value is 0.

arp_filter - BOOLEAN
	- 1 - Allows you to have multiple network interfaces on the same
	  subnet, and have the ARPs for each interface be answered
	  based on whether or not the kernel would route a packet from
	  the ARP'd IP out that interface (therefore you must use source
	  based routing for this to work). In other words it allows control
	  of which cards (usually 1) will respond to an arp request.

	- 0 - (default) The kernel can respond to arp requests with addresses
	  from other interfaces. This may seem wrong but it usually makes
	  sense, because it increases the chance of successful communication.
	  IP addresses are owned by the complete host on Linux, not by
	  particular interfaces. Only for more complex setups like load-
	  balancing, does this behaviour cause problems.

	arp_filter for the interface will be enabled if at least one of
	conf/{all,interface}/arp_filter is set to TRUE,
	it will be disabled otherwise

arp_announce - INTEGER
	Define different restriction levels for announcing the local
	source IP address from IP packets in ARP requests sent on
	interface:

	- 0 - (default) Use any local address, configured on any interface
	- 1 - Try to avoid local addresses that are not in the target's
	  subnet for this interface. This mode is useful when target
	  hosts reachable via this interface require the source IP
	  address in ARP requests to be part of their logical network
	  configured on the receiving interface. When we generate the
	  request we will check all our subnets that include the
	  target IP and will preserve the source address if it is from
	  such subnet. If there is no such subnet we select source
	  address according to the rules for level 2.
	- 2 - Always use the best local address for this target.
	  In this mode we ignore the source address in the IP packet
	  and try to select local address that we prefer for talks with
	  the target host. Such local address is selected by looking
	  for primary IP addresses on all our subnets on the outgoing
	  interface that include the target IP address. If no suitable
	  local address is found we select the first local address
	  we have on the outgoing interface or on all other interfaces,
	  with the hope we will receive reply for our request and
	  even sometimes no matter the source IP address we announce.

	The max value from conf/{all,interface}/arp_announce is used.

	Increasing the restriction level gives more chance for
	receiving answer from the resolved target while decreasing
	the level announces more valid sender's information.

arp_ignore - INTEGER
	Define different modes for sending replies in response to
	received ARP requests that resolve local target IP addresses:

	- 0 - (default): reply for any local target IP address, configured
	  on any interface
	- 1 - reply only if the target IP address is local address
	  configured on the incoming interface
	- 2 - reply only if the target IP address is local address
	  configured on the incoming interface and both with the
	  sender's IP address are part from same subnet on this interface
	- 3 - do not reply for local addresses configured with scope host,
	  only resolutions for global and link addresses are replied
	- 4-7 - reserved
	- 8 - do not reply for all local addresses

	The max value from conf/{all,interface}/arp_ignore is used
	when ARP request is received on the {interface}

arp_notify - BOOLEAN
	Define mode for notification of address and device changes.

	 ==  ==========================================================
	  0  (default): do nothing
	  1  Generate gratuitous arp requests when device is brought up
	     or hardware address changes.
	 ==  ==========================================================

arp_accept - BOOLEAN
	Define behavior for gratuitous ARP frames who's IP is not
	already present in the ARP table:

	- 0 - don't create new entries in the ARP table
	- 1 - create new entries in the ARP table

	Both replies and requests type gratuitous arp will trigger the
	ARP table to be updated, if this setting is on.

	If the ARP table already contains the IP address of the
	gratuitous arp frame, the arp table will be updated regardless
	if this setting is on or off.

arp_evict_nocarrier - BOOLEAN
	Clears the ARP cache on NOCARRIER events. This option is important for
	wireless devices where the ARP cache should not be cleared when roaming
	between access points on the same network. In most cases this should
	remain as the default (1).

	- 1 - (default): Clear the ARP cache on NOCARRIER events
	- 0 - Do not clear ARP cache on NOCARRIER events

mcast_solicit - INTEGER
	The maximum number of multicast probes in INCOMPLETE state,
	when the associated hardware address is unknown.  Defaults
	to 3.

ucast_solicit - INTEGER
	The maximum number of unicast probes in PROBE state, when
	the hardware address is being reconfirmed.  Defaults to 3.

app_solicit - INTEGER
	The maximum number of probes to send to the user space ARP daemon
	via netlink before dropping back to multicast probes (see
	mcast_resolicit).  Defaults to 0.

mcast_resolicit - INTEGER
	The maximum number of multicast probes after unicast and
	app probes in PROBE state.  Defaults to 0.

disable_policy - BOOLEAN
	Disable IPSEC policy (SPD) for this interface

disable_xfrm - BOOLEAN
	Disable IPSEC encryption on this interface, whatever the policy

igmpv2_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	IGMPv1 or IGMPv2 report retransmit will take place.

	Default: 10000 (10 seconds)

igmpv3_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	IGMPv3 report retransmit will take place.

	Default: 1000 (1 seconds)

ignore_routes_with_linkdown - BOOLEAN
        Ignore routes whose link is down when performing a FIB lookup.

promote_secondaries - BOOLEAN
	When a primary IP address is removed from this interface
	promote a corresponding secondary IP address instead of
	removing all the corresponding secondary IP addresses.

drop_unicast_in_l2_multicast - BOOLEAN
	Drop any unicast IP packets that are received in link-layer
	multicast (or broadcast) frames.

	This behavior (for multicast) is actually a SHOULD in RFC
	1122, but is disabled by default for compatibility reasons.

	Default: off (0)

drop_gratuitous_arp - BOOLEAN
	Drop all gratuitous ARP frames, for example if there's a known
	good ARP proxy on the network and such frames need not be used
	(or in the case of 802.11, must not be used to prevent attacks.)

	Default: off (0)


tag - INTEGER
	Allows you to write a number, which can be used as required.

	Default value is 0.

xfrm4_gc_thresh - INTEGER
	(Obsolete since linux-4.14)
	The threshold at which we will start garbage collecting for IPv4
	destination cache entries.  At twice this value the system will
	refuse new allocations.

igmp_link_local_mcast_reports - BOOLEAN
	Enable IGMP reports for link local multicast groups in the
	224.0.0.X range.

	Default TRUE

Alexey Kuznetsov.
kuznet@ms2.inr.ac.ru

Updated by:

- Andi Kleen
  ak@muc.de
- Nicolas Delon
  delon.nicolas@wanadoo.fr




/proc/sys/net/ipv6/* Variables
==============================

IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
apply to IPv6 [XXX?].

bindv6only - BOOLEAN
	Default value for IPV6_V6ONLY socket option,
	which restricts use of the IPv6 socket to IPv6 communication
	only.

		- TRUE: disable IPv4-mapped address feature
		- FALSE: enable IPv4-mapped address feature

	Default: FALSE (as specified in RFC3493)

flowlabel_consistency - BOOLEAN
	Protect the consistency (and unicity) of flow label.
	You have to disable it to use IPV6_FL_F_REFLECT flag on the
	flow label manager.

	- TRUE: enabled
	- FALSE: disabled

	Default: TRUE

auto_flowlabels - INTEGER
	Automatically generate flow labels based on a flow hash of the
	packet. This allows intermediate devices, such as routers, to
	identify packet flows for mechanisms like Equal Cost Multipath
	Routing (see RFC 6438).

	=  ===========================================================
	0  automatic flow labels are completely disabled
	1  automatic flow labels are enabled by default, they can be
	   disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
	   socket option
	2  automatic flow labels are allowed, they may be enabled on a
	   per socket basis using the IPV6_AUTOFLOWLABEL socket option
	3  automatic flow labels are enabled and enforced, they cannot
	   be disabled by the socket option
	=  ===========================================================

	Default: 1

flowlabel_state_ranges - BOOLEAN
	Split the flow label number space into two ranges. 0-0x7FFFF is
	reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
	is reserved for stateless flow labels as described in RFC6437.

	- TRUE: enabled
	- FALSE: disabled

	Default: true

flowlabel_reflect - INTEGER
	Control flow label reflection. Needed for Path MTU
	Discovery to work with Equal Cost Multipath Routing in anycast
	environments. See RFC 7690 and:
	https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01

	This is a bitmask.

	- 1: enabled for established flows

	  Note that this prevents automatic flowlabel changes, as done
	  in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
	  and "tcp: Change txhash on every SYN and RTO retransmit"

	- 2: enabled for TCP RESET packets (no active listener)
	  If set, a RST packet sent in response to a SYN packet on a closed
	  port will reflect the incoming flow label.

	- 4: enabled for ICMPv6 echo reply messages.

	Default: 0

fib_multipath_hash_policy - INTEGER
	Controls which hash policy to use for multipath routes.

	Default: 0 (Layer 3)

	Possible values:

	- 0 - Layer 3 (source and destination addresses plus flow label)
	- 1 - Layer 4 (standard 5-tuple)
	- 2 - Layer 3 or inner Layer 3 if present
	- 3 - Custom multipath hash. Fields used for multipath hash calculation
	  are determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER
	When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
	fields used for multipath hash calculation are determined by this
	sysctl.

	This value is a bitmask which enables various fields for multipath hash
	calculation.

	Possible fields are:

	====== ============================
	0x0001 Source IP address
	0x0002 Destination IP address
	0x0004 IP protocol
	0x0008 Flow Label
	0x0010 Source port
	0x0020 Destination port
	0x0040 Inner source IP address
	0x0080 Inner destination IP address
	0x0100 Inner IP protocol
	0x0200 Inner Flow Label
	0x0400 Inner source port
	0x0800 Inner destination port
	====== ============================

	Default: 0x0007 (source IP, destination IP and IP protocol)

anycast_src_echo_reply - BOOLEAN
	Controls the use of anycast addresses as source addresses for ICMPv6
	echo reply

	- TRUE:  enabled
	- FALSE: disabled

	Default: FALSE

idgen_delay - INTEGER
	Controls the delay in seconds after which time to retry
	privacy stable address generation if a DAD conflict is
	detected.

	Default: 1 (as specified in RFC7217)

idgen_retries - INTEGER
	Controls the number of retries to generate a stable privacy
	address if a DAD conflict is detected.

	Default: 3 (as specified in RFC7217)

mld_qrv - INTEGER
	Controls the MLD query robustness variable (see RFC3810 9.1).

	Default: 2 (as specified by RFC3810 9.1)

	Minimum: 1 (as specified by RFC6636 4.5)

max_dst_opts_number - INTEGER
	Maximum number of non-padding TLVs allowed in a Destination
	options extension header. If this value is less than zero
	then unknown options are disallowed and the number of known
	TLVs allowed is the absolute value of this number.

	Default: 8

max_hbh_opts_number - INTEGER
	Maximum number of non-padding TLVs allowed in a Hop-by-Hop
	options extension header. If this value is less than zero
	then unknown options are disallowed and the number of known
	TLVs allowed is the absolute value of this number.

	Default: 8

max_dst_opts_length - INTEGER
	Maximum length allowed for a Destination options extension
	header.

	Default: INT_MAX (unlimited)

max_hbh_length - INTEGER
	Maximum length allowed for a Hop-by-Hop options extension
	header.

	Default: INT_MAX (unlimited)

skip_notify_on_dev_down - BOOLEAN
	Controls whether an RTM_DELROUTE message is generated for routes
	removed when a device is taken down or deleted. IPv4 does not
	generate this message; IPv6 does by default. Setting this sysctl
	to true skips the message, making IPv4 and IPv6 on par in relying
	on userspace caches to track link events and evict routes.

	Default: false (generate message)

nexthop_compat_mode - BOOLEAN
	New nexthop API provides a means for managing nexthops independent of
	prefixes. Backwards compatibilty with old route format is enabled by
	default which means route dumps and notifications contain the new
	nexthop attribute but also the full, expanded nexthop definition.
	Further, updates or deletes of a nexthop configuration generate route
	notifications for each fib entry using the nexthop. Once a system
	understands the new API, this sysctl can be disabled to achieve full
	performance benefits of the new API by disabling the nexthop expansion
	and extraneous notifications.
	Default: true (backward compat mode)

fib_notify_on_flag_change - INTEGER
        Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
        RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

        After installing a route to the kernel, user space receives an
        acknowledgment, which means the route was installed in the kernel,
        but not necessarily in hardware.
        It is also possible for a route already installed in hardware to change
        its action and therefore its flags. For example, a host route that is
        trapping packets can be "promoted" to perform decapsulation following
        the installation of an IPinIP/VXLAN tunnel.
        The notifications will indicate to user-space the state of the route.

        Default: 0 (Do not emit notifications.)

        Possible values:

        - 0 - Do not emit notifications.
        - 1 - Emit notifications.
        - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

ioam6_id - INTEGER
        Define the IOAM id of this node. Uses only 24 bits out of 32 in total.

        Min: 0
        Max: 0xFFFFFF

        Default: 0xFFFFFF

ioam6_id_wide - LONG INTEGER
        Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
        total. Can be different from ioam6_id.

        Min: 0
        Max: 0xFFFFFFFFFFFFFF

        Default: 0xFFFFFFFFFFFFFF

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER
	Maximum memory used to reassemble IPv6 fragments. When
	ip6frag_high_thresh bytes of memory is allocated for this purpose,
	the fragment handler will toss packets until ip6frag_low_thresh
	is reached.

ip6frag_low_thresh - INTEGER
	See ip6frag_high_thresh

ip6frag_time - INTEGER
	Time in seconds to keep an IPv6 fragment in memory.

``conf/default/*``:
	Change the interface-specific default settings.

	These settings would be used during creating new interfaces.


``conf/all/*``:
	Change all the interface-specific settings.

	[XXX:  Other special features than forwarding?]

conf/all/disable_ipv6 - BOOLEAN
	Changing this value is same as changing ``conf/default/disable_ipv6``
	setting and also all per-interface ``disable_ipv6`` settings to the same
	value.

	Reading this value does not have any particular meaning. It does not say
	whether IPv6 support is enabled or disabled. Returned value can be 1
	also in the case when some interface has ``disable_ipv6`` set to 0 and
	has configured IPv6 addresses.

conf/all/forwarding - BOOLEAN
	Enable global IPv6 forwarding between all interfaces.

	IPv4 and IPv6 work differently here; e.g. netfilter must be used
	to control which interfaces may forward packets and which not.

	This also sets all interfaces' Host/Router setting
	'forwarding' to the specified value.  See below for details.

	This referred to as global forwarding.

proxy_ndp - BOOLEAN
	Do proxy ndp.

fwmark_reflect - BOOLEAN
	Controls the fwmark of kernel-generated IPv6 reply packets that are not
	associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
	If unset, these packets have a fwmark of zero. If set, they have the
	fwmark of the packet they are replying to.

	Default: 0

``conf/interface/*``:
	Change special settings per interface.

	The functional behaviour for certain settings is different
	depending on whether local forwarding is enabled or not.

accept_ra - INTEGER
	Accept Router Advertisements; autoconfigure using them.

	It also determines whether or not to transmit Router
	Solicitations. If and only if the functional setting is to
	accept Router Advertisements, Router Solicitations will be
	transmitted.

	Possible values are:

		==  ===========================================================
		 0  Do not accept Router Advertisements.
		 1  Accept Router Advertisements if forwarding is disabled.
		 2  Overrule forwarding behaviour. Accept Router Advertisements
		    even if forwarding is enabled.
		==  ===========================================================

	Functional default:

		- enabled if local forwarding is disabled.
		- disabled if local forwarding is enabled.

accept_ra_defrtr - BOOLEAN
	Learn default router in Router Advertisement.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

ra_defrtr_metric - UNSIGNED INTEGER
	Route metric for default route learned in Router Advertisement. This value
	will be assigned as metric for the default route learned via IPv6 Router
	Advertisement. Takes affect only if accept_ra_defrtr is enabled.

	Possible values:
		1 to 0xFFFFFFFF

		Default: IP6_RT_PRIO_USER i.e. 1024.

accept_ra_from_local - BOOLEAN
	Accept RA with source-address that is found on local machine
	if the RA is otherwise proper and able to be accepted.

	Default is to NOT accept these as it may be an un-intended
	network loop.

	Functional default:

	   - enabled if accept_ra_from_local is enabled
	     on a specific interface.
	   - disabled if accept_ra_from_local is disabled
	     on a specific interface.

accept_ra_min_hop_limit - INTEGER
	Minimum hop limit Information in Router Advertisement.

	Hop limit Information in Router Advertisement less than this
	variable shall be ignored.

	Default: 1

accept_ra_pinfo - BOOLEAN
	Learn Prefix Information in Router Advertisement.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_ra_rt_info_min_plen - INTEGER
	Minimum prefix length of Route Information in RA.

	Route Information w/ prefix smaller than this variable shall
	be ignored.

	Functional default:

		* 0 if accept_ra_rtr_pref is enabled.
		* -1 if accept_ra_rtr_pref is disabled.

accept_ra_rt_info_max_plen - INTEGER
	Maximum prefix length of Route Information in RA.

	Route Information w/ prefix larger than this variable shall
	be ignored.

	Functional default:

		* 0 if accept_ra_rtr_pref is enabled.
		* -1 if accept_ra_rtr_pref is disabled.

accept_ra_rtr_pref - BOOLEAN
	Accept Router Preference in RA.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_ra_mtu - BOOLEAN
	Apply the MTU value specified in RA option 5 (RFC4861). If
	disabled, the MTU specified in the RA will be ignored.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_redirects - BOOLEAN
	Accept Redirects.

	Functional default:

		- enabled if local forwarding is disabled.
		- disabled if local forwarding is enabled.

accept_source_route - INTEGER
	Accept source routing (routing extension header).

	- >= 0: Accept only routing header type 2.
	- < 0: Do not accept routing header.

	Default: 0

autoconf - BOOLEAN
	Autoconfigure addresses using Prefix Information in Router
	Advertisements.

	Functional default:

		- enabled if accept_ra_pinfo is enabled.
		- disabled if accept_ra_pinfo is disabled.

dad_transmits - INTEGER
	The amount of Duplicate Address Detection probes to send.

	Default: 1

forwarding - INTEGER
	Configure interface-specific Host/Router behaviour.

	.. note::

	   It is recommended to have the same setting on all
	   interfaces; mixed router/host scenarios are rather uncommon.

	Possible values are:

		- 0 Forwarding disabled
		- 1 Forwarding enabled

	**FALSE (0)**:

	By default, Host behaviour is assumed.  This means:

	1. IsRouter flag is not set in Neighbour Advertisements.
	2. If accept_ra is TRUE (default), transmit Router
	   Solicitations.
	3. If accept_ra is TRUE (default), accept Router
	   Advertisements (and do autoconfiguration).
	4. If accept_redirects is TRUE (default), accept Redirects.

	**TRUE (1)**:

	If local forwarding is enabled, Router behaviour is assumed.
	This means exactly the reverse from the above:

	1. IsRouter flag is set in Neighbour Advertisements.
	2. Router Solicitations are not sent unless accept_ra is 2.
	3. Router Advertisements are ignored unless accept_ra is 2.
	4. Redirects are ignored.

	Default: 0 (disabled) if global forwarding is disabled (default),
	otherwise 1 (enabled).

hop_limit - INTEGER
	Default Hop Limit to set.

	Default: 64

mtu - INTEGER
	Default Maximum Transfer Unit

	Default: 1280 (IPv6 required minimum)

ip_nonlocal_bind - BOOLEAN
	If set, allows processes to bind() to non-local IPv6 addresses,
	which can be quite useful - but may break some applications.

	Default: 0

router_probe_interval - INTEGER
	Minimum interval (in seconds) between Router Probing described
	in RFC4191.

	Default: 60

router_solicitation_delay - INTEGER
	Number of seconds to wait after interface is brought up
	before sending Router Solicitations.

	Default: 1

router_solicitation_interval - INTEGER
	Number of seconds to wait between Router Solicitations.

	Default: 4

router_solicitations - INTEGER
	Number of Router Solicitations to send until assuming no
	routers are present.

	Default: 3

use_oif_addrs_only - BOOLEAN
	When enabled, the candidate source addresses for destinations
	routed via this interface are restricted to the set of addresses
	configured on this interface (vis. RFC 6724, section 4).

	Default: false

use_tempaddr - INTEGER
	Preference for Privacy Extensions (RFC3041).

	  * <= 0 : disable Privacy Extensions
	  * == 1 : enable Privacy Extensions, but prefer public
	    addresses over temporary addresses.
	  * >  1 : enable Privacy Extensions and prefer temporary
	    addresses over public addresses.

	Default:

		* 0 (for most devices)
		* -1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER
	valid lifetime (in seconds) for temporary addresses.

	Default: 172800 (2 days)

temp_prefered_lft - INTEGER
	Preferred lifetime (in seconds) for temporary addresses.

	Default: 86400 (1 day)

keep_addr_on_down - INTEGER
	Keep all IPv6 addresses on an interface down event. If set static
	global addresses with no expiration time are not flushed.

	*   >0 : enabled
	*    0 : system default
	*   <0 : disabled

	Default: 0 (addresses are removed)

max_desync_factor - INTEGER
	Maximum value for DESYNC_FACTOR, which is a random value
	that ensures that clients don't synchronize with each
	other and generate new addresses at exactly the same time.
	value is in seconds.

	Default: 600

regen_max_retry - INTEGER
	Number of attempts before give up attempting to generate
	valid temporary addresses.

	Default: 5

max_addresses - INTEGER
	Maximum number of autoconfigured addresses per interface.  Setting
	to zero disables the limitation.  It is not recommended to set this
	value too large (or to zero) because it would be an easy way to
	crash the kernel by allowing too many addresses to be created.

	Default: 16

disable_ipv6 - BOOLEAN
	Disable IPv6 operation.  If accept_dad is set to 2, this value
	will be dynamically set to TRUE if DAD fails for the link-local
	address.

	Default: FALSE (enable IPv6 operation)

	When this value is changed from 1 to 0 (IPv6 is being enabled),
	it will dynamically create a link-local address on the given
	interface and start Duplicate Address Detection, if necessary.

	When this value is changed from 0 to 1 (IPv6 is being disabled),
	it will dynamically delete all addresses and routes on the given
	interface. From now on it will not possible to add addresses/routes
	to the selected interface.

accept_dad - INTEGER
	Whether to accept DAD (Duplicate Address Detection).

	 == ==============================================================
	  0  Disable DAD
	  1  Enable DAD (default)
	  2  Enable DAD, and disable IPv6 operation if MAC-based duplicate
	     link-local address has been found.
	 == ==============================================================

	DAD operation and mode on a given interface will be selected according
	to the maximum value of conf/{all,interface}/accept_dad.

force_tllao - BOOLEAN
	Enable sending the target link-layer address option even when
	responding to a unicast neighbor solicitation.

	Default: FALSE

	Quoting from RFC 2461, section 4.4, Target link-layer address:

	"The option MUST be included for multicast solicitations in order to
	avoid infinite Neighbor Solicitation "recursion" when the peer node
	does not have a cache entry to return a Neighbor Advertisements
	message.  When responding to unicast solicitations, the option can be
	omitted since the sender of the solicitation has the correct link-
	layer address; otherwise it would not have be able to send the unicast
	solicitation in the first place. However, including the link-layer
	address in this case adds little overhead and eliminates a potential
	race condition where the sender deletes the cached link-layer address
	prior to receiving a response to a previous solicitation."

ndisc_notify - BOOLEAN
	Define mode for notification of address and device changes.

	* 0 - (default): do nothing
	* 1 - Generate unsolicited neighbour advertisements when device is brought
	  up or hardware address changes.

ndisc_tclass - INTEGER
	The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
	Discovery (Router Solicitation, Router Advertisement, Neighbor
	Solicitation, Neighbor Advertisement, Redirect) messages.
	These 8 bits can be interpreted as 6 high order bits holding the DSCP
	value and 2 low order bits representing ECN (which you probably want
	to leave cleared).

	* 0 - (default)

ndisc_evict_nocarrier - BOOLEAN
	Clears the neighbor discovery table on NOCARRIER events. This option is
	important for wireless devices where the neighbor discovery cache should
	not be cleared when roaming between access points on the same network.
	In most cases this should remain as the default (1).

	- 1 - (default): Clear neighbor discover cache on NOCARRIER events.
	- 0 - Do not clear neighbor discovery cache on NOCARRIER events.

mldv1_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	MLDv1 report retransmit will take place.

	Default: 10000 (10 seconds)

mldv2_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	MLDv2 report retransmit will take place.

	Default: 1000 (1 second)

force_mld_version - INTEGER
	* 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
	* 1 - Enforce to use MLD version 1
	* 2 - Enforce to use MLD version 2

suppress_frag_ndisc - INTEGER
	Control RFC 6980 (Security Implications of IPv6 Fragmentation
	with IPv6 Neighbor Discovery) behavior:

	* 1 - (default) discard fragmented neighbor discovery packets
	* 0 - allow fragmented neighbor discovery packets

optimistic_dad - BOOLEAN
	Whether to perform Optimistic Duplicate Address Detection (RFC 4429).

	* 0: disabled (default)
	* 1: enabled

	Optimistic Duplicate Address Detection for the interface will be enabled
	if at least one of conf/{all,interface}/optimistic_dad is set to 1,
	it will be disabled otherwise.

use_optimistic - BOOLEAN
	If enabled, do not classify optimistic addresses as deprecated during
	source address selection.  Preferred addresses will still be chosen
	before optimistic addresses, subject to other ranking in the source
	address selection algorithm.

	* 0: disabled (default)
	* 1: enabled

	This will be enabled if at least one of
	conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.

stable_secret - IPv6 address
	This IPv6 address will be used as a secret to generate IPv6
	addresses for link-local addresses and autoconfigured
	ones. All addresses generated after setting this secret will
	be stable privacy ones by default. This can be changed via the
	addrgenmode ip-link. conf/default/stable_secret is used as the
	secret for the namespace, the interface specific ones can
	overwrite that. Writes to conf/all/stable_secret are refused.

	It is recommended to generate this secret during installation
	of a system and keep it stable after that.

	By default the stable secret is unset.

addr_gen_mode - INTEGER
	Defines how link-local and autoconf addresses are generated.

	=  =================================================================
	0  generate address based on EUI64 (default)
	1  do no generate a link-local address, use EUI64 for addresses
	   generated from autoconf
	2  generate stable privacy addresses, using the secret from
	   stable_secret (RFC7217)
	3  generate stable privacy addresses, using a random secret if unset
	=  =================================================================

drop_unicast_in_l2_multicast - BOOLEAN
	Drop any unicast IPv6 packets that are received in link-layer
	multicast (or broadcast) frames.

	By default this is turned off.

drop_unsolicited_na - BOOLEAN
	Drop all unsolicited neighbor advertisements, for example if there's
	a known good NA proxy on the network and such frames need not be used
	(or in the case of 802.11, must not be used to prevent attacks.)

	By default this is turned off.

enhanced_dad - BOOLEAN
	Include a nonce option in the IPv6 neighbor solicitation messages used for
	duplicate address detection per RFC7527. A received DAD NS will only signal
	a duplicate address if the nonce is different. This avoids any false
	detection of duplicates due to loopback of the NS messages that we send.
	The nonce option will be sent on an interface unless both of
	conf/{all,interface}/enhanced_dad are set to FALSE.

	Default: TRUE

``icmp/*``:
===========

ratelimit - INTEGER
	Limit the maximal rates for sending ICMPv6 messages.

	0 to disable any limiting,
	otherwise the minimal space between responses in milliseconds.

	Default: 1000

ratemask - list of comma separated ranges
	For ICMPv6 message types matching the ranges in the ratemask, limit
	the sending of the message according to ratelimit parameter.

	The format used for both input and output is a comma separated
	list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
	129). Writing to the file will clear all previous ranges of ICMPv6
	message types and update the current list with the input.

	Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
	for numerical values of ICMPv6 message types, e.g. echo request is 128
	and echo reply is 129.

	Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)

echo_ignore_all - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol.

	Default: 0

echo_ignore_multicast - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol via multicast.

	Default: 0

echo_ignore_anycast - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol destined to anycast address.

	Default: 0

xfrm6_gc_thresh - INTEGER
	(Obsolete since linux-4.14)
	The threshold at which we will start garbage collecting for IPv6
	destination cache entries.  At twice this value the system will
	refuse new allocations.


IPv6 Update by:
Pekka Savola <pekkas@netcore.fi>
YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>


/proc/sys/net/bridge/* Variables:
=================================

bridge-nf-call-arptables - BOOLEAN
	- 1 : pass bridged ARP traffic to arptables' FORWARD chain.
	- 0 : disable this.

	Default: 1

bridge-nf-call-iptables - BOOLEAN
	- 1 : pass bridged IPv4 traffic to iptables' chains.
	- 0 : disable this.

	Default: 1

bridge-nf-call-ip6tables - BOOLEAN
	- 1 : pass bridged IPv6 traffic to ip6tables' chains.
	- 0 : disable this.

	Default: 1

bridge-nf-filter-vlan-tagged - BOOLEAN
	- 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
	- 0 : disable this.

	Default: 0

bridge-nf-filter-pppoe-tagged - BOOLEAN
	- 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
	- 0 : disable this.

	Default: 0

bridge-nf-pass-vlan-input-dev - BOOLEAN
	- 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
	  interface on the bridge and set the netfilter input device to the
	  vlan. This allows use of e.g. "iptables -i br0.1" and makes the
	  REDIRECT target work with vlan-on-top-of-bridge interfaces.  When no
	  matching vlan interface is found, or this switch is off, the input
	  device is set to the bridge interface.

	- 0: disable bridge netfilter vlan interface lookup.

	Default: 0

``proc/sys/net/sctp/*`` Variables:
==================================

addip_enable - BOOLEAN
	Enable or disable extension of  Dynamic Address Reconfiguration
	(ADD-IP) functionality specified in RFC5061.  This extension provides
	the ability to dynamically add and remove new addresses for the SCTP
	associations.

	1: Enable extension.

	0: Disable extension.

	Default: 0

pf_enable - INTEGER
	Enable or disable pf (pf is short for potentially failed) state. A value
	of pf_retrans > path_max_retrans also disables pf state. That is, one of
	both pf_enable and pf_retrans > path_max_retrans can disable pf state.
	Since pf_retrans and path_max_retrans can be changed by userspace
	application, sometimes user expects to disable pf state by the value of
	pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
	or path_max_retrans is changed by the user application, this pf state is
	enabled. As such, it is necessary to add this to dynamically enable
	and disable pf state. See:
	https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
	details.

	1: Enable pf.

	0: Disable pf.

	Default: 1

pf_expose - INTEGER
	Unset or enable/disable pf (pf is short for potentially failed) state
	exposure.  Applications can control the exposure of the PF path state
	in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
	sockopt.   When it's unset, no SCTP_PEER_ADDR_CHANGE event with
	SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
	can be got via SCTP_GET_PEER_ADDR_INFO sockopt;  When it's enabled,
	a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
	SCTP_PF state and a SCTP_PF-state transport info can be got via
	SCTP_GET_PEER_ADDR_INFO sockopt;  When it's diabled, no
	SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
	trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
	sockopt.

	0: Unset pf state exposure, Compatible with old applications.

	1: Disable pf state exposure.

	2: Enable pf state exposure.

	Default: 0

addip_noauth_enable - BOOLEAN
	Dynamic Address Reconfiguration (ADD-IP) requires the use of
	authentication to protect the operations of adding or removing new
	addresses.  This requirement is mandated so that unauthorized hosts
	would not be able to hijack associations.  However, older
	implementations may not have implemented this requirement while
	allowing the ADD-IP extension.  For reasons of interoperability,
	we provide this variable to control the enforcement of the
	authentication requirement.

	== ===============================================================
	1  Allow ADD-IP extension to be used without authentication.  This
	   should only be set in a closed environment for interoperability
	   with older implementations.

	0  Enforce the authentication requirement
	== ===============================================================

	Default: 0

auth_enable - BOOLEAN
	Enable or disable Authenticated Chunks extension.  This extension
	provides the ability to send and receive authenticated chunks and is
	required for secure operation of Dynamic Address Reconfiguration
	(ADD-IP) extension.

	- 1: Enable this extension.
	- 0: Disable this extension.

	Default: 0

prsctp_enable - BOOLEAN
	Enable or disable the Partial Reliability extension (RFC3758) which
	is used to notify peers that a given DATA should no longer be expected.

	- 1: Enable extension
	- 0: Disable

	Default: 1

max_burst - INTEGER
	The limit of the number of new packets that can be initially sent.  It
	controls how bursty the generated traffic can be.

	Default: 4

association_max_retrans - INTEGER
	Set the maximum number for retransmissions that an association can
	attempt deciding that the remote end is unreachable.  If this value
	is exceeded, the association is terminated.

	Default: 10

max_init_retransmits - INTEGER
	The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
	that an association will attempt before declaring the destination
	unreachable and terminating.

	Default: 8

path_max_retrans - INTEGER
	The maximum number of retransmissions that will be attempted on a given
	path.  Once this threshold is exceeded, the path is considered
	unreachable, and new traffic will use a different path when the
	association is multihomed.

	Default: 5

pf_retrans - INTEGER
	The number of retransmissions that will be attempted on a given path
	before traffic is redirected to an alternate transport (should one
	exist).  Note this is distinct from path_max_retrans, as a path that
	passes the pf_retrans threshold can still be used.  Its only
	deprioritized when a transmission path is selected by the stack.  This
	setting is primarily used to enable fast failover mechanisms without
	having to reduce path_max_retrans to a very low value.  See:
	http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
	for details.  Note also that a value of pf_retrans > path_max_retrans
	disables this feature. Since both pf_retrans and path_max_retrans can
	be changed by userspace application, a variable pf_enable is used to
	disable pf state.

	Default: 0

ps_retrans - INTEGER
	Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
	from section-5 "Primary Path Switchover" in rfc7829.  The primary path
	will be changed to another active path when the path error counter on
	the old primary path exceeds PSMR, so that "the SCTP sender is allowed
	to continue data transmission on a new working path even when the old
	primary destination address becomes active again".   Note this feature
	is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
	and its value can't be less than 'pf_retrans' when changing by sysctl.

	Default: 0xffff

rto_initial - INTEGER
	The initial round trip timeout value in milliseconds that will be used
	in calculating round trip times.  This is the initial time interval
	for retransmissions.

	Default: 3000

rto_max - INTEGER
	The maximum value (in milliseconds) of the round trip timeout.  This
	is the largest time interval that can elapse between retransmissions.

	Default: 60000

rto_min - INTEGER
	The minimum value (in milliseconds) of the round trip timeout.  This
	is the smallest time interval the can elapse between retransmissions.

	Default: 1000

hb_interval - INTEGER
	The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
	are sent at the specified interval on idle paths to probe the state of
	a given path between 2 associations.

	Default: 30000

sack_timeout - INTEGER
	The amount of time (in milliseconds) that the implementation will wait
	to send a SACK.

	Default: 200

valid_cookie_life - INTEGER
	The default lifetime of the SCTP cookie (in milliseconds).  The cookie
	is used during association establishment.

	Default: 60000

cookie_preserve_enable - BOOLEAN
	Enable or disable the ability to extend the lifetime of the SCTP cookie
	that is used during the establishment phase of SCTP association

	- 1: Enable cookie lifetime extension.
	- 0: Disable

	Default: 1

cookie_hmac_alg - STRING
	Select the hmac algorithm used when generating the cookie value sent by
	a listening sctp socket to a connecting client in the INIT-ACK chunk.
	Valid values are:

	* md5
	* sha1
	* none

	Ability to assign md5 or sha1 as the selected alg is predicated on the
	configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
	CONFIG_CRYPTO_SHA1).

	Default: Dependent on configuration.  MD5 if available, else SHA1 if
	available, else none.

rcvbuf_policy - INTEGER
	Determines if the receive buffer is attributed to the socket or to
	association.   SCTP supports the capability to create multiple
	associations on a single socket.  When using this capability, it is
	possible that a single stalled association that's buffering a lot
	of data may block other associations from delivering their data by
	consuming all of the receive buffer space.  To work around this,
	the rcvbuf_policy could be set to attribute the receiver buffer space
	to each association instead of the socket.  This prevents the described
	blocking.

	- 1: rcvbuf space is per association
	- 0: rcvbuf space is per socket

	Default: 0

sndbuf_policy - INTEGER
	Similar to rcvbuf_policy above, this applies to send buffer space.

	- 1: Send buffer is tracked per association
	- 0: Send buffer is tracked per socket.

	Default: 0

sctp_mem - vector of 3 INTEGERs: min, pressure, max
	Number of pages allowed for queueing by all SCTP sockets.

	min: Below this number of pages SCTP is not bothered about its
	memory appetite. When amount of memory allocated by SCTP exceeds
	this number, SCTP starts to moderate memory usage.

	pressure: This value was introduced to follow format of tcp_mem.

	max: Number of pages allowed for queueing by all SCTP sockets.

	Default is calculated at boot time from amount of available memory.

sctp_rmem - vector of 3 INTEGERs: min, default, max
	Only the first value ("min") is used, "default" and "max" are
	ignored.

	min: Minimal size of receive buffer used by SCTP socket.
	It is guaranteed to each SCTP socket (but not association) even
	under moderate memory pressure.

	Default: 4K

sctp_wmem  - vector of 3 INTEGERs: min, default, max
	Currently this tunable has no effect.

addr_scope_policy - INTEGER
	Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00

	- 0   - Disable IPv4 address scoping
	- 1   - Enable IPv4 address scoping
	- 2   - Follow draft but allow IPv4 private addresses
	- 3   - Follow draft but allow IPv4 link local addresses

	Default: 1

udp_port - INTEGER
	The listening port for the local UDP tunneling sock. Normally it's
	using the IANA-assigned UDP port number 9899 (sctp-tunneling).

	This UDP sock is used for processing the incoming UDP-encapsulated
	SCTP packets (from RFC6951), and shared by all applications in the
	same net namespace. This UDP sock will be closed when the value is
	set to 0.

	The value will also be used to set the src port of the UDP header
	for the outgoing UDP-encapsulated SCTP packets. For the dest port,
	please refer to 'encap_port' below.

	Default: 0

encap_port - INTEGER
	The default remote UDP encapsulation port.

	This value is used to set the dest port of the UDP header for the
	outgoing UDP-encapsulated SCTP packets by default. Users can also
	change the value for each sock/asoc/transport by using setsockopt.
	For further information, please refer to RFC6951.

	Note that when connecting to a remote server, the client should set
	this to the port that the UDP tunneling sock on the peer server is
	listening to and the local UDP tunneling sock on the client also
	must be started. On the server, it would get the encap_port from
	the incoming packet's source port.

	Default: 0

plpmtud_probe_interval - INTEGER
        The time interval (in milliseconds) for the PLPMTUD probe timer,
        which is configured to expire after this period to receive an
        acknowledgment to a probe packet. This is also the time interval
        between the probes for the current pmtu when the probe search
        is done.

        PLPMTUD will be disabled when 0 is set, and other values for it
        must be >= 5000.

	Default: 0


``/proc/sys/net/core/*``
========================

	Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.


``/proc/sys/net/unix/*``
========================

max_dgram_qlen - INTEGER
	The maximum length of dgram socket receive queue

	Default: 10