summaryrefslogtreecommitdiff
path: root/doc/bird.sgml
blob: 991fe224429e1db7a594ce1258a36d4ca3e4c95b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
<!doctype birddoc system>

<!--
	BIRD documentation

This documentation can have 4 forms: sgml (this is master copy), html, ASCII
text and dvi/postscript (generated from sgml using sgmltools). You should always
edit master copy.

This is a slightly modified linuxdoc dtd. Anything in <descrip> tags is
considered definition of configuration primitives, <cf> is fragment of
configuration within normal text, <m> is "meta" information within fragment of
configuration - something in config which is not keyword.

    (set-fill-column 80)

    Copyright 1999,2000 Pavel Machek <pavel@ucw.cz>, distribute under GPL version 2 or later.

 -->

<book>

<title>BIRD User's Guide
<author>
Ondrej Filip <it/&lt;feela@network.cz&gt;/,
Pavel Machek <it/&lt;pavel@ucw.cz&gt;/,
Martin Mares <it/&lt;mj@ucw.cz&gt;/,
Ondrej Zajicek <it/&lt;santiago@crfreenet.org&gt;/
</author>

<abstract>
This document contains user documentation for the BIRD Internet Routing Daemon project.
</abstract>

<!-- Table of contents -->
<toc>

<!-- Begin the document -->


<chapt>Introduction
<label id="intro">

<sect>What is BIRD
<label id="what-is-bird">

<p>The name `BIRD' is actually an acronym standing for `BIRD Internet Routing
Daemon'. Let's take a closer look at the meaning of the name:

<p><em/BIRD/: Well, we think we have already explained that. It's an acronym
standing for `BIRD Internet Routing Daemon', you remember, don't you? :-)

<p><em/Internet Routing/: It's a program (well, a daemon, as you are going to
discover in a moment) which works as a dynamic router in an Internet type
network (that is, in a network running either the IPv4 or the IPv6 protocol).
Routers are devices which forward packets between interconnected networks in
order to allow hosts not connected directly to the same local area network to
communicate with each other. They also communicate with the other routers in the
Internet to discover the topology of the network which allows them to find
optimal (in terms of some metric) rules for forwarding of packets (which are
called routing tables) and to adapt themselves to the changing conditions such
as outages of network links, building of new connections and so on. Most of
these routers are costly dedicated devices running obscure firmware which is
hard to configure and not open to any changes (on the other hand, their special
hardware design allows them to keep up with lots of high-speed network
interfaces, better than general-purpose computer does). Fortunately, most
operating systems of the UNIX family allow an ordinary computer to act as a
router and forward packets belonging to the other hosts, but only according to a
statically configured table.

<p>A <em/Routing Daemon/ is in UNIX terminology a non-interactive program
running on background which does the dynamic part of Internet routing, that is
it communicates with the other routers, calculates routing tables and sends them
to the OS kernel which does the actual packet forwarding. There already exist
other such routing daemons: routed (RIP only), GateD (non-free),
<HTMLURL URL="http://www.zebra.org" name="Zebra"> and
<HTMLURL URL="http://sourceforge.net/projects/mrt" name="MRTD">,
but their capabilities are limited and they are relatively hard to configure
and maintain.

<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
to support all the routing technology used in the today's Internet or planned to
be used in near future and to have a clean extensible architecture allowing new
routing protocols to be incorporated easily. Among other features, BIRD
supports:

<itemize>
	<item>both IPv4 and IPv6 protocols
	<item>multiple routing tables
	<item>the Border Gateway Protocol (BGPv4)
	<item>the Routing Information Protocol (RIPv2)
	<item>the Open Shortest Path First protocol (OSPFv2, OSPFv3)
	<item>the Router Advertisements for IPv6 hosts
	<item>a virtual protocol for exchange of routes between different
		routing tables on a single host
	<item>a command-line interface allowing on-line control and inspection
		of status of the daemon
	<item>soft reconfiguration (no need to use complex online commands to
		change the configuration, just edit the configuration file and
		notify BIRD to re-read it and it will smoothly switch itself to
		the new configuration, not disturbing routing protocols unless
		they are affected by the configuration changes)
	<item>a powerful language for route filtering
</itemize>

<p>BIRD has been developed at the Faculty of Math and Physics, Charles
University, Prague, Czech Republic as a student project. It can be freely
distributed under the terms of the GNU General Public License.

<p>BIRD has been designed to work on all UNIX-like systems. It has been
developed and tested under Linux 2.0 to 2.6, and then ported to FreeBSD, NetBSD
and OpenBSD, porting to other systems (even non-UNIX ones) should be relatively
easy due to its highly modular architecture.

<p>BIRD supports either IPv4 or IPv6 protocol, but have to be compiled separately
for each one. Therefore, a dualstack router would run two instances of BIRD (one
for IPv4 and one for IPv6), with completely separate setups (configuration
files, tools ...).


<sect>Installing BIRD
<label id="install">

<p>On a recent UNIX system with GNU development tools (GCC, binutils, m4, make)
and Perl, installing BIRD should be as easy as:

<code>
	./configure
	make
	make install
	vi /usr/local/etc/bird.conf
	bird
</code>

<p>You can use <tt>./configure --help</tt> to get a list of configure
options. The most important ones are: <tt/--enable-ipv6/ which enables building
of an IPv6 version of BIRD, <tt/--with-protocols=/ to produce a slightly smaller
BIRD executable by configuring out routing protocols you don't use, and
<tt/--prefix=/ to install BIRD to a place different from <file>/usr/local</file>.


<sect>Running BIRD
<label id="argv">

<p>You can pass several command-line options to bird:

<descrip>
	<tag><label id="argv-config">-c <m/config name/</tag>
	use given configuration file instead of <it/prefix/<file>/etc/bird.conf</file>.

	<tag><label id="argv-debug">-d</tag>
	enable debug messages and run bird in foreground.

	<tag><label id="argv-log-file">-D <m/filename of debug log/</tag>
	log debugging information to given file instead of stderr.

	<tag><label id="argv-foreground">-f</tag>
	run bird in foreground.

	<tag><label id="argv-group">-g <m/group/</tag>
	use that group ID, see the next section for details.

	<tag><label id="argv-help">-h, --help</tag>
	display command-line options to bird.

	<tag><label id="argv-local">-l</tag>
	look for a configuration file and a communication socket in the current
	working directory instead of in default system locations. However, paths
	specified by options <cf/-c/, <cf/-s/ have higher priority.

	<tag><label id="argv-parse">-p</tag>
	just parse the config file and exit. Return value is zero if the config
	file is valid, nonzero if there are some errors.

	<tag><label id="argv-pid">-P <m/name of PID file/</tag>
	create a PID file with given filename.

	<tag><label id="argv-recovery">-R</tag>
	apply graceful restart recovery after start.

	<tag><label id="argv-socket">-s <m/name of communication socket/</tag>
	use given filename for a socket for communications with the client,
	default is <it/prefix/<file>/var/run/bird.ctl</file>.

	<tag><label id="argv-user">-u <m/user/</tag>
	drop privileges and use that user ID, see the next section for details.

	<tag><label id="argv-version">--version</tag>
	display bird version.
</descrip>

<p>BIRD writes messages about its work to log files or syslog (according to config).


<sect>Privileges
<label id="privileges">

<p>BIRD, as a routing daemon, uses several privileged operations (like setting
routing table and using raw sockets). Traditionally, BIRD is executed and runs
with root privileges, which may be prone to security problems. The recommended
way is to use a privilege restriction (options <cf/-u/, <cf/-g/). In that case
BIRD is executed with root privileges, but it changes its user and group ID to
an unprivileged ones, while using Linux capabilities to retain just required
privileges (capabilities CAP_NET_*). Note that the control socket is created
before the privileges are dropped, but the config file is read after that. The
privilege restriction is not implemented in BSD port of BIRD.

<p>An unprivileged user (as an argument to <cf/-u/ options) may be the user
<cf/nobody/, but it is suggested to use a new dedicated user account (like
<cf/bird/). The similar considerations apply for the group option, but there is
one more condition -- the users in the same group can use <file/birdc/ to
control BIRD.

<p>Finally, there is a possibility to use external tools to run BIRD in an
environment with restricted privileges. This may need some configuration, but it
is generally easy -- BIRD needs just the standard library, privileges to read
the config file and create the control socket and the CAP_NET_* capabilities.


<chapt>About routing tables
<label id="routing-tables">

<p>BIRD has one or more routing tables which may or may not be synchronized with
OS kernel and which may or may not be synchronized with each other (see the Pipe
protocol). Each routing table contains a list of known routes. Each route
consists of:

<itemize>
	<item>network prefix this route is for (network address and prefix
		length -- the number of bits forming the network part of the
		address; also known as a netmask)
	<item>preference of this route
	<item>IP address of router which told us about this route
	<item>IP address of router we should forward the packets to using this
		route
	<item>other attributes common to all routes
	<item>dynamic attributes defined by protocols which may or may not be
		present (typically protocol metrics)
</itemize>

Routing table maintains multiple entries for a network, but at most one entry
for one network and one protocol. The entry with the highest preference is used
for routing (we will call such an entry the <it/selected route/). If there are
more entries with the same preference and they are from the same protocol, the
protocol decides (typically according to metrics). If they aren't, an internal
ordering is used to break the tie. You can get the list of route attributes in
the Route attributes section.

<p>Each protocol is connected to a routing table through two filters which can
accept, reject and modify the routes. An <it/export/ filter checks routes passed
from the routing table to the protocol, an <it/import/ filter checks routes in
the opposite direction. When the routing table gets a route from a protocol, it
recalculates the selected route and broadcasts it to all protocols connected to
the table. The protocols typically send the update to other routers in the
network. Note that although most protocols are interested in receiving just
selected routes, some protocols (e.g. the <cf/Pipe/ protocol) receive and
process all entries in routing tables (accepted by filters).

<p><label id="dsc-table-sorted">Usually, a routing table just chooses a selected route
from a list of entries for one network. But if the <cf/sorted/ option is
activated, these lists of entries are kept completely sorted (according to
preference or some protocol-dependent metric). This is needed for some features
of some protocols (e.g. <cf/secondary/ option of BGP protocol, which allows to
accept not just a selected route, but the first route (in the sorted list) that
is accepted by filters), but it is incompatible with some other features (e.g.
<cf/deterministic med/ option of BGP protocol, which activates a way of choosing
selected route that cannot be described using comparison and ordering). Minor
advantage is that routes are shown sorted in <cf/show route/, minor disadvantage
is that it is slightly more computationally expensive.


<sect>Graceful restart
<label id="graceful-restart">

<p>When BIRD is started after restart or crash, it repopulates routing tables in
an uncoordinated manner, like after clean start. This may be impractical in some
cases, because if the forwarding plane (i.e. kernel routing tables) remains
intact, then its synchronization with BIRD would temporarily disrupt packet
forwarding until protocols converge. Graceful restart is a mechanism that could
help with this issue. Generally, it works by starting protocols and letting them
repopulate routing tables while deferring route propagation until protocols
acknowledge their convergence. Note that graceful restart behavior have to be
configured for all relevant protocols and requires protocol-specific support
(currently implemented for Kernel and BGP protocols), it is activated for
particular boot by option <cf/-R/.


<chapt>Configuration
<label id="config">

<sect>Introduction
<label id="config-intro">

<p>BIRD is configured using a text configuration file. Upon startup, BIRD reads
<it/prefix/<file>/etc/bird.conf</file> (unless the <tt/-c/ command line option
is given). Configuration may be changed at user's request: if you modify the
config file and then signal BIRD with <tt/SIGHUP/, it will adjust to the new
config. Then there's the client which allows you to talk with BIRD in an
extensive way.

<p>In the config, everything on a line after <cf/#/ or inside <cf>/* */</cf> is
a comment, whitespace characters are treated as a single space. If there's a
variable number of options, they are grouped using the <cf/{ }/ brackets. Each
option is terminated by a <cf/;/. Configuration is case sensitive. There are two
ways how to name symbols (like protocol names, filter names, constants etc.). You
can either use a simple string starting with a letter followed by any
combination of letters and numbers (e.g. "R123", "myfilter", "bgp5") or you can
enclose the name into apostrophes (<cf/'/) and than you can use any combination
of numbers, letters. hyphens, dots and colons (e.g. "'1:strange-name'",
"'-NAME-'", "'cool::name'").

<p>Here is an example of a simple config file. It enables synchronization of
routing tables with OS kernel, scans for new network interfaces every 10 seconds
and runs RIP on all network interfaces found.

<code>
protocol kernel {
	persist;		# Don't remove routes on BIRD shutdown
	scan time 20;		# Scan kernel routing table every 20 seconds
	export all;		# Default is export none
}

protocol device {
	scan time 10;		# Scan interfaces every 10 seconds
}

protocol rip {
	export all;
	import all;
	interface "*";
}
</code>


<sect>Global options
<label id="global-opts">

<p><descrip>
	<tag><label id="opt-include">include "<m/filename/"</tag>
	This statement causes inclusion of a new file. <m/Filename/ could also
	be a wildcard, in that case matching files are included in alphabetic
	order. The maximal depth is 8. Note that this statement could be used
	anywhere in the config file, not just as a top-level option.

	<tag><label id="opt-log">log "<m/filename/"|syslog [name <m/name/]|stderr all|{ <m/list of classes/ }</tag>
	Set logging of messages having the given class (either <cf/all/ or
	<cf/{ error|trace [, <m/.../] }/ etc.) into selected destination (a file specified
	as a filename string, syslog with optional name argument, or the stderr
	output). Classes are:
	<cf/info/, <cf/warning/, <cf/error/ and <cf/fatal/ for messages about local problems,
	<cf/debug/ for debugging messages,
	<cf/trace/ when you want to know what happens in the network,
	<cf/remote/ for messages about misbehavior of remote machines,
	<cf/auth/ about authentication failures,
	<cf/bug/ for internal BIRD bugs.
	You may specify more than one <cf/log/ line to establish logging to
	multiple destinations. Default: log everything to the system log.

	<tag><label id="opt-debug-protocols">debug protocols all|off|{ states|routes|filters|interfaces|events|packets [, <m/.../] }</tag>
	Set global defaults of protocol debugging options. See <cf/debug/ in the
	following section. Default: off.

	<tag><label id="opt-debug-commands">debug commands <m/number/</tag>
	Control logging of client connections (0 for no logging, 1 for logging
	of connects and disconnects, 2 and higher for logging of all client
	commands). Default: 0.

	<tag><label id="opt-debug-latency">debug latency <m/switch/</tag>
	Activate tracking of elapsed time for internal events. Recent events
	could be examined using <cf/dump events/ command. Default: off.

	<tag><label id="opt-debug-latency-limit">debug latency limit <m/time/</tag>
	If <cf/debug latency/ is enabled, this option allows to specify a limit
	for elapsed time. Events exceeding the limit are logged. Default: 1 s.

	<tag><label id="opt-watchdog-warn">watchdog warning <m/time/</tag>
	Set time limit for I/O loop cycle. If one iteration took more time to
	complete, a warning is logged. Default: 5 s.

	<tag><label id="opt-watchdog-timeout">watchdog timeout <m/time/</tag>
	Set time limit for I/O loop cycle. If the limit is breached, BIRD is
	killed by abort signal. The timeout has effective granularity of
	seconds, zero means disabled. Default: disabled (0).

	<tag><label id="opt-mrtdump">mrtdump "<m/filename/"</tag>
	Set MRTdump file name. This option must be specified to allow MRTdump
	feature. Default: no dump file.

	<tag><label id="opt-mrtdump-protocols">mrtdump protocols all|off|{ states|messages [, <m/.../] }</tag>
	Set global defaults of MRTdump options. See <cf/mrtdump/ in the
	following section. Default: off.

	<tag><label id="opt-filter">filter <m/name local variables/{ <m/commands/ }</tag>
	Define a filter. You can learn more about filters in the following
	chapter.

	<tag><label id="opt-function">function <m/name/ (<m/parameters/) <m/local variables/ { <m/commands/ }</tag>
	Define a function. You can learn more about functions in the following chapter.

	<tag><label id="opt-protocol">protocol rip|ospf|bgp|<m/.../ [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
	Define a protocol instance called <cf><m/name/</cf> (or with a name like
	"rip5" generated automatically if you don't specify any
	<cf><m/name/</cf>). You can learn more about configuring protocols in
	their own chapters. When <cf>from <m/name2/</cf> expression is used,
	initial protocol options are taken from protocol or template
	<cf><m/name2/</cf> You can run more than one instance of most protocols
	(like RIP or BGP). By default, no instances are configured.

	<tag><label id="opt-template">template rip|bgp|<m/.../ [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
	Define a protocol template instance called <m/name/ (or with a name like
	"bgp1" generated automatically if you don't specify any	<m/name/).
	Protocol templates can be used to group common options when many
	similarly configured protocol instances are to be defined. Protocol
	instances (and other templates) can use templates by using <cf/from/
	expression and the name of the template. At the moment templates (and
	<cf/from/ expression) are not implemented for OSPF protocol.

	<tag><label id="opt-define">define <m/constant/ = <m/expression/</tag>
	Define a constant. You can use it later in every place you could use a
	value of the same type. Besides, there are some predefined numeric
	constants based on /etc/iproute2/rt_* files. A list of defined constants
	can be seen (together with other symbols) using 'show symbols' command.

	<tag><label id="opt-router-id">router id <m/IPv4 address/</tag>
	Set BIRD's router ID. It's a world-wide unique identification of your
	router, usually one of router's IPv4 addresses. Default: in IPv4
	version, the lowest IP address of a non-loopback interface. In IPv6
	version, this option is mandatory.

	<tag><label id="opt-router-id-from">router id from [-] [ "<m/mask/" ] [ <m/prefix/ ] [, <m/.../]</tag>
	Set BIRD's router ID based on an IP address of an interface specified by
	an interface pattern. The option is applicable for IPv4 version only.
	See <ref id="proto-iface" name="interface"> section for detailed
	description of interface patterns with extended clauses.

	<tag><label id="opt-listen-bgp">listen bgp [address <m/address/] [port <m/port/] [dual]</tag>
	This option allows to specify address and port where BGP protocol should
	listen. It is global option as listening socket is common to all BGP
	instances. Default is to listen on all addresses (0.0.0.0) and port 179.
	In IPv6 mode, option <cf/dual/ can be used to specify that BGP socket
	should accept both IPv4 and IPv6 connections (but even in that case,
	BIRD would accept IPv6 routes only). Such behavior was default in older
	versions of BIRD.

	<tag><label id="opt-graceful-restart">graceful restart wait <m/number/</tag>
	During graceful restart recovery, BIRD waits for convergence of routing
	protocols. This option allows to specify a timeout for the recovery to
	prevent waiting indefinitely if some protocols cannot converge. Default:
	240 seconds.

	<tag><label id="opt-timeformat">timeformat route|protocol|base|log "<m/format1/" [<m/limit/ "<m/format2/"]</tag>
	This option allows to specify a format of date/time used by BIRD. The
	first argument specifies for which purpose such format is used.
	<cf/route/ is a format used in 'show route' command output,
	<cf/protocol/ is used in 'show protocols' command output, <cf/base/ is
	used for other commands and <cf/log/ is used in a log file.

	"<m/format1/" is a format string using <it/strftime(3)/ notation (see
	<it/man strftime/ for details). <m/limit> and "<m/format2/" allow to
	specify the second format string for times in past deeper than <m/limit/
 	seconds. There are few shorthands: <cf/iso long/ is a ISO 8601 date/time
	format (YYYY-MM-DD hh:mm:ss) that can be also specified using <cf/"%F %T"/.
	<cf/iso short/ is a variant of ISO 8601 that uses just the time format
	(hh:mm:ss) for near times (up to 20 hours in the past) and the date
	format (YYYY-MM-DD) for far times. This is a shorthand for
	<cf/"%T" 72000 "%F"/.

	By default, BIRD uses the <cf/iso short/ format for <cf/route/ and
	<cf/protocol/ times, and the <cf/iso long/ format for <cf/base/ and
	<cf/log/ times.

	In pre-1.4.0 versions, BIRD used an short, ad-hoc format for <cf/route/
	and <cf/protocol/ times, and a <cf/iso long/ similar format (DD-MM-YYYY
	hh:mm:ss) for <cf/base/ and <cf/log/. These timeformats could be set by
	<cf/old short/ and <cf/old long/ compatibility shorthands.

	<tag><label id="opt-table">table <m/name/ [sorted]</tag>
	Create a new routing table. The default routing table is created
	implicitly, other routing tables have to be added by this command.
	Option <cf/sorted/ can be used to enable sorting of routes, see
	<ref id="dsc-table-sorted" name="sorted table"> description for details.

	<tag><label id="opt-roa-table">roa table <m/name/ [ { <m/roa table options .../ } ]</tag>
	Create a new ROA (Route Origin Authorization) table. ROA tables can be
	used to validate route origination of BGP routes. A ROA table contains
	ROA entries, each consist of a network prefix, a max prefix length and
	an AS number. A ROA entry specifies prefixes which could be originated
	by that AS number. ROA tables could be filled with data from RPKI (<rfc
	id="6480">) or from public databases like Whois. ROA tables are
	examined by <cf/roa_check()/ operator in filters.

	Currently, there is just one option, <cf>roa <m/prefix/ max <m/num/ as
	<m/num/</cf>, which can be used to populate the ROA table with static
	ROA entries. The option may be used multiple times. Other entries can be
	added dynamically by <cf/add roa/ command.

	<tag><label id="opt-eval">eval <m/expr/</tag>
	Evaluates given filter expression. It is used by us for	testing of filters.
</descrip>


<sect>Protocol options
<label id="protocol-opts">

<p>For each protocol instance, you can configure a bunch of options. Some of
them (those described in this section) are generic, some are specific to the
protocol (see sections talking about the protocols).

<p>Several options use a <m/switch/ argument. It can be either <cf/on/,
<cf/yes/ or a numeric expression with a non-zero value for the option to be
enabled or <cf/off/, <cf/no/ or a numeric expression evaluating to zero to
disable it. An empty <m/switch/ is equivalent to <cf/on/ ("silence means
agreement").

<descrip>
	<tag><label id="proto-preference">preference <m/expr/</tag>
	Sets the preference of routes generated by this protocol. Default:
	protocol dependent.

	<tag><label id="proto-disabled">disabled <m/switch/</tag>
	Disables the protocol. You can change the disable/enable status from the
	command line interface without needing to touch the configuration.
	Disabled protocols are not activated. Default: protocol is enabled.

	<tag><label id="proto-debug">debug all|off|{ states|routes|filters|interfaces|events|packets [, <m/.../] }</tag>
	Set protocol debugging options. If asked, each protocol is capable of
	writing trace messages about its work to the log (with category
	<cf/trace/). You can either request printing of <cf/all/ trace messages
	or only of the types selected: <cf/states/ for protocol state changes
	(protocol going up, down, starting, stopping etc.), <cf/routes/ for
	routes exchanged with the routing table, <cf/filters/ for details on
	route filtering, <cf/interfaces/ for interface change events sent to the
	protocol, <cf/events/ for events internal to the protocol and <cf/packets/
	for packets sent and received by the protocol. Default: off.

	<tag><label id="proto-mrtdump">mrtdump all|off|{ states|messages [, <m/.../] }</tag>
	Set protocol MRTdump flags. MRTdump is a standard binary format for
	logging information from routing protocols and daemons. These flags
	control what kind of information is logged from the protocol to the
	MRTdump file (which must be specified by global <cf/mrtdump/ option, see
	the previous section). Although these flags are similar to flags of
	<cf/debug/ option, their meaning is different and protocol-specific. For
	BGP protocol, <cf/states/ logs BGP state changes and <cf/messages/ logs
	received BGP messages. Other protocols does not support MRTdump yet.

	<tag><label id="proto-router-id">router id <m/IPv4 address/</tag>
	This option can be used to override global router id for a given
	protocol. Default: uses global router id.

	<tag><label id="proto-import">import all | none | filter <m/name/ | filter { <m/filter commands/ } | where <m/filter expression/</tag>
	Specify a filter to be used for filtering routes coming from the
	protocol to the routing table. <cf/all/ is shorthand for <cf/where true/
	and <cf/none/ is shorthand for <cf/where false/. Default: <cf/all/.

	<tag><label id="proto-export">export <m/filter/</tag>
	This is similar to the <cf>import</cf> keyword, except that it works in
	the direction from the routing table to the protocol. Default: <cf/none/.

	<tag><label id="proto-import-keep-filtered">import keep filtered <m/switch/</tag>
	Usually, if an import filter rejects a route, the route is forgotten.
	When this option is active, these routes are kept in the routing table,
	but they are hidden and not propagated to other protocols. But it is
	possible to show them using <cf/show route filtered/. Note that this
	option does not work for the pipe protocol. Default: off.

	<tag><label id="proto-import-limit">import limit [<m/number/ | off ] [action warn | block | restart | disable]</tag>
	Specify an import route limit (a maximum number of routes imported from
	the protocol) and optionally the action to be taken when the limit is
	hit. Warn action just prints warning log message. Block action discards
	new routes coming from the protocol. Restart and disable actions shut
	the protocol down like appropriate commands. Disable is the default
	action if an action is not explicitly specified. Note that limits are
	reset during protocol reconfigure, reload or restart. Default: <cf/off/.

	<tag><label id="proto-receive-limit">receive limit [<m/number/ | off ] [action warn | block | restart | disable]</tag>
	Specify an receive route limit (a maximum number of routes received from
	the protocol and remembered). It works almost identically to <cf>import
	limit</cf> option, the only difference is that if <cf/import keep
	filtered/ option is active, filtered routes are counted towards the
	limit and blocked routes are forgotten, as the main purpose of the
	receive limit is to protect routing tables from overflow. Import limit,
	on the contrary, counts accepted routes only and routes blocked by the
	limit are handled like filtered routes. Default: <cf/off/.

	<tag><label id="proto-export-limit">export limit [ <m/number/ | off ] [action warn | block | restart | disable]</tag>
	Specify an export route limit, works similarly to the <cf>import
	limit</cf> option, but for the routes exported to the protocol. This
	option is experimental, there are some problems in details of its
	behavior -- the number of exported routes can temporarily exceed the
	limit without triggering it during protocol reload, exported routes
	counter ignores route blocking and block action also blocks route
	updates of already accepted routes -- and these details will probably
	change in the future. Default: <cf/off/.

	<tag><label id="proto-description">description "<m/text/"</tag>
	This is an optional description of the protocol. It is displayed as a
	part of the output of 'show route all' command.

	<tag><label id="proto-table">table <m/name/</tag>
	Connect this protocol to a non-default routing table.

	<tag><label id="proto-vrf">vrf "<m/text/"</tag>
	Associate the protocol with specific VRF. The protocol will be
	restricted to interfaces assigned to the VRF and will use sockets bound
	to the VRF. Appropriate VRF interface must exist on OS level. For kernel
	protocol, an appropriate table still must be explicitly selected by
	<cf/table/ option. Note that for proper VRF support it is necessary to
	use Linux kernel version at least 4.14, older versions have limited
	VRF implementation.
</descrip>

<p>There are several options that give sense only with certain protocols:

<descrip>
	<tag><label id="proto-iface">interface [-] [ "<m/mask/" ] [ <m/prefix/ ] [, <m/.../] [ { <m/option/; [<m/.../] } ]</tag>
	Specifies a set of interfaces on which the protocol is activated with
	given interface-specific options. A set of interfaces specified by one
	interface option is described using an interface pattern. The interface
	pattern consists of a sequence of clauses (separated by commas), each
	clause is a mask specified as a shell-like pattern. Interfaces are
	matched by their name.

	An interface matches the pattern if it matches any of its clauses. If
	the clause begins with <cf/-/, matching interfaces are excluded. Patterns
	are processed left-to-right, thus <cf/interface "eth0", -"eth*", "*";/
	means eth0 and all non-ethernets.

	Some protocols (namely OSPFv2 and Direct) support extended clauses that
	may contain a mask, a prefix, or both of them. An interface matches such
	clause if its name matches the mask (if specified) and its address
	matches the prefix (if specified). Extended clauses are used when the
	protocol handles multiple addresses on an interface independently.

	An interface option can be used more times with different interface-specific
	options, in that case for given interface the first matching interface
	option is used.

	This option is allowed in Babel, BFD, Direct, OSPF, RAdv and RIP
	protocols, but in OSPF protocol it is used in the <cf/area/ subsection.

	Default: none.

	Examples:

	<cf>interface "*" { type broadcast; };</cf> - start the protocol on all
	interfaces with <cf>type broadcast</cf> option.

	<cf>interface "eth1", "eth4", "eth5" { type ptp; };</cf> - start the
	protocol on enumerated interfaces with <cf>type ptp</cf> option.

	<cf>interface -192.168.1.0/24, 192.168.0.0/16;</cf> - start the protocol
	on all interfaces that have address from 192.168.0.0/16, but not from
	192.168.1.0/24.

	<cf>interface -192.168.1.0/24, 192.168.0.0/16;</cf> - start the protocol
	on all interfaces that have address from 192.168.0.0/16, but not from
	192.168.1.0/24.

	<cf>interface "eth*" 192.168.1.0/24;</cf> - start the protocol on all
	ethernet interfaces that have address from 192.168.1.0/24.

	<tag><label id="proto-tx-class">tx class|dscp <m/num/</tag>
	This option specifies the value of ToS/DS/Class field in IP headers of
	the outgoing protocol packets. This may affect how the protocol packets
	are processed by the network relative to the other network traffic. With
	<cf/class/ keyword, the value (0-255) is used for the whole ToS/Class
	octet (but two bits reserved for ECN are ignored). With	<cf/dscp/
	keyword, the value (0-63) is used just for the DS field in the octet.
	Default value is 0xc0 (DSCP 0x30 - CS6).

	<tag><label id="proto-tx-priority">tx priority <m/num/</tag>
	This option specifies the local packet priority. This may affect how the
	protocol packets are processed in the local TX queues. This option is
	Linux specific. Default value is 7 (highest priority, privileged traffic).

	<tag><label id="proto-pass">password "<m/password/" [ { <m>password options</m> } ]</tag>
	Specifies a password that can be used by the protocol as a shared secret
	key. Password option can be used more times to specify more passwords.
	If more passwords are specified, it is a protocol-dependent decision
	which one is really used. Specifying passwords does not mean that
	authentication is enabled, authentication can be enabled by separate,
	protocol-dependent <cf/authentication/ option.

	This option is allowed in BFD, OSPF and RIP protocols. BGP has also
	<cf/password/ option, but it is slightly different and described
	separately.
	Default: none.
</descrip>

<p>Password option can contain section with some (not necessary all) password sub-options:

<descrip>
	<tag><label id="proto-pass-id">id <M>num</M></tag>
	ID of the password, (1-255). If it is not used, BIRD will choose ID based
	on an order of the password item in the interface. For example, second
	password item in one interface will have default ID 2. ID is used by
	some routing protocols to identify which password was used to
	authenticate protocol packets.

	<tag><label id="proto-pass-gen-from">generate from "<m/time/"</tag>
	The start time of the usage of the password for packet signing.
	The format of <cf><m/time/</cf> is <tt>dd-mm-yyyy HH:MM:SS</tt>.

	<tag><label id="proto-pass-gen-to">generate to "<m/time/"</tag>
	The last time of the usage of the password for packet signing.

	<tag><label id="proto-pass-accept-from">accept from "<m/time/"</tag>
	The start time of the usage of the password for packet verification.

	<tag><label id="proto-pass-accept-to">accept to "<m/time/"</tag>
	The last time of the usage of the password for packet verification.

	<tag><label id="proto-pass-from">from "<m/time/"</tag>
	Shorthand for setting both <cf/generate from/ and <cf/accept from/.

	<tag><label id="proto-pass-to">to "<m/time/"</tag>
	Shorthand for setting both <cf/generate to/ and <cf/accept to/.

	<tag><label id="proto-pass-algorithm">algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 )</tag>
	The message authentication algorithm for the password when cryptographic
	authentication is enabled. The default value depends on the protocol.
	For RIP and OSPFv2 it is Keyed-MD5 (for compatibility), for OSPFv3
	protocol it is HMAC-SHA-256.

</descrip>

<chapt>Remote control
<label id="remote-control">

<p>You can use the command-line client <file>birdc</file> to talk with a running
BIRD. Communication is done using a <file/bird.ctl/ UNIX domain socket (unless
changed with the <tt/-s/ option given to both the server and the client). The
commands can perform simple actions such as enabling/disabling of protocols,
telling BIRD to show various information, telling it to show routing table
filtered by filter, or asking BIRD to reconfigure. Press <tt/?/ at any time to
get online help. Option <tt/-r/ can be used to enable a restricted mode of BIRD
client, which allows just read-only commands (<cf/show .../). Option <tt/-v/ can
be passed to the client, to make it dump numeric return codes along with the
messages. You do not necessarily need to use <file/birdc/ to talk to BIRD, your
own applications could do that, too -- the format of communication between BIRD
and <file/birdc/ is stable (see the programmer's documentation).

<p>There is also lightweight variant of BIRD client called <file/birdcl/, which
does not support command line editing and history and has minimal dependencies.
This is useful for running BIRD in resource constrained environments, where
Readline library (required for regular BIRD client) is not available.

<p>Many commands have the <m/name/ of the protocol instance as an argument.
This argument can be omitted if there exists only a single instance.

<p>Here is a brief list of supported functions:

<descrip>
	<tag><label id="cli-show-status">show status</tag>
	Show router status, that is BIRD version, uptime and time from last
	reconfiguration.

	<tag><label id="cli-show-interfaces">show interfaces [summary]</tag>
	Show the list of interfaces. For each interface, print its type, state,
	MTU and addresses assigned.

	<tag><label id="cli-show-protocols">show protocols [all]</tag>
	Show list of protocol instances along with tables they are connected to
	and protocol status, possibly giving verbose information, if <cf/all/ is
	specified.

	<tag><label id="cli-show-ospf-iface">show ospf interface [<m/name/] ["<m/interface/"]</tag>
	Show detailed information about OSPF interfaces.

	<tag><label id="cli-show-ospf-neighbors">show ospf neighbors [<m/name/] ["<m/interface/"]</tag>
	Show a list of OSPF neighbors and a state of adjacency to them.

	<tag><label id="cli-show-ospf-state">show ospf state [all] [<m/name/]</tag>
	Show detailed information about OSPF areas based on a content of the
	link-state database. It shows network topology, stub networks,
	aggregated networks and routers from other areas and external routes.
	The command shows information about reachable network nodes, use option
	<cf/all/ to show information about all network nodes in the link-state
	database.

	<tag><label id="cli-show-ospf-topology">show ospf topology [all] [<m/name/]</tag>
	Show a topology of OSPF areas based on a content of the link-state
	database. It is just a stripped-down version of 'show ospf state'.

	<tag><label id="cli-show-ospf-lsadb">show ospf lsadb [global | area <m/id/ | link] [type <m/num/] [lsid <m/id/] [self | router <m/id/] [<m/name/] </tag>
	Show contents of an OSPF LSA database. Options could be used to filter
	entries.

	<tag><label id="cli-show-rip-interfaces">show rip interfaces [<m/name/] ["<m/interface/"]</tag>
	Show detailed information about RIP interfaces.

	<tag><label id="cli-show-rip-neighbors">show rip neighbors [<m/name/] ["<m/interface/"]</tag>
	Show a list of RIP neighbors and associated state.

	<tag><label id="cli-show-static">show static [<m/name/]</tag>
	Show detailed information about static routes.

	<tag><label id="cli-show-bfd-sessions">show bfd sessions [<m/name/]</tag>
	Show information about BFD sessions.

	<tag><label id="cli-show-symbols">show symbols [table|filter|function|protocol|template|roa|<m/symbol/]</tag>
	Show the list of symbols defined in the configuration (names of
	protocols, routing tables etc.).

	<tag><label id="cli-show-route">show route [[for] <m/prefix/|<m/IP/] [table <m/t/] [filter <m/f/|where <m/c/] [(export|preexport|noexport) <m/p/] [protocol <m/p/] [<m/options/]</tag>
	Show contents of a routing table (by default of the main one or the
	table attached to a respective protocol), that is routes, their metrics
	and (in case the <cf/all/ switch is given) all their attributes.

	<p>You can specify a <m/prefix/ if you want to print routes for a
	specific network. If you use <cf>for <m/prefix or IP/</cf>, you'll get
	the entry which will be used for forwarding of packets to the given
	destination. By default, all routes for each network are printed with
	the selected one at the top, unless <cf/primary/ is given in which case
	only the selected route is shown.

	<p>You can also ask for printing only routes processed and accepted by
	a given filter (<cf>filter <m/name/</cf> or <cf>filter { <m/filter/ }
	</cf> or matching a given condition (<cf>where <m/condition/</cf>).

	The <cf/export/, <cf/preexport/ and <cf/noexport/ switches ask for
	printing of routes that are exported to the specified protocol.
	With <cf/preexport/, the export filter of the protocol is skipped.
	With <cf/noexport/, routes rejected by the export filter are printed
	instead. Note that routes not exported to the protocol for other reasons
	(e.g. secondary routes or routes imported from that protocol) are not
	printed even with <cf/noexport/.

	<p>You can also select just routes added by a specific protocol.
	<cf>protocol <m/p/</cf>.

	<p>If BIRD is configured to keep filtered routes (see <cf/import keep
	filtered/ option), you can show them instead of routes by using
	<cf/filtered/ switch.

	<p>The <cf/stats/ switch requests showing of route statistics (the
	number of networks, number of routes before and after filtering). If
	you use <cf/count/ instead, only the statistics will be printed.

	<tag><label id="cli-mrt-dump">mrt dump table <m/name/|"<m/pattern/" to "<m/filename/" [filter <m/f/|where <m/c/]</tag>
	Dump content of a routing table to a specified file in MRT table dump
	format. See <ref id="mrt" name="MRT protocol"> for details.

	<tag><label id="cli-show-roa">show roa [<m/prefix/ | in <m/prefix/ | for <m/prefix/] [as <m/num/] [table <m/t/]</tag>
	Show contents of a ROA table (by default of the first one). You can
	specify a <m/prefix/ to print ROA entries for a specific network. If you
	use <cf>for <m/prefix/</cf>, you'll get all entries relevant for route
	validation of the network prefix; i.e., ROA entries whose prefixes cover
	the network prefix. Or you can use <cf>in <m/prefix/</cf> to get ROA
	entries covered by the network prefix. You could also use <cf/as/ option
	to show just entries for given AS.

	<tag><label id="cli-add-roa">add roa <m/prefix/ max <m/num/ as <m/num/ [table <m/t/]</tag>
	Add a new ROA entry to a ROA table. Such entry is called <it/dynamic/
	compared to <it/static/ entries specified in the config file. These
	dynamic entries survive reconfiguration.

	<tag><label id="cli-delete-roa">delete roa <m/prefix/ max <m/num/ as <m/num/ [table <m/t/]</tag>
	Delete the specified ROA entry from a ROA table. Only dynamic ROA
	entries (i.e., the ones added by <cf/add roa/ command) can be deleted.

	<tag><label id="cli-flush-roa">flush roa [table <m/t/]</tag>
	Remove all dynamic ROA entries from a ROA table.

	<tag><label id="cli-configure">configure [soft] ["<m/config file/"] [timeout [<m/num/]]</tag>
	Reload configuration from a given file. BIRD will smoothly switch itself
	to the new configuration, protocols are reconfigured if possible,
	restarted otherwise. Changes in filters usually lead to restart of
	affected protocols.

	If <cf/soft/ option is used, changes in filters does not cause BIRD to
	restart affected protocols, therefore already accepted routes (according
	to old filters) would be still propagated, but new routes would be
	processed according to the new filters.

	If <cf/timeout/ option is used, config timer is activated. The new
	configuration could be either confirmed using <cf/configure confirm/
	command, or it will be reverted to the old one when the config timer
	expires. This is useful for cases when reconfiguration breaks current
	routing and a router becomes inaccessible for an administrator. The
	config timeout expiration is equivalent to <cf/configure undo/
	command. The timeout duration could be specified, default is 300 s.

	<tag><label id="cli-configure-confirm">configure confirm</tag>
	Deactivate the config undo timer and therefore confirm the current
	configuration.

	<tag><label id="cli-configure-undo">configure undo</tag>
	Undo the last configuration change and smoothly switch back to the
	previous (stored) configuration. If the last configuration change was
	soft, the undo change is also soft. There is only one level of undo, but
	in some specific cases when several reconfiguration requests are given
	immediately in a row and the intermediate ones are skipped then the undo
	also skips them back.

	<tag><label id="cli-configure-check">configure check ["<m/config file/"]</tag>
	Read and parse given config file, but do not use it. useful for checking
	syntactic and some semantic validity of an config file.

	<tag><label id="cli-enable-disable-restart">enable|disable|restart <m/name/|"<m/pattern/"|all</tag>
	Enable, disable or restart a given protocol instance, instances matching
	the <cf><m/pattern/</cf> or <cf/all/ instances.

	<tag><label id="cli-reload">reload [in|out] <m/name/|"<m/pattern/"|all</tag>
	Reload a given protocol instance, that means re-import routes from the
	protocol instance and re-export preferred routes to the instance. If
	<cf/in/ or <cf/out/ options are used, the command is restricted to one
	direction (re-import or re-export).

	This command is useful if appropriate filters have changed but the
	protocol instance was not restarted (or reloaded), therefore it still
	propagates the old set of routes. For example when <cf/configure soft/
	command was used to change filters.

	Re-export always succeeds, but re-import is protocol-dependent and might
	fail (for example, if BGP neighbor does not support route-refresh
	extension). In that case, re-export is also skipped. Note that for the
	pipe protocol, both directions are always reloaded together (<cf/in/ or
	<cf/out/ options are ignored in that case).

	<tag><label id="cli-down">down</tag>
	Shut BIRD down.

	<tag><label id="cli-debug">debug <m/protocol/|<m/pattern/|all all|off|{ states|routes|filters|events|packets [, <m/.../] }</tag>
	Control protocol debugging.

	<tag><label id="cli-dump">dump resources|sockets|interfaces|neighbors|attributes|routes|protocols</tag>
	Dump contents of internal data structures to the debugging output.

	<tag><label id="cli-echo">echo all|off|{ <m/list of log classes/ } [ <m/buffer-size/ ]</tag>
	Control echoing of log messages to the command-line output.
	See <ref id="opt-log" name="log option"> for a list of log classes.

	<tag><label id="cli-eval">eval <m/expr/</tag>
	Evaluate given expression.
</descrip>


<chapt>Filters
<label id="filters">

<sect>Introduction
<label id="filters-intro">

<p>BIRD contains a simple programming language. (No, it can't yet read mail :-).
There are two objects in this language: filters and functions. Filters are
interpreted by BIRD core when a route is being passed between protocols and
routing tables. The filter language contains control structures such as if's and
switches, but it allows no loops. An example of a filter using many features can
be found in <file>filter/test.conf</file>.

<p>Filter gets the route, looks at its attributes and modifies some of them if
it wishes. At the end, it decides whether to pass the changed route through
(using <cf/accept/) or whether to <cf/reject/ it. A simple filter looks like
this:

<code>
filter not_too_far
int var;
{
	if defined( rip_metric ) then
		var = rip_metric;
	else {
		var = 1;
		rip_metric = 1;
	}
	if rip_metric &gt; 10 then
		reject "RIP metric is too big";
	else
		accept "ok";
}
</code>

<p>As you can see, a filter has a header, a list of local variables, and a body.
The header consists of the <cf/filter/ keyword followed by a (unique) name of
filter. The list of local variables consists of <cf><M>type name</M>;</cf>
pairs where each pair defines one local variable. The body consists of <cf>
{ <M>statements</M> }</cf>. Each <m/statement/ is terminated by a <cf/;/. You
can group several statements to a single compound statement by using braces
(<cf>{ <M>statements</M> }</cf>) which is useful if you want to make a bigger
block of code conditional.

<p>BIRD supports functions, so that you don't have to repeat the same blocks of
code over and over. Functions can have zero or more parameters and they can have
local variables. Recursion is not allowed. Function definitions look like this:

<code>
function name ()
int local_variable;
{
	local_variable = 5;
}

function with_parameters (int parameter)
{
	print parameter;
}
</code>

<p>Unlike in C, variables are declared after the <cf/function/ line, but before
the first <cf/{/. You can't declare variables in nested blocks. Functions are
called like in C: <cf>name(); with_parameters(5);</cf>. Function may return
values using the <cf>return <m/[expr]/</cf> command. Returning a value exits
from current function (this is similar to C).

<p>Filters are declared in a way similar to functions except they can't have
explicit parameters. They get a route table entry as an implicit parameter, it
is also passed automatically to any functions called. The filter must terminate
with either <cf/accept/ or <cf/reject/ statement. If there's a runtime error in
filter, the route is rejected.

<p>A nice trick to debug filters is to use <cf>show route filter <m/name/</cf>
from the command line client. An example session might look like:

<code>
pavel@bug:~/bird$ ./birdc -s bird.ctl
BIRD 0.0.0 ready.
bird> show route
10.0.0.0/8         dev eth0 [direct1 23:21] (240)
195.113.30.2/32    dev tunl1 [direct1 23:21] (240)
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird> show route ?
show route [<prefix>] [table <t>] [filter <f>] [all] [primary]...
bird> show route filter { if 127.0.0.5 &tilde; net then accept; }
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird>
</code>


<sect>Data types
<label id="data-types">

<p>Each variable and each value has certain type. Booleans, integers and enums
are incompatible with each other (that is to prevent you from shooting in the
foot).

<descrip>
	<tag><label id="type-bool">bool</tag>
	This is a boolean type, it can have only two values, <cf/true/ and
	<cf/false/. Boolean is the only type you can use in <cf/if/ statements.

	<tag><label id="type-int">int</tag>
	This is a general integer type. It is an unsigned 32bit type; i.e., you
	can expect it to store values from 0 to 4294967295. Overflows are not
	checked. You can use <cf/0x1234/ syntax to write hexadecimal values.

	<tag><label id="type-pair">pair</tag>
	This is a pair of two short integers. Each component can have values
	from 0 to 65535. Literals of this type are written as <cf/(1234,5678)/.
	The same syntax can also be used to construct a pair from two arbitrary
	integer expressions (for example <cf/(1+2,a)/).

	<tag><label id="type-quad">quad</tag>
	This is a dotted quad of numbers used to represent router IDs (and
	others). Each component can have a value from 0 to 255. Literals of
	this type are written like IPv4 addresses.

	<tag><label id="type-string">string</tag>
	This is a string of characters. There are no ways to modify strings in
	filters. You can pass them between functions, assign them to variables
	of type <cf/string/, print such variables, use standard string
	comparison operations (e.g. <cf/=, !=, &lt;, &gt;, &lt;=, &gt;=/), but
	you can't concatenate two strings. String literals are written as
	<cf/"This is a string constant"/. Additionally matching (<cf/&tilde;,
	!&tilde;/) operators could be used to match a string value against
	a shell pattern (represented also as a string).

	<tag><label id="type-ip">ip</tag>
	This type can hold a single IP address. Depending on the compile-time
	configuration of BIRD you are using, it is either an IPv4 or IPv6
	address. IP addresses are written in the standard notation
	(<cf/10.20.30.40/ or <cf/fec0:3:4::1/). You can apply special operator
	<cf>.mask(<M>num</M>)</cf> on values of type ip. It masks out all but
	first <cf><M>num</M></cf> bits from the IP address. So
	<cf/1.2.3.4.mask(8) = 1.0.0.0/ is true.

	<tag><label id="type-prefix">prefix</tag>
	This type can hold a network prefix consisting of IP address and prefix
	length. Prefix literals are written as <cf><m/ipaddress//<m/pxlen/</cf>,
	or <cf><m>ipaddress</m>/<m>netmask</m></cf>. There are two special
	operators on prefixes: <cf/.ip/ which extracts the IP address from the
	pair, and <cf/.len/, which separates prefix length from the pair.
	So <cf>1.2.0.0/16.len = 16</cf> is true.

	<tag><label id="type-ec">ec</tag>
	This is a specialized type used to represent BGP extended community
	values. It is essentially a 64bit value, literals of this type are
	usually written as <cf>(<m/kind/, <m/key/, <m/value/)</cf>, where
	<cf/kind/ is a kind of extended community (e.g. <cf/rt/ / <cf/ro/ for a
	route target / route origin communities), the format and possible values
	of <cf/key/ and <cf/value/ are usually integers, but it depends on the
	used kind. Similarly to pairs, ECs can be constructed using expressions
	for <cf/key/ and <cf/value/ parts, (e.g. <cf/(ro, myas, 3*10)/, where
	<cf/myas/ is an integer variable).

	<tag><label id="type-lc">lc</tag>
	This is a specialized type used to represent BGP large community
	values. It is essentially a triplet of 32bit values, where the first
	value is reserved for the AS number of the issuer, while meaning of
	remaining parts is defined by the issuer. Literals of this type are
	written as <cf/(123, 456, 789)/, with any integer values. Similarly to
	pairs, LCs can be constructed using expressions for its parts, (e.g.
	<cf/(myas, 10+20, 3*10)/, where <cf/myas/ is an integer variable).

	<tag><label id="type-set">int|pair|quad|ip|prefix|ec|lc|enum set</tag>
	Filters recognize four types of sets. Sets are similar to strings: you
	can pass them around but you can't modify them. Literals of type <cf>int
	set</cf> look like <cf> [ 1, 2, 5..7 ]</cf>. As you can see, both simple
	values and ranges are permitted in sets.

	For pair sets, expressions like <cf/(123,*)/ can be used to denote
	ranges (in that case <cf/(123,0)..(123,65535)/). You can also use
	<cf/(123,5..100)/ for range <cf/(123,5)..(123,100)/. You can also use
	<cf/*/ and <cf/a..b/ expressions in the first part of a pair, note that
	such expressions are translated to a set of intervals, which may be
	memory intensive. E.g. <cf/(*,4..20)/ is translated to <cf/(0,4..20),
	(1,4..20), (2,4..20), ... (65535, 4..20)/.

	EC sets use similar expressions like pair sets, e.g. <cf/(rt, 123,
	10..20)/ or <cf/(ro, 123, *)/. Expressions requiring the translation
	(like <cf/(rt, *, 3)/) are not allowed (as they usually have 4B range
	for ASNs).

	Also LC sets use similar expressions like pair sets. You can use ranges
	and wildcards, but if one field uses that, more specific (later) fields
	must be wildcards. E.g., <cf/(10, 20..30, *)/ or <cf/(10, 20, 30..40)/
	is valid, while <cf/(10, *, 20..30)/ or <cf/(10, 20..30, 40)/ is not
	valid.

	You can also use expressions for int, pair, EC and LC set values.
	However, it must be possible to evaluate these expressions before daemon
	boots. So you can use only constants inside them. E.g.

	<code>
	 define one=1;
	 define myas=64500;
	 int set odds;
	 pair set ps;
	 ec set es;

	 odds = [ one, 2+1, 6-one, 2*2*2-1, 9, 11 ];
	 ps = [ (1,one+one), (3,4)..(4,8), (5,*), (6,3..6), (7..9,*) ];
	 es = [ (rt, myas, 3*10), (rt, myas+one, 0..16*16*16-1), (ro, myas+2, *) ];
	</code>

	Sets of prefixes are special: their literals does not allow ranges, but
	allows prefix patterns that are written
	as <cf><M>ipaddress</M>/<M>pxlen</M>{<M>low</M>,<M>high</M>}</cf>.
	Prefix <cf><m>ip1</m>/<m>len1</m></cf> matches prefix
	pattern <cf><m>ip2</m>/<m>len2</m>{<m>l</m>,<m>h</m>}</cf> if the
	first <cf>min(len1, len2)</cf> bits of <cf/ip1/ and <cf/ip2/ are
	identical and <cf>len1 &lt;= ip1 &lt;= len2</cf>. A valid prefix pattern
	has to satisfy <cf>low &lt;= high</cf>, but <cf/pxlen/ is not
	constrained by <cf/low/ or <cf/high/. Obviously, a prefix matches a
	prefix set literal if it matches any prefix pattern in the prefix set
	literal.

	There are also two shorthands for prefix patterns: <cf><m/address//<m/len/+</cf>
	is a shorthand for <cf><m/address//<m/len/{<m/len/,<m/maxlen/}</cf>
	(where <cf><m/maxlen/</cf> is 32 for IPv4 and 128 for IPv6), that means
	network prefix <cf><m/address//<m/len/</cf> and all its	subnets.
	<cf><m/address//<m/len/-</cf> is a shorthand for
	<cf><m/address//<m/len/{0,<m/len/}</cf>, that means network prefix
	<cf><m/address//<m/len/</cf> and all its supernets (network prefixes
	that contain it).

	For example, <cf>[ 1.0.0.0/8, 2.0.0.0/8+, 3.0.0.0/8-, 4.0.0.0/8{16,24}
	]</cf> matches prefix <cf>1.0.0.0/8</cf>, all subprefixes of
	<cf>2.0.0.0/8</cf>, all superprefixes of <cf>3.0.0.0/8</cf> and prefixes
	<cf/4.X.X.X/ whose prefix length is 16 to 24. <cf>[ 0.0.0.0/0{20,24} ]</cf>
	matches all prefixes (regardless of IP address) whose prefix length is
	20 to 24, <cf>[ 1.2.3.4/32- ]</cf> matches any prefix that contains IP
	address <cf>1.2.3.4</cf>. <cf>1.2.0.0/16 &tilde; [ 1.0.0.0/8{15,17} ]</cf>
	is true, but <cf>1.0.0.0/16 &tilde; [ 1.0.0.0/8- ]</cf> is false.

	Cisco-style patterns like <cf>10.0.0.0/8 ge 16 le 24</cf> can be expressed
	in BIRD as <cf>10.0.0.0/8{16,24}</cf>, <cf>192.168.0.0/16 le 24</cf> as
	<cf>192.168.0.0/16{16,24}</cf> and <cf>192.168.0.0/16 ge 24</cf> as
	<cf>192.168.0.0/16{24,32}</cf>.

	<tag><label id="type-enum">enum</tag>
	Enumeration types are fixed sets of possibilities. You can't define your
	own variables of such type, but some route attributes are of enumeration
	type. Enumeration types are incompatible with each other.

	<tag><label id="type-bgppath">bgppath</tag>
	BGP path is a list of autonomous system numbers. You can't write
	literals of this type. There are several special operators on bgppaths:

	<cf><m/P/.first</cf> returns the first ASN (the neighbor ASN) in path <m/P/.

	<cf><m/P/.last</cf> returns the last ASN (the source ASN) in path <m/P/.

	<cf><m/P/.last_nonaggregated</cf> returns the last ASN in the non-aggregated part of the path <m/P/.

	Both <cf/first/ and <cf/last/ return zero if there is no appropriate
	ASN, for example if the path contains an AS set element as the first (or
	the last) part. If the path ends with an AS set, <cf/last_nonaggregated/
	may be used to get last ASN before any AS set.

	<cf><m/P/.len</cf> returns the length of path <m/P/.

	<cf><m/P/.empty</cf> resets path <m/P/ to empty path.

	<cf>prepend(<m/P/,<m/A/)</cf> prepends ASN <m/A/ to path <m/P/ and
	returns the result.

	<cf>delete(<m/P/,<m/A/)</cf> deletes all instances of ASN <m/A/ from
	from path <m/P/ and returns the result. <m/A/ may also be an integer
	set, in that case the operator deletes all ASNs from path <m/P/ that are
	also members of set <m/A/.

	<cf>filter(<m/P/,<m/A/)</cf> deletes all ASNs from path <m/P/ that are
	not members of integer set <m/A/. I.e., <cf/filter/ do the same as
	<cf/delete/ with inverted set <m/A/.

	Statement <cf><m/P/ = prepend(<m/P/, <m/A/);</cf> can be shortened to
	<cf><m/P/.prepend(<m/A/);</cf> if <m/P/ is appropriate route attribute
	(for example <cf/bgp_path/). Similarly for <cf/delete/ and <cf/filter/.

	<tag><label id="type-bgpmask">bgpmask</tag>
	BGP masks are patterns used for BGP path matching (using <cf>path
	&tilde; [= 2 3 5 * =]</cf> syntax). The masks resemble wildcard patterns
	as used by UNIX shells. Autonomous system numbers match themselves,
	<cf/*/ matches any (even empty) sequence of arbitrary AS numbers and
	<cf/?/ matches one arbitrary AS number. For example, if <cf>bgp_path</cf>
 	is 4 3 2 1, then: <tt>bgp_path &tilde; [= * 4 3 * =]</tt> is true,
	but <tt>bgp_path &tilde; [= * 4 5 * =]</tt> is false. BGP mask
	expressions can also contain integer expressions enclosed in parenthesis
	and integer variables, for example <tt>[= * 4 (1+2) a =]</tt>. You can
        also use ranges, for example <tt>[= * 3..5 2 100..200 * =]</tt>.
        There is also old (deprecated) syntax that uses / .. / instead of [= .. =]
        and ? instead of *.

	<tag><label id="type-clist">clist</tag>
	Clist is similar to a set, except that unlike other sets, it can be
	modified. The type is used for community list (a set of pairs) and for
	cluster list (a set of quads). There exist no literals of this type.
	There are three special operators on clists:

	<cf><m/C/.len</cf> returns the length of clist <m/C/.

	<cf><m/C/.empty</cf> resets clist <m/C/ to empty clist.

	<cf>add(<m/C/,<m/P/)</cf> adds pair (or quad) <m/P/ to clist <m/C/ and
	returns the result. If item <m/P/ is already in clist <m/C/, it does
	nothing. <m/P/ may also be a clist, in that case all its members are
	added; i.e., it works as clist union.

	<cf>delete(<m/C/,<m/P/)</cf> deletes pair (or quad) <m/P/ from clist
	<m/C/ and returns the result. If clist <m/C/ does not contain item
	<m/P/, it does nothing. <m/P/ may also be a pair (or quad) set, in that
	case the operator deletes all items from clist <m/C/ that are also
	members of set <m/P/. Moreover, <m/P/ may also be a clist, which works
	analogously; i.e., it works as clist difference.

	<cf>filter(<m/C/,<m/P/)</cf> deletes all items from clist <m/C/ that are
	not members of pair (or quad) set <m/P/. I.e., <cf/filter/ do the same
	as <cf/delete/ with inverted set <m/P/. <m/P/ may also be a clist, which
	works analogously; i.e., it works as clist intersection.

	Statement <cf><m/C/ = add(<m/C/, <m/P/);</cf> can be shortened to
	<cf><m/C/.add(<m/P/);</cf> if <m/C/ is appropriate route attribute (for
	example <cf/bgp_community/). Similarly for <cf/delete/ and <cf/filter/.

	<tag><label id="type-eclist">eclist</tag>
	Eclist is a data type used for BGP extended community lists. Eclists
	are very similar to clists, but they are sets of ECs instead of pairs.
	The same operations (like <cf/add/, <cf/delete/ or <cf/&tilde;/ and
	<cf/!&tilde;/ membership operators) can be used to modify or test
	eclists, with ECs instead of pairs as arguments.

	<tag/lclist/
	Lclist is a data type used for BGP large community lists. Like eclists,
	lclists are very similar to clists, but they are sets of LCs instead of
	pairs. The same operations (like <cf/add/, <cf/delete/ or <cf/&tilde;/
	and <cf/!&tilde;/ membership operators) can be used to modify or test
	lclists, with LCs instead of pairs as arguments.
</descrip>


<sect>Operators
<label id="operators">

<p>The filter language supports common integer operators <cf>(+,-,*,/)</cf>,
parentheses <cf/(a*(b+c))/, comparison <cf/(a=b, a!=b, a&lt;b, a&gt;=b)/.
Logical operations include unary not (<cf/!/), and (<cf/&amp;&amp;/) and or
(<cf/&verbar;&verbar;/). Special operators include (<cf/&tilde;/,
<cf/!&tilde;/) for "is (not) element of a set" operation - it can be used on
element and set of elements of the same type (returning true if element is
contained in the given set), or on two strings (returning true if first string
matches a shell-like pattern stored in second string) or on IP and prefix
(returning true if IP is within the range defined by that prefix), or on prefix
and prefix (returning true if first prefix is more specific than second one) or
on bgppath and bgpmask (returning true if the path matches the mask) or on
number and bgppath (returning true if the number is in the path) or on bgppath
and int (number) set (returning true if any ASN from the path is in the set) or
on pair/quad and clist (returning true if the pair/quad is element of the
clist) or on clist and pair/quad set (returning true if there is an element of
the clist that is also a member of the pair/quad set).

<p>There is one operator related to ROA infrastructure - <cf/roa_check()/. It
examines a ROA table and does <rfc id="6483"> route origin validation for a
given network prefix. The basic usage is <cf>roa_check(<m/table/)</cf>, which
checks current route (which should be from BGP to have AS_PATH argument) in the
specified ROA table and returns ROA_UNKNOWN if there is no relevant ROA,
ROA_VALID if there is a matching ROA, or ROA_INVALID if there are some relevant
ROAs but none of them match. There is also an extended variant
<cf>roa_check(<m/table/, <m/prefix/, <m/asn/)</cf>, which allows to specify a
prefix and an ASN as arguments.


<sect>Control structures
<label id="control-structures">

<p>Filters support two control structures: conditions and case switches.

<p>Syntax of a condition is: <cf>if <M>boolean expression</M> then <m/command1/;
else <m/command2/;</cf> and you can use <cf>{ <m/command_1/; <m/command_2/;
<M>...</M> }</cf> instead of either command. The <cf>else</cf> clause may be
omitted. If the <cf><m>boolean expression</m></cf> is true, <m/command1/ is
executed, otherwise <m/command2/ is executed.

<p>The <cf>case</cf> is similar to case from Pascal. Syntax is <cf>case
<m/expr/ { else: | <m/num_or_prefix [ .. num_or_prefix]/: <m/statement/ ; [
... ] }</cf>. The expression after <cf>case</cf> can be of any type which can be
on the left side of the &tilde; operator and anything that could be a member of
a set is allowed before <cf/:/. Multiple commands are allowed without <cf/{}/
grouping. If <cf><m/expr/</cf> matches one of the <cf/:/ clauses, statements
between it and next <cf/:/ statement are executed. If <cf><m/expr/</cf> matches
neither of the <cf/:/ clauses, the statements after <cf/else:/ are executed.

<p>Here is example that uses <cf/if/ and <cf/case/ structures:

<code>
case arg1 {
	2: print "two"; print "I can do more commands without {}";
	3 .. 5: print "three to five";
	else: print "something else";
}

if 1234 = i then printn "."; else {
  print "not 1234";
  print "You need {} around multiple commands";
}
</code>


<sect>Route attributes
<label id="route-attributes">

<p>A filter is implicitly passed a route, and it can access its attributes just
like it accesses variables. Attempts to access undefined attribute result in a
runtime error; you can check if an attribute is defined by using the
<cf>defined( <m>attribute</m> )</cf> operator. One notable exception to this
rule are attributes of clist type, where undefined value is regarded as empty
clist for most purposes.

<descrip>
	<tag><label id="rta-net"><m/prefix/ net</tag>
	Network the route is talking about. Read-only. (See the chapter about
	routing tables.)

	<tag><label id="rta-scope"><m/enum/ scope</tag>
	The scope of the route. Possible values: <cf/SCOPE_HOST/ for routes
	local to this host, <cf/SCOPE_LINK/ for those specific for a physical
	link, <cf/SCOPE_SITE/ and <cf/SCOPE_ORGANIZATION/ for private routes and
	<cf/SCOPE_UNIVERSE/ for globally visible routes. This attribute is not
	interpreted by BIRD and can be used to mark routes in filters. The
	default value for new routes is <cf/SCOPE_UNIVERSE/.

	<tag><label id="rta-preference"><m/int/ preference</tag>
	Preference of the route. Valid values are 0-65535. (See the chapter
	about routing tables.)

	<tag><label id="rta-from"><m/ip/ from</tag>
	The router which the route has originated from.

	<tag><label id="rta-gw"><m/ip/ gw</tag>
	Next hop packets routed using this route should be forwarded to.

	<tag><label id="rta-proto"><m/string/ proto</tag>
	The name of the protocol which the route has been imported from.
	Read-only.

	<tag><label id="rta-source"><m/enum/ source</tag>
	what protocol has told me about this route. Possible values:
	<cf/RTS_DUMMY/, <cf/RTS_STATIC/, <cf/RTS_INHERIT/, <cf/RTS_DEVICE/,
	<cf/RTS_STATIC_DEVICE/, <cf/RTS_REDIRECT/, <cf/RTS_RIP/, <cf/RTS_OSPF/,
	<cf/RTS_OSPF_IA/, <cf/RTS_OSPF_EXT1/, <cf/RTS_OSPF_EXT2/, <cf/RTS_BGP/,
	<cf/RTS_PIPE/, <cf/RTS_BABEL/.

	<tag><label id="rta-cast"><m/enum/ cast</tag>
	Route type (Currently <cf/RTC_UNICAST/ for normal routes,
	<cf/RTC_BROADCAST/, <cf/RTC_MULTICAST/, <cf/RTC_ANYCAST/ will be used in
	the future for broadcast, multicast and anycast routes). Read-only.

	<tag><label id="rta-dest"><m/enum/ dest</tag>
	Type of destination the packets should be sent to
	(<cf/RTD_ROUTER/ for forwarding to a neighboring router,
	<cf/RTD_DEVICE/ for routing to a directly-connected network,
	<cf/RTD_MULTIPATH/ for multipath destinations,
	<cf/RTD_BLACKHOLE/ for packets to be silently discarded,
	<cf/RTD_UNREACHABLE/, <cf/RTD_PROHIBIT/ for packets that should be
	returned with ICMP host unreachable / ICMP administratively prohibited
	messages). Can be changed, but only to <cf/RTD_BLACKHOLE/,
	<cf/RTD_UNREACHABLE/ or <cf/RTD_PROHIBIT/.

	<tag><label id="rta-ifname"><m/string/ ifname</tag>
	Name of the outgoing interface. Sink routes (like blackhole, unreachable
	or prohibit) and multipath routes have no interface associated with
	them, so <cf/ifname/ returns an empty string for such routes. Setting it
	would also change route to a direct one (remove gateway).

	<tag><label id="rta-ifindex"><m/int/ ifindex</tag>
	Index of the outgoing interface. System wide index of the interface. May
	be used for interface matching, however indexes might change on interface
	creation/removal. Zero is returned for routes with undefined outgoing
	interfaces. Read-only.

	<tag><label id="rta-igp-metric"><m/int/ igp_metric</tag>
	The optional attribute that can be used to specify a distance to the
	network for routes that do not have a native protocol metric attribute
	(like <cf/ospf_metric1/ for OSPF routes). It is used mainly by BGP to
	compare internal distances to boundary routers (see below). It is also
	used when the route is exported to OSPF as a default value for OSPF type
	1 metric.
</descrip>

<p>There also exist some protocol-specific attributes which are described in the
corresponding protocol sections.


<sect>Other statements
<label id="other-statements">

<p>The following statements are available:

<descrip>
	<tag><label id="assignment"><m/variable/ = <m/expr/</tag>
	Set variable to a given value.

	<tag><label id="filter-accept-reject">accept|reject [ <m/expr/ ]</tag>
	Accept or reject the route, possibly printing <cf><m>expr</m></cf>.

	<tag><label id="return">return <m/expr/</tag>
	Return <cf><m>expr</m></cf> from the current function, the function ends
	at this point.

	<tag><label id="print">print|printn <m/expr/ [<m/, expr.../]</tag>
	Prints given expressions; useful mainly while debugging filters. The
	<cf/printn/ variant does not terminate the line.

	<tag><label id="quitbird">quitbird</tag>
	Terminates BIRD. Useful when debugging the filter interpreter.
</descrip>


<chapt>Protocols
<label id="protocols">

<sect>Babel
<label id="babel">

<sect1>Introduction
<label id="babel-intro">

<p>The Babel protocol
(<rfc id="6126">) is a loop-avoiding distance-vector routing protocol that is
robust and efficient both in ordinary wired networks and in wireless mesh
networks. Babel is conceptually very simple in its operation and "just works"
in its default configuration, though some configuration is possible and in some
cases desirable.

<p>While the Babel protocol is dual stack (i.e., can carry both IPv4 and IPv6
routes over the same IPv6 transport), BIRD presently implements only the IPv6
subset of the protocol. No Babel extensions are implemented, but the BIRD
implementation can coexist with implementations using the extensions (and will
just ignore extension messages).

<p>The Babel protocol implementation in BIRD is currently in alpha stage.

<sect1>Configuration
<label id="babel-config">

<p>Babel supports no global configuration options apart from those common to all
other protocols, but supports the following per-interface configuration options:

<code>
protocol babel [<name>] {
	interface <interface pattern> {
		type <wired|wireless>;
		rxcost <number>;
		hello interval <number>;
		update interval <number>;
		port <number>;
		tx class|dscp <number>;
		tx priority <number>;
		rx buffer <number>;
		tx length <number>;
		check link <switch>;
	};
}
</code>

<descrip>
      <tag><label id="babel-type">type wired|wireless </tag>
      This option specifies the interface type: Wired or wireless. Wired
      interfaces are considered more reliable, and so the default hello
      interval is higher, and a neighbour is considered unreachable after only
      a small number of "hello" packets are lost. On wireless interfaces,
      hello packets are sent more often, and the ETX link quality estimation
      technique is used to compute the metrics of routes discovered over this
      interface. This technique will gradually degrade the metric of routes
      when packets are lost rather than the more binary up/down mechanism of
      wired type links. Default: <cf/wired/.

      <tag><label id="babel-rxcost">rxcost <m/num/</tag>
      This specifies the RX cost of the interface. The route metrics will be
      computed from this value with a mechanism determined by the interface
      <cf/type/. Default: 96 for wired interfaces, 256 for wireless.

      <tag><label id="babel-hello">hello interval <m/num/</tag>
      Interval at which periodic "hello" messages are sent on this interface,
      in seconds. Default: 4 seconds.

      <tag><label id="babel-update">update interval <m/num/</tag>
      Interval at which periodic (full) updates are sent. Default: 4 times the
      hello interval.

      <tag><label id="babel-port">port <m/number/</tag>
      This option selects an UDP port to operate on. The default is to operate
      on port 6696 as specified in the Babel RFC.

      <tag><label id="babel-tx-class">tx class|dscp|priority <m/number/</tag>
      These options specify the ToS/DiffServ/Traffic class/Priority of the
      outgoing Babel packets. See <ref id="proto-tx-class" name="tx class"> common
      option for detailed description.

      <tag><label id="babel-rx-buffer">rx buffer <m/number/</tag>
      This option specifies the size of buffers used for packet processing.
      The buffer size should be bigger than maximal size of received packets.
      The default value is the interface MTU, and the value will be clamped to a
      minimum of 512 bytes + IP packet overhead.

      <tag><label id="babel-tx-length">tx length <m/number/</tag>
      This option specifies the maximum length of generated Babel packets. To
      avoid IP fragmentation, it should not exceed the interface MTU value.
      The default value is the interface MTU value, and the value will be
      clamped to a minimum of 512 bytes + IP packet overhead.

      <tag><label id="babel-check-link">check link <m/switch/</tag>
      If set, the hardware link state (as reported by OS) is taken into
      consideration. When the link disappears (e.g. an ethernet cable is
      unplugged), neighbors are immediately considered unreachable and all
      routes received from them are withdrawn. It is possible that some
      hardware drivers or platforms do not implement this feature. Default:
      yes.
</descrip>

<sect1>Attributes
<label id="babel-attr">

<p>Babel defines just one attribute: the internal babel metric of the route. It
is exposed as the <cf/babel_metric/ attribute and has range from 1 to infinity
(65535).

<sect1>Example
<label id="babel-exam">

<p><code>
protocol babel {
	interface "eth*" {
		type wired;
	};
	interface "wlan0", "wlan1" {
		type wireless;
		hello interval 1;
		rxcost 512;
	};
	interface "tap0";

	# This matches the default of babeld: redistribute all addresses
	# configured on local interfaces, plus re-distribute all routes received
	# from other babel peers.

	export where (source = RTS_DEVICE) || (source = RTS_BABEL);
}
</code>


<sect>BFD
<label id="bfd">

<sect1>Introduction
<label id="bfd-intro">

<p>Bidirectional Forwarding Detection (BFD) is not a routing protocol itself, it
is an independent tool providing liveness and failure detection. Routing
protocols like OSPF and BGP use integrated periodic "hello" messages to monitor
liveness of neighbors, but detection times of these mechanisms are high (e.g. 40
seconds by default in OSPF, could be set down to several seconds). BFD offers
universal, fast and low-overhead mechanism for failure detection, which could be
attached to any routing protocol in an advisory role.

<p>BFD consists of mostly independent BFD sessions. Each session monitors an
unicast bidirectional path between two BFD-enabled routers. This is done by
periodically sending control packets in both directions. BFD does not handle
neighbor discovery, BFD sessions are created on demand by request of other
protocols (like OSPF or BGP), which supply appropriate information like IP
addresses and associated interfaces. When a session changes its state, these
protocols are notified and act accordingly (e.g. break an OSPF adjacency when
the BFD session went down).

<p>BIRD implements basic BFD behavior as defined in <rfc id="5880"> (some
advanced features like the echo mode or authentication are not implemented), IP
transport for BFD as defined in <rfc id="5881"> and <rfc id="5883"> and
interaction with client protocols as defined in <rfc id="5882">.

<p>Note that BFD implementation in BIRD is currently a new feature in
development, expect some rough edges and possible UI and configuration changes
in the future. Also note that we currently support at most one protocol instance.

<p>BFD packets are sent with a dynamic source port number. Linux systems use by
default a bit different dynamic port range than the IANA approved one
(49152-65535). If you experience problems with compatibility, please adjust
<cf>/proc/sys/net/ipv4/ip_local_port_range</cf>

<sect1>Configuration
<label id="bfd-config">

<p>BFD configuration consists mainly of multiple definitions of interfaces.
Most BFD config options are session specific. When a new session is requested
and dynamically created, it is configured from one of these definitions. For
sessions to directly connected neighbors, <cf/interface/ definitions are chosen
based on the interface associated with the session, while <cf/multihop/
definition is used for multihop sessions. If no definition is relevant, the
session is just created with the default configuration. Therefore, an empty BFD
configuration is often sufficient.

<p>Note that to use BFD for other protocols like OSPF or BGP, these protocols
also have to be configured to request BFD sessions, usually by <cf/bfd/ option.

<p>Some of BFD session options require <m/time/ value, which has to be specified
with the appropriate unit: <m/num/ <cf/s/|<cf/ms/|<cf/us/. Although microseconds
are allowed as units, practical minimum values are usually in order of tens of
milliseconds.

<code>
protocol bfd [&lt;name&gt;] {
	interface &lt;interface pattern&gt; {
		interval &lt;time&gt;;
		min rx interval &lt;time&gt;;
		min tx interval &lt;time&gt;;
		idle tx interval &lt;time&gt;;
		multiplier &lt;num&gt;;
		passive &lt;switch&gt;;
		authentication none;
		authentication simple;
		authentication [meticulous] keyed md5|sha1;
		password "&lt;text&gt;";
		password "&lt;text&gt;" {
			id &lt;num&gt;;
			generate from "&lt;date&gt;";
			generate to "&lt;date&gt;";
			accept from "&lt;date&gt;";
			accept to "&lt;date&gt;";
			from "&lt;date&gt;";
			to "&lt;date&gt;";
		};
	};
	multihop {
		interval &lt;time&gt;;
		min rx interval &lt;time&gt;;
		min tx interval &lt;time&gt;;
		idle tx interval &lt;time&gt;;
		multiplier &lt;num&gt;;
		passive &lt;switch&gt;;
	};
	neighbor &lt;ip&gt; [dev "&lt;interface&gt;"] [local &lt;ip&gt;] [multihop &lt;switch&gt;];
}
</code>

<descrip>
	<tag><label id="bfd-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
	Interface definitions allow to specify options for sessions associated
	with such interfaces and also may contain interface specific options.
	See <ref id="proto-iface" name="interface"> common option for a detailed
	description of interface patterns. Note that contrary to the behavior of
	<cf/interface/ definitions of other protocols, BFD protocol would accept
	sessions (in default configuration) even on interfaces not covered by
	such definitions.

	<tag><label id="bfd-multihop">multihop { <m/options/ }</tag>
	Multihop definitions allow to specify options for multihop BFD sessions,
	in the same manner as <cf/interface/ definitions are used for directly
	connected sessions. Currently only one such definition (for all multihop
	sessions) could be used.

	<tag><label id="bfd-neighbor">neighbor <m/ip/ [dev "<m/interface/"] [local <m/ip/] [multihop <m/switch/]</tag>
	BFD sessions are usually created on demand as requested by other
	protocols (like OSPF or BGP). This option allows to explicitly add
	a BFD session to the specified neighbor regardless of such requests.

	The session is identified by the IP address of the neighbor, with
	optional specification of used interface and local IP. By default
	the neighbor must be directly connected, unless the session is
	configured as multihop. Note that local IP must be specified for
	multihop sessions.
</descrip>

<p>Session specific options (part of <cf/interface/ and <cf/multihop/ definitions):

<descrip>
	<tag><label id="bfd-interval">interval <m/time/</tag>
	BFD ensures availability of the forwarding path associated with the
	session by periodically sending BFD control packets in both
	directions. The rate of such packets is controlled by two options,
	<cf/min rx interval/ and <cf/min tx interval/ (see below). This option
	is just a shorthand to set both of these options together.

	<tag><label id="bfd-min-rx-interval">min rx interval <m/time/</tag>
	This option specifies the minimum RX interval, which is announced to the
	neighbor and used there to limit the neighbor's rate of generated BFD
	control packets. Default: 10 ms.

	<tag><label id="bfd-min-tx-interval">min tx interval <m/time/</tag>
	This option specifies the desired TX interval, which controls the rate
	of generated BFD control packets (together with <cf/min rx interval/
	announced by the neighbor). Note that this value is used only if the BFD
	session is up, otherwise the value of <cf/idle tx interval/ is used
	instead. Default: 100 ms.

	<tag><label id="bfd-idle-tx-interval">idle tx interval <m/time/</tag>
	In order to limit unnecessary traffic in cases where a neighbor is not
	available or not running BFD, the rate of generated BFD control packets
	is lower when the BFD session is not up. This option specifies the
	desired TX interval in such cases instead of <cf/min tx interval/.
	Default: 1 s.

	<tag><label id="bfd-multiplier">multiplier <m/num/</tag>
	Failure detection time for BFD sessions is based on established rate of
	BFD control packets (<cf>min rx/tx interval</cf>) multiplied by this
	multiplier, which is essentially (ignoring jitter) a number of missed
	packets after which the session is declared down. Note that rates and
	multipliers could be different in each direction of a BFD session.
	Default: 5.

	<tag><label id="bfd-passive">passive <m/switch/</tag>
	Generally, both BFD session endpoints try to establish the session by
	sending control packets to the other side. This option allows to enable
	passive mode, which means that the router does not send BFD packets
	until it has received one from the other side. Default: disabled.

	<tag>authentication none</tag>
	No passwords are sent in BFD packets. This is the default value.

	<tag>authentication simple</tag>
	Every packet carries 16 bytes of password. Received packets lacking this
	password are ignored. This authentication mechanism is very weak.

	<tag>authentication [meticulous] keyed md5|sha1</tag>
	An authentication code is appended to each packet. The cryptographic
	algorithm is keyed MD5 or keyed SHA-1. Note that the algorithm is common
	for all keys (on one interface), in contrast to OSPF or RIP, where it
	is a per-key option. Passwords (keys) are not sent open via network.

	The <cf/meticulous/ variant means that cryptographic sequence numbers
	are increased for each sent packet, while in the basic variant they are
	increased about once per second. Generally, the <cf/meticulous/ variant
	offers better resistance to replay attacks but may require more
	computation.

	<tag>password "<M>text</M>"</tag>
	Specifies a password used for authentication. See <ref id="proto-pass"
	name="password"> common option for detailed description. Note that
	password option <cf/algorithm/ is not available in BFD protocol. The
	algorithm is selected by <cf/authentication/ option for all passwords.

</descrip>

<sect1>Example
<label id="bfd-exam">

<p><code>
protocol bfd {
	interface "eth*" {
		min rx interval 20 ms;
		min tx interval 50 ms;
		idle tx interval 300 ms;
	};
	interface "gre*" {
		interval 200 ms;
		multiplier 10;
		passive;
	};
	multihop {
		interval 200 ms;
		multiplier 10;
	};

	neighbor 192.168.1.10;
	neighbor 192.168.2.2 dev "eth2";
	neighbor 192.168.10.1 local 192.168.1.1 multihop;
}
</code>


<sect>BGP
<label id="bgp">

<p>The Border Gateway Protocol is the routing protocol used for backbone level
routing in the today's Internet. Contrary to other protocols, its convergence
does not rely on all routers following the same rules for route selection,
making it possible to implement any routing policy at any router in the network,
the only restriction being that if a router advertises a route, it must accept
and forward packets according to it.

<p>BGP works in terms of autonomous systems (often abbreviated as AS). Each AS
is a part of the network with common management and common routing policy. It is
identified by a unique 16-bit number (ASN). Routers within each AS usually
exchange AS-internal routing information with each other using an interior
gateway protocol (IGP, such as OSPF or RIP). Boundary routers at the border of
the AS communicate global (inter-AS) network reachability information with their
neighbors in the neighboring AS'es via exterior BGP (eBGP) and redistribute
received information to other routers in the AS via interior BGP (iBGP).

<p>Each BGP router sends to its neighbors updates of the parts of its routing
table it wishes to export along with complete path information (a list of AS'es
the packet will travel through if it uses the particular route) in order to
avoid routing loops.

<p>BIRD supports all requirements of the BGP4 standard as defined in
<rfc id="4271"> It also supports the community attributes (<rfc id="1997">),
capability negotiation (<rfc id="5492">), MD5 password authentication (<rfc
id="2385">), extended communities (<rfc id="4360">), route reflectors (<rfc
id="4456">), graceful restart (<rfc id="4724">), multiprotocol extensions
(<rfc id="4760">), 4B AS numbers (<rfc id="4893">), and 4B AS numbers in
extended communities (<rfc id="5668">).


For IPv6, it uses the standard multiprotocol extensions defined in
<rfc id="4760"> and applied to IPv6 according to <rfc id="2545">.

<sect1>Route selection rules
<label id="bgp-route-select-rules">

<p>BGP doesn't have any simple metric, so the rules for selection of an optimal
route among multiple BGP routes with the same preference are a bit more complex
and they are implemented according to the following algorithm. It starts the
first rule, if there are more "best" routes, then it uses the second rule to
choose among them and so on.

<itemize>
	<item>Prefer route with the highest Local Preference attribute.
	<item>Prefer route with the shortest AS path.
	<item>Prefer IGP origin over EGP and EGP origin over incomplete.
	<item>Prefer the lowest value of the Multiple Exit Discriminator.
	<item>Prefer routes received via eBGP over ones received via iBGP.
	<item>Prefer routes with lower internal distance to a boundary router.
	<item>Prefer the route with the lowest value of router ID of the
	advertising router.
</itemize>

<sect1>IGP routing table
<label id="bgp-igp-routing-table">

<p>BGP is mainly concerned with global network reachability and with routes to
other autonomous systems. When such routes are redistributed to routers in the
AS via BGP, they contain IP addresses of a boundary routers (in route attribute
NEXT_HOP). BGP depends on existing IGP routing table with AS-internal routes to
determine immediate next hops for routes and to know their internal distances to
boundary routers for the purpose of BGP route selection. In BIRD, there is
usually one routing table used for both IGP routes and BGP routes.

<sect1>Configuration
<label id="bgp-config">

<p>Each instance of the BGP corresponds to one neighboring router. This allows
to set routing policy and all the other parameters differently for each neighbor
using the following configuration parameters:

<descrip>
	<tag><label id="bgp-local">local [<m/ip/] as <m/number/</tag>
	Define which AS we are part of. (Note that contrary to other IP routers,
	BIRD is able to act as a router located in multiple AS'es simultaneously,
	but in such cases you need to tweak the BGP paths manually in the filters
	to get consistent behavior.) Optional <cf/ip/ argument specifies a source
	address, equivalent to the <cf/source address/ option (see below). This
	parameter is mandatory.

	<tag><label id="bgp-neighbor">neighbor [<m/ip/] [port <m/number/] [as <m/number/]</tag>
	Define neighboring router this instance will be talking to and what AS
	it is located in. In case the neighbor is in the same AS as we are, we
	automatically switch to iBGP. Optionally, the remote port may also be
	specified. The parameter may be used multiple times with different
	sub-options (e.g., both <cf/neighbor 10.0.0.1 as 65000;/ and
	<cf/neighbor 10.0.0.1; neighbor as 65000;/ are valid). This parameter is
	mandatory.

	<tag><label id="bgp-iface">interface <m/string/</tag>
	Define interface we should use for link-local BGP IPv6 sessions.
	Interface can also be specified as a part of <cf/neighbor address/
	(e.g., <cf/neighbor fe80::1234%eth0 as 65000;/). The option may also be
	used for non link-local sessions when it is necessary to explicitly
	specify an interface, but only for direct (not multihop) sessions.

	<tag><label id="bgp-direct">direct</tag>
	Specify that the neighbor is directly connected. The IP address of the
	neighbor must be from a directly reachable IP range (i.e. associated
	with one of your router's interfaces), otherwise the BGP session
	wouldn't start but it would wait for such interface to appear. The
	alternative is the <cf/multihop/ option. Default: enabled for eBGP.

	<tag><label id="bgp-multihop">multihop [<m/number/]</tag>
	Configure multihop BGP session to a neighbor that isn't directly
	connected. Accurately, this option should be used if the configured
	neighbor IP address does not match with any local network subnets. Such
	IP address have to be reachable through system routing table. The
	alternative is the <cf/direct/ option. For multihop BGP it is
	recommended to explicitly configure the source address to have it
	stable. Optional <cf/number/ argument can be used to specify the number
	of hops (used for TTL). Note that the number of networks (edges) in a
	path is counted; i.e., if two BGP speakers are separated by one router,
	the number of hops is 2. Default: enabled for iBGP.

	<tag><label id="bgp-source-address">source address <m/ip/</tag>
	Define local address we should use for next hop calculation and as a
	source address for the BGP session. Default: the address of the local
	end of the interface our neighbor is connected to.

	<tag><label id="bgp-next-hop-self">next hop self</tag>
	Avoid calculation of the Next Hop attribute and always advertise our own
	source address as a next hop. This needs to be used only occasionally to
	circumvent misconfigurations of other routers. Default: disabled.

	<tag><label id="bgp-next-hop-keep">next hop keep</tag>
	Forward the received Next Hop attribute even in situations where the
	local address should be used instead, like when the route is sent to an
	interface with a different subnet. Default: disabled.

	<tag><label id="bgp-missing-lladdr">missing lladdr self|drop|ignore</tag>
	Next Hop attribute in BGP-IPv6 sometimes contains just the global IPv6
	address, but sometimes it has to contain both global and link-local IPv6
	addresses. This option specifies what to do if BIRD have to send both
	addresses but does not know link-local address. This situation might
	happen when routes from other protocols are exported to BGP, or when
	improper updates are received from BGP peers. <cf/self/ means that BIRD
	advertises its own local address instead. <cf/drop/ means that BIRD
	skips that prefixes and logs error. <cf/ignore/ means that BIRD ignores
	the problem and sends just the global address (and therefore forms
	improper BGP update). Default: <cf/self/, unless BIRD is configured as a
	route server (option <cf/rs client/), in that case default is <cf/ignore/,
	because route servers usually do not forward packets themselves.

	<tag><label id="bgp-gateway">gateway direct|recursive</tag>
	For received routes, their <cf/gw/ (immediate next hop) attribute is
	computed from received <cf/bgp_next_hop/ attribute. This option
	specifies how it is computed. Direct mode means that the IP address from
	<cf/bgp_next_hop/ is used if it is directly reachable, otherwise the
	neighbor IP address is used. Recursive mode means that the gateway is
	computed by an IGP routing table lookup for the IP address from
	<cf/bgp_next_hop/. Note that there is just one level of indirection in
	recursive mode - the route obtained by the lookup must not be recursive
	itself, to prevent mutually recursive routes.

	Recursive mode is the behavior specified by the BGP
	standard. Direct mode is simpler, does not require any routes in a
	routing table, and was used in older versions of BIRD, but does not
	handle well nontrivial iBGP setups and multihop. Recursive mode is
	incompatible with <ref id="dsc-table-sorted" name="sorted tables">. Default:
	<cf/direct/ for direct sessions, <cf/recursive/ for multihop sessions.

	<tag><label id="bgp-igp-table">igp table <m/name/</tag>
	Specifies a table that is used as an IGP routing table. Default: the
	same as the table BGP is connected to.

	<tag><label id="bgp-check-link">check link <M>switch</M></tag>
	BGP could use hardware link state into consideration.  If enabled,
	BIRD tracks the link state of the associated interface and when link
	disappears (e.g. an ethernet cable is unplugged), the BGP session is
	immediately shut down. Note that this option cannot be used with
	multihop BGP. Default: disabled.

	<tag><label id="bgp-bfd">bfd <M>switch</M>|graceful</tag>
	BGP could use BFD protocol as an advisory mechanism for neighbor
	liveness and failure detection. If enabled, BIRD setups a BFD session
	for the BGP neighbor and tracks its liveness by it. This has an
	advantage of an order of magnitude lower detection times in case of
	failure. When a neighbor failure is detected, the BGP session is
	restarted. Optionally, it can be configured (by <cf/graceful/ argument)
	to trigger graceful restart instead of regular restart.  Note that BFD
	protocol also has to be configured, see <ref id="bfd" name="BFD">
	section for details. Default: disabled.

	<tag><label id="bgp-ttl-security">ttl security <m/switch/</tag>
	Use GTSM (<rfc id="5082"> - the generalized TTL security mechanism). GTSM
	protects against spoofed packets by ignoring received packets with a
	smaller than expected TTL. To work properly, GTSM have to be enabled on
	both sides of a BGP session. If both <cf/ttl security/ and
	<cf/multihop/ options are enabled, <cf/multihop/ option should specify
	proper hop value to compute expected TTL. Kernel support required:
	Linux: 2.6.34+ (IPv4), 2.6.35+ (IPv6), BSD: since long ago, IPv4 only.
	Note that full (ICMP protection, for example) <rfc id="5082"> support is
	provided by Linux only. Default: disabled.

	<tag><label id="bgp-pass">password <m/string/</tag>
	Use this password for MD5 authentication of BGP sessions (<rfc id="2385">). When
	used on BSD systems, see also <cf/setkey/ option below. Default: no
	authentication.

	<tag><label id="bgp-setkey">setkey <m/switch/</tag>
	On BSD systems, keys for TCP MD5 authentication are stored in the global
	SA/SP database, which can be accessed by external utilities (e.g.
	setkey(8)). BIRD configures security associations in the SA/SP database
	automatically based on <cf/password/ options (see above), this option
	allows to disable automatic updates by BIRD when manual configuration by
	external utilities is preferred. Note that automatic SA/SP database
	updates are currently implemented only for FreeBSD. Passwords have to be
	set manually by an external utility on NetBSD and OpenBSD. Default:
	enabled (ignored on non-FreeBSD).

	<tag><label id="bgp-passive">passive <m/switch/</tag>
	Standard BGP behavior is both initiating outgoing connections and
	accepting incoming connections. In passive mode, outgoing connections
	are not initiated. Default: off.

	<tag><label id="bgp-rr-client">rr client</tag>
	Be a route reflector and treat the neighbor as a route reflection
	client. Default: disabled.

	<tag><label id="bgp-rr-cluster-id">rr cluster id <m/IPv4 address/</tag>
	Route reflectors use cluster id to avoid route reflection loops. When
	there is one route reflector in a cluster it usually uses its router id
	as a cluster id, but when there are more route reflectors in a cluster,
	these need to be configured (using this option) to use a common cluster
	id. Clients in a cluster need not know their cluster id and this option
	is not allowed for them. Default: the same as router id.

	<tag><label id="bgp-rs-client">rs client</tag>
	Be a route server and treat the neighbor as a route server client.
	A route server is used as a replacement for full mesh EBGP routing in
	Internet exchange points in a similar way to route reflectors used in
	IBGP routing. BIRD does not implement obsoleted <rfc id="1863">, but
	uses ad-hoc implementation, which behaves like plain EBGP but reduces
	modifications to advertised route attributes to be transparent (for
	example does not prepend its AS number to AS PATH attribute and
	keeps MED attribute). Default: disabled.

	<tag><label id="bgp-secondary">secondary <m/switch/</tag>
	Usually, if an export filter rejects a selected route, no other route is
	propagated for that network. This option allows to try the next route in
	order until one that is accepted is found or all routes for that network
	are rejected. This can be used for route servers that need to propagate
	different tables to each client but do not want to have these tables
	explicitly (to conserve memory). This option requires that the connected
	routing table is <ref id="dsc-table-sorted" name="sorted">. Default: off.

	<tag><label id="bgp-add-paths">add paths <m/switch/|rx|tx</tag>
	Standard BGP can propagate only one path (route) per destination network
	(usually the selected one). This option controls the add-path protocol
	extension, which allows to advertise any number of paths to a
	destination. Note that to be active, add-path has to be enabled on both
	sides of the BGP session, but it could be enabled separately for RX and
	TX direction. When active, all available routes accepted by the export
	filter are advertised to the neighbor. Default: off.

	<tag><label id="bgp-allow-local-pref">allow bgp_local_pref <m/switch/</tag>
	A standard BGP implementation do not send the Local Preference attribute
	to eBGP neighbors and ignore this attribute if received from eBGP
	neighbors, as per <rfc id="4271">.  When this option is enabled on an
	eBGP session, this attribute will be sent to and accepted from the peer,
	which is useful for example if you have a setup like in <rfc id="7938">.
	The option does not affect iBGP sessions. Default: off.

	<tag><label id="bgp-allow-local-as">allow local as [<m/number/]</tag>
	BGP prevents routing loops by rejecting received routes with the local
	AS number in the AS path. This option allows to loose or disable the
	check. Optional <cf/number/ argument can be used to specify the maximum
	number of local ASNs in the AS path that is allowed for received
	routes. When the option is used without the argument, the check is
	completely disabled and you should ensure loop-free behavior by some
	other means. Default: 0 (no local AS number allowed).

	<tag><label id="bgp-enable-route-refresh">enable route refresh <m/switch/</tag>
	After the initial route exchange, BGP protocol uses incremental updates
	to keep BGP speakers synchronized. Sometimes (e.g., if BGP speaker
	changes its import filter, or if there is suspicion of inconsistency) it
	is necessary to do a new complete route exchange. BGP protocol extension
	Route Refresh (<rfc id="2918">) allows BGP speaker to request
	re-advertisement of all routes from its neighbor. BGP protocol
	extension Enhanced Route Refresh (<rfc id="7313">) specifies explicit
	begin and end for such exchanges, therefore the receiver can remove
	stale routes that were not advertised during the exchange. This option
	specifies whether BIRD advertises these capabilities and supports
	related procedures. Note that even when disabled, BIRD can send route
	refresh requests.  Default: on.

	<tag><label id="bgp-graceful-restart">graceful restart <m/switch/|aware</tag>
	When a BGP speaker restarts or crashes, neighbors will discard all
	received paths from the speaker, which disrupts packet forwarding even
	when the forwarding plane of the speaker remains intact. <rfc
	id="4724"> specifies an optional graceful restart mechanism to
	alleviate this issue. This option controls the mechanism. It has three
	states: Disabled, when no support is provided. Aware, when the graceful
	restart support is announced and the support for restarting neighbors
	is provided, but no local graceful restart is allowed (i.e.
	receiving-only role). Enabled, when the full graceful restart
	support is provided (i.e. both restarting and receiving role). Note
	that proper support for local graceful restart requires also
	configuration of other protocols.  Default: aware.

	<tag><label id="bgp-graceful-restart-time">graceful restart time <m/number/</tag>
	The restart time is announced in the BGP graceful restart capability
	and specifies how long the neighbor would wait for the BGP session to
	re-establish after a restart before deleting stale routes. Default:
	120 seconds.

	<tag><label id="bgp-long-lived-graceful-restart">long lived graceful restart <m/switch/|aware</tag>
	The long-lived graceful restart is an extension of the traditional
	<ref id="bgp-graceful-restart" name="BGP graceful restart">, where stale
	routes are kept even after the <ref id="bgp-graceful-restart-time"
	name="restart time"> expires for additional long-lived stale time, but
	they are marked with the LLGR_STALE community, depreferenced, and
	withdrawn from routers not supporting LLGR. Like traditional BGP
	graceful restart, it has three states: disabled, aware (receiving-only),
	and enabled. Note that long-lived graceful restart requires at least
	aware level of traditional BGP graceful restart. Default: aware, unless
	graceful restart is disabled.

	<tag><label id="bgp-long-lived-stale-time">long lived stale time <m/number/</tag>
	The long-lived stale time is announced in the BGP long-lived graceful
	restart capability and specifies how long the neighbor would keep stale
	routes depreferenced during long-lived graceful restart until either the
	session is re-stablished and synchronized or the stale time expires and
	routes are removed. Default: 3600 seconds.

	<tag><label id="bgp-interpret-communities">interpret communities <m/switch/</tag>
	<rfc id="1997"> demands that BGP speaker should process well-known
	communities like no-export (65535, 65281) or no-advertise (65535,
	65282). For example, received route carrying a no-adverise community
	should not be advertised to any of its neighbors. If this option is
	enabled (which is by default), BIRD has such behavior automatically (it
	is evaluated when a route is exported to the BGP protocol just before
	the export filter).  Otherwise, this integrated processing of
	well-known communities is disabled. In that case, similar behavior can
	be implemented in the export filter.  Default: on.

	<tag><label id="bgp-enable-as4">enable as4 <m/switch/</tag>
	BGP protocol was designed to use 2B AS numbers and was extended later to
	allow 4B AS number. BIRD supports 4B AS extension, but by disabling this
	option it can be persuaded not to advertise it and to maintain old-style
	sessions with its neighbors. This might be useful for circumventing bugs
	in neighbor's implementation of 4B AS extension. Even when disabled
	(off), BIRD behaves internally as AS4-aware BGP router. Default: on.

	<tag><label id="bgp-enable-extended-messages">enable extended messages <m/switch/</tag>
	The BGP protocol uses maximum message length of 4096 bytes. This option
	provides an extension to allow extended messages with length up
	to 65535 bytes. Default: off.

	<tag><label id="bgp-capabilities">capabilities <m/switch/</tag>
	Use capability advertisement to advertise optional capabilities. This is
	standard behavior for newer BGP implementations, but there might be some
	older BGP implementations that reject such connection attempts. When
	disabled (off), features that request it (4B AS support) are also
	disabled. Default: on, with automatic fallback to off when received
	capability-related error.

	<tag><label id="bgp-advertise-ipv4">advertise ipv4 <m/switch/</tag>
	Advertise IPv4 multiprotocol capability. This is not a correct behavior
	according to the strict interpretation of <rfc id="4760">, but it is
	widespread and required by some BGP implementations (Cisco and Quagga).
	This option is relevant to IPv4 mode with enabled capability
	advertisement only. Default: on.

	<tag><label id="bgp-route-limit">route limit <m/number/</tag>
	The maximal number of routes that may be imported from the protocol. If
	the route limit is exceeded, the connection is closed with an error.
	Limit is currently implemented as <cf>import limit <m/number/ action
	restart</cf>. This option is obsolete and it is replaced by
	<ref id="proto-import-limit" name="import limit option">. Default: no limit.

	<tag><label id="bgp-disable-after-error">disable after error <m/switch/</tag>
	When an error is encountered (either locally or by the other side),
	disable the instance automatically and wait for an administrator to fix
	the problem manually. Default: off.

	<tag><label id="bgp-hold-time">hold time <m/number/</tag>
	Time in seconds to wait for a Keepalive message from the other side
	before considering the connection stale. Default: depends on agreement
	with the neighboring router, we prefer 240 seconds if the other side is
	willing to accept it.

	<tag><label id="bgp-startup-hold-time">startup hold time <m/number/</tag>
	Value of the hold timer used before the routers have a chance to exchange
	open messages and agree on the real value. Default: 240	seconds.

	<tag><label id="bgp-keepalive-time">keepalive time <m/number/</tag>
	Delay in seconds between sending of two consecutive Keepalive messages.
	Default: One third of the hold time.

	<tag><label id="bgp-connect-delay-time">connect delay time <m/number/</tag>
	Delay in seconds between protocol startup and the first attempt to
	connect. Default: 5 seconds.

	<tag><label id="bgp-connect-retry-time">connect retry time <m/number/</tag>
	Time in seconds to wait before retrying a failed attempt to connect.
	Default: 120 seconds.

	<tag><label id="bgp-error-wait-time">error wait time <m/number/,<m/number/</tag>
	Minimum and maximum delay in seconds between a protocol failure (either
	local or reported by the peer) and automatic restart. Doesn't apply
	when <cf/disable after error/ is configured. If consecutive errors
	happen, the delay is increased exponentially until it reaches the
	maximum. Default: 60, 300.

	<tag><label id="bgp-error-forget-time">error forget time <m/number/</tag>
	Maximum time in seconds between two protocol failures to treat them as a
	error sequence which makes <cf/error wait time/ increase exponentially.
	Default: 300 seconds.

	<tag><label id="bgp-path-metric">path metric <m/switch/</tag>
	Enable comparison of path lengths when deciding which BGP route is the
	best one. Default: on.

	<tag><label id="bgp-med-metric">med metric <m/switch/</tag>
	Enable comparison of MED attributes (during best route selection) even
	between routes received from different ASes. This may be useful if all
	MED attributes contain some consistent metric, perhaps enforced in
	import filters of AS boundary routers. If this option is disabled, MED
	attributes are compared only if routes are received from the same AS
	(which is the standard behavior). Default: off.

	<tag><label id="bgp-deterministic-med">deterministic med <m/switch/</tag>
	BGP route selection algorithm is often viewed as a comparison between
	individual routes (e.g. if a new route appears and is better than the
	current best one, it is chosen as the new best one). But the proper
	route selection, as specified by <rfc id="4271">, cannot be fully
	implemented in that way. The problem is mainly in handling the MED
	attribute. BIRD, by default, uses an simplification based on individual
	route comparison, which in some cases may lead to temporally dependent
	behavior (i.e. the selection is dependent on the order in which routes
	appeared). This option enables a different (and slower) algorithm
	implementing proper <rfc id="4271"> route selection, which is
	deterministic. Alternative way how to get deterministic behavior is to
	use <cf/med metric/ option. This option is incompatible with <ref
	id="dsc-table-sorted" name="sorted tables">.  Default: off.

	<tag><label id="bgp-igp-metric">igp metric <m/switch/</tag>
	Enable comparison of internal distances to boundary routers during best
 	route selection. Default: on.

	<tag><label id="bgp-prefer-older">prefer older <m/switch/</tag>
	Standard route selection algorithm breaks ties by comparing router IDs.
	This changes the behavior to prefer older routes (when both are external
	and from different peer). For details, see <rfc id="5004">. Default: off.

	<tag><label id="bgp-default-med">default bgp_med <m/number/</tag>
	Value of the Multiple Exit Discriminator to be used during route
	selection when the MED attribute is missing. Default: 0.

	<tag><label id="bgp-default-local-pref">default bgp_local_pref <m/number/</tag>
	A default value for the Local Preference attribute. It is used when
	a new Local Preference attribute is attached to a route by the BGP
	protocol itself (for example, if a route is received through eBGP and
	therefore does not have such attribute). Default: 100 (0 in pre-1.2.0
	versions of BIRD).
</descrip>

<sect1>Attributes
<label id="bgp-attr">

<p>BGP defines several route attributes. Some of them (those marked with
`<tt/I/' in the table below) are available on internal BGP connections only,
some of them (marked with `<tt/O/') are optional.

<descrip>
	<tag><label id="rta-bgp-path">bgppath bgp_path</tag>
	Sequence of AS numbers describing the AS path the packet will travel
	through when forwarded according to the particular route. In case of
	internal BGP it doesn't contain the number of the local AS.

	<tag><label id="rta-bgp-local-pref">int bgp_local_pref [I]</tag>
	Local preference value used for selection among multiple BGP routes (see
	the selection rules above). It's used as an additional metric which is
	propagated through the whole local AS.

	<tag><label id="rta-bgp-med">int bgp_med [O]</tag>
	The Multiple Exit Discriminator of the route is an optional attribute
	which is used on external (inter-AS) links to convey to an adjacent AS
	the optimal entry point into the local AS. The received attribute is
	also propagated over internal BGP links. The attribute value is zeroed
	when a route is exported to an external BGP instance to ensure that the
	attribute received from a neighboring AS is not propagated to other
	neighboring ASes. A new value might be set in the export filter of an
	external BGP instance. See <rfc id="4451"> for further discussion of
	BGP MED attribute.

	<tag><label id="rta-bgp-origin">enum bgp_origin</tag>
	Origin of the route: either <cf/ORIGIN_IGP/ if the route has originated
	in an interior routing protocol or <cf/ORIGIN_EGP/ if it's been imported
	from the <tt>EGP</tt> protocol (nowadays it seems to be obsolete) or
	<cf/ORIGIN_INCOMPLETE/ if the origin is unknown.

	<tag><label id="rta-bgp-next-hop">ip bgp_next_hop</tag>
	Next hop to be used for forwarding of packets to this destination. On
	internal BGP connections, it's an address of the originating router if
	it's inside the local AS or a boundary router the packet will leave the
	AS through if it's an exterior route, so each BGP speaker within the AS
	has a chance to use the shortest interior path possible to this point.

	<tag><label id="rta-bgp-atomic-aggr">void bgp_atomic_aggr [O]</tag>
	This is an optional attribute which carries no value, but the sole
	presence of which indicates that the route has been aggregated from
	multiple routes by some router on the path from the originator.

<!-- we don't handle aggregators right since they are of a very obscure type
	<tag>bgp_aggregator</tag>
-->
	<tag><label id="rta-bgp-community">clist bgp_community [O]</tag>
	List of community values associated with the route. Each such value is a
	pair (represented as a <cf/pair/ data type inside the filters) of 16-bit
	integers, the first of them containing the number of the AS which
	defines the community and the second one being a per-AS identifier.
	There are lots of uses of the community mechanism, but generally they
	are used to carry policy information like "don't export to USA peers".
	As each AS can define its own routing policy, it also has a complete
	freedom about which community attributes it defines and what will their
	semantics be.

	<tag><label id="rta-bgp-ext-community">eclist bgp_ext_community [O]</tag>
	List of extended community values associated with the route. Extended
	communities have similar usage as plain communities, but they have an
	extended range (to allow 4B ASNs) and a nontrivial structure with a type
	field. Individual community values are represented using an <cf/ec/ data
	type inside the filters.

	<tag><label id="rta-bgp-large-community">lclist bgp_large_community [O]</tag>
	List of large community values associated with the route. Large BGP
	communities is another variant of communities, but contrary to extended
	communities they behave very much the same way as regular communities,
	just larger -- they are uniform untyped triplets of 32bit numbers.
	Individual community values are represented using an <cf/lc/ data type
	inside the filters.

	<tag><label id="rta-bgp-originator-id">quad bgp_originator_id [I, O]</tag>
	This attribute is created by the route reflector when reflecting the
	route and contains the router ID of the originator of the route in the
	local AS.

	<tag><label id="rta-bgp-cluster-list">clist bgp_cluster_list [I, O]</tag>
	This attribute contains a list of cluster IDs of route reflectors. Each
	route reflector prepends its cluster ID when reflecting the route.
</descrip>

<sect1>Example
<label id="bgp-exam">

<p><code>
protocol bgp {
	local as 65000;			     # Use a private AS number
	neighbor 198.51.100.130 as 64496;    # Our neighbor ...
	multihop;			     # ... which is connected indirectly
	export filter {			     # We use non-trivial export rules
		if source = RTS_STATIC then { # Export only static routes
			# Assign our community
			bgp_community.add((65000,64501));
			# Artificially increase path length
			# by advertising local AS number twice
			if bgp_path ~ [= 65000 =] then
				bgp_path.prepend(65000);
			accept;
		}
		reject;
	};
	import all;
	source address 198.51.100.14;	# Use a non-standard source address
}
</code>


<sect>Device
<label id="device">

<p>The Device protocol is not a real routing protocol. It doesn't generate any
routes and it only serves as a module for getting information about network
interfaces from the kernel.

<p>Except for very unusual circumstances, you probably should include this
protocol in the configuration since almost all other protocols require network
interfaces to be defined for them to work with.

<sect1>Configuration
<label id="device-config">

<p><descrip>

	<tag><label id="device-scan-time">scan time <m/number/</tag>
	Time in seconds between two scans of the network interface list. On
	systems where we are notified about interface status changes
	asynchronously (such as newer versions of Linux), we need to scan the
	list only in order to avoid confusion by lost notification messages,
	so the default time is set to a large value.

	<tag><label id="device-primary">primary [ "<m/mask/" ] <m/prefix/</tag>
	If a network interface has more than one network address, BIRD has to
	choose one of them as a primary one. By default, BIRD chooses the
	lexicographically smallest address as the primary one.

	This option allows to specify which network address should be chosen as
	a primary one. Network addresses that match <m/prefix/ are preferred to
	non-matching addresses. If more <cf/primary/ options are used, the first
	one has the highest preference. If "<m/mask/" is specified, then such
	<cf/primary/ option is relevant only to matching network interfaces.

	In all cases, an address marked by operating system as secondary cannot
	be chosen as the primary one.
</descrip>

<p>As the Device protocol doesn't generate any routes, it cannot have
any attributes. Example configuration looks like this:

<p><code>
protocol device {
	scan time 10;		# Scan the interfaces often
	primary "eth0" 192.168.1.1;
	primary 192.168.0.0/16;
}
</code>


<sect>Direct
<label id="direct">

<p>The Direct protocol is a simple generator of device routes for all the
directly connected networks according to the list of interfaces provided by the
kernel via the Device protocol.

<p>The question is whether it is a good idea to have such device routes in BIRD
routing table. OS kernel usually handles device routes for directly connected
networks by itself so we don't need (and don't want) to export these routes to
the kernel protocol. OSPF protocol creates device routes for its interfaces
itself and BGP protocol is usually used for exporting aggregate routes. Although
there are some use cases that use the direct protocol (like abusing eBGP as an
IGP routing protocol), in most cases it is not needed to have these device
routes in BIRD routing table and to use the direct protocol.

<p>There is one notable case when you definitely want to use the direct protocol
-- running BIRD on BSD systems. Having high priority device routes for directly
connected networks from the direct protocol protects kernel device routes from
being overwritten or removed by IGP routes during some transient network
conditions, because a lower priority IGP route for the same network is not
exported to the kernel routing table. This is an issue on BSD systems only, as
on Linux systems BIRD cannot change non-BIRD route in the kernel routing table.

<p>There are just few configuration options for the Direct protocol:

<p><descrip>
	<tag><label id="direct-iface">interface <m/pattern/ [, <m/.../]</tag>
	By default, the Direct protocol will generate device routes for all the
	interfaces available. If you want to restrict it to some subset of
	interfaces or addresses (e.g. if you're using multiple routing tables
	for policy routing and some of the policy domains don't contain all
	interfaces), just use this clause. See <ref id="proto-iface" name="interface">
	common option for detailed description. The Direct protocol uses
	extended interface clauses.

	<tag><label id="direct-check-link">check link <m/switch/</tag>
	If enabled, a hardware link state (reported by OS) is taken into
	consideration. Routes for directly connected networks are generated only
	if link up is reported and they are withdrawn when link disappears
	(e.g., an ethernet cable is unplugged). Default value is no.
</descrip>

<p>Direct device routes don't contain any specific attributes.

<p>Example config might look like this:

<p><code>
protocol direct {
	interface "-arc*", "*";		# Exclude the ARCnets
}
</code>


<sect>Kernel
<label id="krt">

<p>The Kernel protocol is not a real routing protocol. Instead of communicating
with other routers in the network, it performs synchronization of BIRD's routing
tables with the OS kernel. Basically, it sends all routing table updates to the
kernel and from time to time it scans the kernel tables to see whether some
routes have disappeared (for example due to unnoticed up/down transition of an
interface) or whether an `alien' route has been added by someone else (depending
on the <cf/learn/ switch, such routes are either ignored or accepted to our
table).

<p>Unfortunately, there is one thing that makes the routing table synchronization
a bit more complicated. In the kernel routing table there are also device routes
for directly connected networks. These routes are usually managed by OS itself
(as a part of IP address configuration) and we don't want to touch that. They
are completely ignored during the scan of the kernel tables and also the export
of device routes from BIRD tables to kernel routing tables is restricted to
prevent accidental interference. This restriction can be disabled using
<cf/device routes/ switch.

<p>If your OS supports only a single routing table, you can configure only one
instance of the Kernel protocol. If it supports multiple tables (in order to
allow policy routing; such an OS is for example Linux), you can run as many
instances as you want, but each of them must be connected to a different BIRD
routing table and to a different kernel table.

<p>Because the kernel protocol is partially integrated with the connected
routing table, there are two limitations - it is not possible to connect more
kernel protocols to the same routing table and changing route destination
(gateway) in an export filter of a kernel protocol does not work. Both
limitations can be overcome using another routing table and the pipe protocol.

<sect1>Configuration
<label id="krt-config">

<p><descrip>
	<tag><label id="krt-persist">persist <m/switch/</tag>
	Tell BIRD to leave all its routes in the routing tables when it exits
	(instead of cleaning them up).

	<tag><label id="krt-scan-time">scan time <m/number/</tag>
	Time in seconds between two consecutive scans of the kernel routing
	table.

	<tag><label id="krt-learn">learn <m/switch/</tag>
	Enable learning of routes added to the kernel routing tables by other
	routing daemons or by the system administrator. This is possible only on
	systems which support identification of route authorship.

	<tag><label id="krt-device-routes">device routes <m/switch/</tag>
	Enable export of device routes to the kernel routing table. By default,
	such routes are rejected (with the exception of explicitly configured
	device routes from the static protocol) regardless of the export filter
	to protect device routes in kernel routing table (managed by OS itself)
	from accidental overwriting or erasing.

	<tag><label id="krt-kernel-table">kernel table <m/number/</tag>
	Select which kernel table should this particular instance of the Kernel
	protocol work with. Available only on systems supporting multiple
	routing tables.

	<tag><label id="krt-metric">metric <m/number/</tag> (Linux)
	Use specified value as a kernel metric (priority) for all routes sent to
	the kernel. When multiple routes for the same network are in the kernel
	routing table, the Linux kernel chooses one with lower metric. Also,
	routes with different metrics do not clash with each other, therefore
	using dedicated metric value is a reliable way to avoid overwriting
	routes from other sources (e.g. kernel device routes). Metric 0 has a
	special meaning of undefined metric, in which either OS default is used,
	or per-route metric can be set using <cf/krt_metric/ attribute. Default:
	0 (undefined).

	<tag><label id="krt-graceful-restart">graceful restart <m/switch/</tag>
	Participate in graceful restart recovery. If this option is enabled and
	a graceful restart recovery is active, the Kernel protocol will defer
	synchronization of routing tables until the end of the recovery. Note
	that import of kernel routes to BIRD is not affected.

	<tag><label id="krt-merge-paths">merge paths <M>switch</M> [limit <M>number</M>]</tag>
	Usually, only best routes are exported to the kernel protocol. With path
	merging enabled, both best routes and equivalent non-best routes are
	merged during export to generate one ECMP (equal-cost multipath) route
	for each network. This is useful e.g. for BGP multipath. Note that best
	routes are still pivotal for route export (responsible for most
	properties of resulting ECMP routes), while exported non-best routes are
	responsible just for additional multipath next hops. This option also
	allows to specify a limit on maximal number of nexthops in one route. By
	default, multipath merging is disabled. If enabled, default value of the
	limit is 16.
</descrip>

<sect1>Attributes
<label id="krt-attr">

<p>The Kernel protocol defines several attributes. These attributes are
translated to appropriate system (and OS-specific) route attributes. We support
these attributes:

<descrip>
	<tag><label id="rta-krt-source">int krt_source</tag>
	The original source of the imported kernel route. The value is
	system-dependent. On Linux, it is a value of the protocol field of the
	route. See /etc/iproute2/rt_protos for common values. On BSD, it is
	based on STATIC and PROTOx flags. The attribute is read-only.

	<tag><label id="rta-krt-metric">int krt_metric</tag> (Linux)
	The kernel metric of the route. When multiple same routes are in a
	kernel routing table, the Linux kernel chooses one with lower metric.
	Note that preferred way to set kernel metric is to use protocol option
	<cf/metric/, unless per-route metric values are needed.

	<tag><label id="rta-krt-prefsrc">ip krt_prefsrc</tag> (Linux)
	The preferred source address. Used in source address selection for
	outgoing packets. Has to be one of the IP addresses of the router.

	<tag><label id="rta-krt-realm">int krt_realm</tag> (Linux)
	The realm of the route. Can be used for traffic classification.

	<tag><label id="rta-krt-scope">int krt_scope</tag> (Linux IPv4)
	The scope of the route. Valid values are 0-254, although Linux kernel
	may reject some values depending on route type and nexthop. It is
	supposed to represent `indirectness' of the route, where nexthops of
	routes are resolved through routes with a higher scope, but in current
	kernels anything below <it/link/ (253) is treated as <it/global/ (0).
	When not present, global scope is implied for all routes except device
	routes, where link scope is used by default.
</descrip>

<p>In Linux, there is also a plenty of obscure route attributes mostly focused
on tuning TCP performance of local connections. BIRD supports most of these
attributes, see Linux or iproute2 documentation for their meaning. Attributes
<cf/krt_lock_*/ and <cf/krt_feature_*/ have type bool, others have type int.
Supported attributes are:

<cf/krt_mtu/, <cf/krt_lock_mtu/, <cf/krt_window/, <cf/krt_lock_window/,
<cf/krt_rtt/, <cf/krt_lock_rtt/, <cf/krt_rttvar/, <cf/krt_lock_rttvar/,
<cf/krt_sstresh/, <cf/krt_lock_sstresh/, <cf/krt_cwnd/, <cf/krt_lock_cwnd/,
<cf/krt_advmss/, <cf/krt_lock_advmss/, <cf/krt_reordering/, <cf/krt_lock_reordering/,
<cf/krt_hoplimit/, <cf/krt_lock_hoplimit/, <cf/krt_rto_min/, <cf/krt_lock_rto_min/,
<cf/krt_initcwnd/, <cf/krt_initrwnd/, <cf/krt_quickack/,
<cf/krt_feature_ecn/, <cf/krt_feature_allfrag/

<sect1>Example
<label id="krt-exam">

<p>A simple configuration can look this way:

<p><code>
protocol kernel {
	export all;
}
</code>

<p>Or for a system with two routing tables:

<p><code>
protocol kernel {		# Primary routing table
	learn;			# Learn alien routes from the kernel
	persist;		# Don't remove routes on bird shutdown
	scan time 10;		# Scan kernel routing table every 10 seconds
	import all;
	export all;
}

protocol kernel {		# Secondary routing table
	table auxtable;
	kernel table 100;
	export all;
}
</code>


<sect>MRT
<label id="mrt">

<sect1>Introduction
<label id="mrt-intro">

<p>The MRT protocol is a component responsible for handling the Multi-Threaded
Routing Toolkit (MRT) routing information export format, which is mainly used
for collecting and analyzing of routing information from BGP routers. The MRT
protocol can be configured to do periodic dumps of routing tables, created MRT
files can be analyzed later by other tools. Independent MRT table dumps can also
be requested from BIRD client. There is also a feature to save incoming BGP
messages in MRT files, but it is controlled by <ref id="proto-mrtdump"
name="mrtdump"> options independently of MRT protocol, although that might
change in the future.

BIRD implements the main MRT format specification as defined in <rfc id="6396">
and the ADD_PATH extension (<rfc id="8050">).

<sect1>Configuration
<label id="mrt-config">

<p>MRT configuration consists of several statements describing routing table
dumps. Multiple independent periodic dumps can be done as multiple MRT protocol
instances. There are two mandatory statements: <cf/filename/ and <cf/period/.
The behavior can be modified by following configuration parameters:

<descrip>
	<tag><label id="mrt-table">table <m/name/ | "<m/pattern/"</tag>
	Specify a routing table (or a set of routing tables described by a
	wildcard pattern) that are to be dumped by the MRT protocol instance.
	Default: the master table.

	<tag><label id="mrt-filter">filter { <m/filter commands/ }</tag>
	The MRT protocol allows to specify a filter that is applied to routes as
	they are dumped. Rejected routes are ignored and not saved to the MRT
	dump file. Default: no filter.

	<tag><label id="mrt-where">where <m/filter expression/</tag>
	An alternative way to specify a filter for the MRT protocol.

	<tag><label id="mrt-filename">filename "<m/filename/"</tag>
	Specify a filename for MRT dump files. The filename may contain time
	format sequences with <it/strftime(3)/ notation (see <it/man strftime/
	for details), there is also a sequence "%N" that is expanded to the name
	of dumped table. Therefore, each periodic dump of each table can be
	saved to a different file. Mandatory, see example below.

	<tag><label id="mrt-period">period <m/number/</tag>
	Specify the time interval (in seconds) between periodic dumps.
	Mandatory.

	<tag><label id="mrt-always-add-path">always add path <m/switch/</tag>
	The MRT format uses special records (specified in <rfc id="8050">) for
	routes received using BGP ADD_PATH extension to keep Path ID, while
	other routes use regular records. This has advantage of better
	compatibility with tools that do not know special records, but it loses
	information about which route is the best route. When this option is
	enabled, both ADD_PATH and non-ADD_PATH routes are stored in ADD_PATH
	records and order of routes for network is preserved. Default: disabled.
</descrip>

<sect1>Example
<label id="mrt-exam">

<p><code>
protocol mrt {
	table "tab*";
	where source = RTS_BGP;
	filename "/var/log/bird/%N_%F_%T.mrt";
	period 300;
}
</code>


<sect>OSPF
<label id="ospf">

<sect1>Introduction
<label id="ospf-intro">

<p>Open Shortest Path First (OSPF) is a quite complex interior gateway
protocol. The current IPv4 version (OSPFv2) is defined in <rfc id="2328"> and
the current IPv6 version (OSPFv3) is defined in <rfc id="5340"> It's a link
state (a.k.a. shortest path first) protocol -- each router maintains a database
describing the autonomous system's topology. Each participating router has an
identical copy of the database and all routers run the same algorithm
calculating a shortest path tree with themselves as a root. OSPF chooses the
least cost path as the best path.

<p>In OSPF, the autonomous system can be split to several areas in order to
reduce the amount of resources consumed for exchanging the routing information
and to protect the other areas from incorrect routing data. Topology of the area
is hidden to the rest of the autonomous system.

<p>Another very important feature of OSPF is that it can keep routing information
from other protocols (like Static or BGP) in its link state database as external
routes. Each external route can be tagged by the advertising router, making it
possible to pass additional information between routers on the boundary of the
autonomous system.

<p>OSPF quickly detects topological changes in the autonomous system (such as
router interface failures) and calculates new loop-free routes after a short
period of convergence. Only a minimal amount of routing traffic is involved.

<p>Each router participating in OSPF routing periodically sends Hello messages
to all its interfaces. This allows neighbors to be discovered dynamically. Then
the neighbors exchange theirs parts of the link state database and keep it
identical by flooding updates. The flooding process is reliable and ensures that
each router detects all changes.

<sect1>Configuration
<label id="ospf-config">

<p>In the main part of configuration, there can be multiple definitions of OSPF
areas, each with a different id. These definitions includes many other switches
and multiple definitions of interfaces. Definition of interface may contain many
switches and constant definitions and list of neighbors on nonbroadcast
networks.

<code>
protocol ospf &lt;name&gt; {
	rfc1583compat &lt;switch&gt;;
	instance id &lt;num&gt;;
	stub router &lt;switch&gt;;
	tick &lt;num&gt;;
	ecmp &lt;switch&gt; [limit &lt;num&gt;];
	merge external &lt;switch&gt;;
	area &lt;id&gt; {
		stub;
		nssa;
		summary &lt;switch&gt;;
		default nssa &lt;switch&gt;;
		default cost &lt;num&gt;;
		default cost2 &lt;num&gt;;
		translator &lt;switch&gt;;
		translator stability &lt;num&gt;;

                networks {
			&lt;prefix&gt;;
			&lt;prefix&gt; hidden;
		}
                external {
			&lt;prefix&gt;;
			&lt;prefix&gt; hidden;
			&lt;prefix&gt; tag &lt;num&gt;;
		}
		stubnet &lt;prefix&gt;;
		stubnet &lt;prefix&gt; {
			hidden &lt;switch&gt;;
			summary &lt;switch&gt;;
			cost &lt;num&gt;;
		}
		interface &lt;interface pattern&gt; [instance &lt;num&gt;] {
			cost &lt;num&gt;;
			stub &lt;switch&gt;;
			hello &lt;num&gt;;
			poll &lt;num&gt;;
			retransmit &lt;num&gt;;
			priority &lt;num&gt;;
			wait &lt;num&gt;;
			dead count &lt;num&gt;;
			dead &lt;num&gt;;
			secondary &lt;switch&gt;;
			rx buffer [normal|large|&lt;num&gt;];
			tx length &lt;num&gt;;
			type [broadcast|bcast|pointopoint|ptp|
				nonbroadcast|nbma|pointomultipoint|ptmp];
			link lsa suppression &lt;switch&gt;;
			strict nonbroadcast &lt;switch&gt;;
			real broadcast &lt;switch&gt;;
			ptp netmask &lt;switch&gt;;
			check link &lt;switch&gt;;
			bfd &lt;switch&gt;;
			ecmp weight &lt;num&gt;;
			ttl security [&lt;switch&gt;; | tx only]
			tx class|dscp &lt;num&gt;;
			tx priority &lt;num&gt;;
			authentication none|simple|cryptographic;
			password "&lt;text&gt;";
			password "&lt;text&gt;" {
				id &lt;num&gt;;
				generate from "&lt;date&gt;";
				generate to "&lt;date&gt;";
				accept from "&lt;date&gt;";
				accept to "&lt;date&gt;";
				from "&lt;date&gt;";
				to "&lt;date&gt;";
				algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
			};
			neighbors {
				&lt;ip&gt;;
				&lt;ip&gt; eligible;
			};
		};
		virtual link &lt;id&gt; [instance &lt;num&gt;] {
			hello &lt;num&gt;;
			retransmit &lt;num&gt;;
			wait &lt;num&gt;;
			dead count &lt;num&gt;;
			dead &lt;num&gt;;
			authentication none|simple|cryptographic;
			password "&lt;text&gt;";
			password "&lt;text&gt;" {
				id &lt;num&gt;;
				generate from "&lt;date&gt;";
				generate to "&lt;date&gt;";
				accept from "&lt;date&gt;";
				accept to "&lt;date&gt;";
				from "&lt;date&gt;";
				to "&lt;date&gt;";
				algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
			};
		};
	};
}
</code>

<descrip>
	<tag><label id="ospf-rfc1583compat">rfc1583compat <M>switch</M></tag>
	This option controls compatibility of routing table calculation with
	<rfc id="1583">. Default value is no.

	<tag><label id="ospf-instance-id">instance id <m/num/</tag>
	When multiple OSPF protocol instances are active on the same links, they
	should use different instance IDs to distinguish their packets. Although
	it could be done on per-interface basis, it is often preferred to set
	one instance ID to whole OSPF domain/topology (e.g., when multiple
	instances are used to represent separate logical topologies on the same
	physical network). This option specifies the default instance ID for all
	interfaces of the OSPF instance. Note that this option, if used, must
	precede interface definitions. Default value is 0.

	<tag><label id="ospf-stub-router">stub router <M>switch</M></tag>
	This option configures the router to be a stub router, i.e., a router
	that participates in the OSPF topology but does not allow transit
	traffic. In OSPFv2, this is implemented by advertising maximum metric
	for outgoing links. In OSPFv3, the stub router behavior is announced by
	clearing the R-bit in the router LSA. See <rfc id="6987"> for details.
	Default value is no.

	<tag><label id="ospf-tick">tick <M>num</M></tag>
	The routing table calculation and clean-up of areas' databases is not
	performed when a single link state change arrives. To lower the CPU
	utilization, it's processed later at periodical intervals of <m/num/
	seconds. The default value is 1.

	<tag><label id="ospf-ecmp">ecmp <M>switch</M> [limit <M>number</M>]</tag>
	This option specifies whether OSPF is allowed to generate ECMP
	(equal-cost multipath) routes. Such routes are used when there are
	several directions to the destination, each with the same (computed)
	cost. This option also allows to specify a limit on maximum number of
	nexthops in one route. By default, ECMP is disabled. If enabled,
	default	value of the limit is 16.

	<tag><label id="ospf-merge-external">merge external <M>switch</M></tag>
	This option specifies whether OSPF should merge external routes from
	different routers/LSAs for the same destination. When enabled together
	with <cf/ecmp/, equal-cost external routes will be combined to multipath
	routes in the same way as regular routes. When disabled, external routes
	from different LSAs are treated as separate even if they represents the
	same destination. Default value is no.

	<tag><label id="ospf-area">area <M>id</M></tag>
	This defines an OSPF area with given area ID (an integer or an IPv4
	address, similarly to a router ID). The most important area is the
	backbone (ID 0) to which every other area must be connected.

	<tag><label id="ospf-stub">stub</tag>
	This option configures the area to be a stub area. External routes are
	not flooded into stub areas. Also summary LSAs can be limited in stub
	areas (see option <cf/summary/). By default, the area is not a stub
	area.

	<tag><label id="ospf-nssa">nssa</tag>
	This option configures the area to be a NSSA (Not-So-Stubby Area). NSSA
	is a variant of a stub area which allows a limited way of external route
	propagation. Global external routes are not propagated into a NSSA, but
	an external route can be imported into NSSA as a (area-wide) NSSA-LSA
	(and possibly translated and/or aggregated on area boundary). By
	default, the area is not NSSA.

	<tag><label id="ospf-summary">summary <M>switch</M></tag>
	This option controls propagation of summary LSAs into stub or NSSA
	areas. If enabled, summary LSAs are propagated as usual, otherwise just
	the default summary route (0.0.0.0/0) is propagated (this is sometimes
	called totally stubby area). If a stub area has more area boundary
	routers, propagating summary LSAs could lead to more efficient routing
	at the cost of larger link state database. Default value is no.

	<tag><label id="ospf-default-nssa">default nssa <M>switch</M></tag>
	When <cf/summary/ option is enabled, default summary route is no longer
	propagated to the NSSA. In that case, this option allows to originate
	default route as NSSA-LSA to the NSSA. Default value is no.

	<tag><label id="ospf-default-cost">default cost <M>num</M></tag>
	This option controls the cost of a default route propagated to stub and
	NSSA areas. Default value is 1000.

	<tag><label id="ospf-default-cost2">default cost2 <M>num</M></tag>
	When a default route is originated as NSSA-LSA, its cost can use either
	type 1 or type 2 metric. This option allows to specify the cost of a
	default route in type 2 metric. By default, type 1 metric (option
	<cf/default cost/) is used.

	<tag><label id="ospf-translator">translator <M>switch</M></tag>
	This option controls translation of NSSA-LSAs into external LSAs. By
	default, one translator per NSSA is automatically elected from area
	boundary routers. If enabled, this area boundary router would
	unconditionally translate all NSSA-LSAs regardless of translator
	election. Default value is no.

	<tag><label id="ospf-translator-stability">translator stability <M>num</M></tag>
	This option controls the translator stability interval (in seconds).
	When the new translator is elected, the old one keeps translating until
	the interval is over. Default value is 40.

	<tag><label id="ospf-networks">networks { <m/set/ }</tag>
	Definition of area IP ranges. This is used in summary LSA origination.
	Hidden networks are not propagated into other areas.

	<tag><label id="ospf-external">external { <m/set/ }</tag>
	Definition of external area IP ranges for NSSAs. This is used for
	NSSA-LSA translation. Hidden networks are not translated into external
	LSAs. Networks can have configured route tag.

	<tag><label id="ospf-stubnet">stubnet <m/prefix/ { <m/options/ }</tag>
	Stub networks are networks that are not transit networks between OSPF
	routers. They are also propagated through an OSPF area as a part of a
	link state database. By default, BIRD generates a stub network record
	for each primary network address on each OSPF interface that does not
	have any OSPF neighbors, and also for each non-primary network address
	on each OSPF interface. This option allows to alter a set of stub
	networks propagated by this router.

	Each instance of this option adds a stub network with given network
	prefix to the set of propagated stub network, unless option <cf/hidden/
	is used. It also suppresses default stub networks for given network
	prefix. When option <cf/summary/ is used, also default stub networks
	that are subnetworks of given stub network are suppressed. This might be
	used, for example, to aggregate generated stub networks.

	<tag><label id="ospf-iface">interface <M>pattern</M> [instance <m/num/]</tag>
	Defines that the specified interfaces belong to the area being defined.
	See <ref id="proto-iface" name="interface"> common option for detailed
	description. In OSPFv2, extended interface clauses are used, because
	each network prefix is handled as a separate virtual interface.

	You can specify alternative instance ID for the interface definition,
	therefore it is possible to have several instances of that interface
	with different options or even in different areas. For OSPFv2, instance
	ID support is an extension (<rfc id="6549">) and is supposed to be set
	per-protocol. For OSPFv3, it is an integral feature.

	<tag><label id="ospf-virtual-link">virtual link <M>id</M> [instance <m/num/]</tag>
	Virtual link to router with the router id. Virtual link acts as a
	point-to-point interface belonging to backbone. The actual area is used
	as a transport area. This item cannot be in the backbone. Like with
	<cf/interface/ option, you could also use several virtual links to one
	destination with different instance IDs.

	<tag><label id="ospf-cost">cost <M>num</M></tag>
	Specifies output cost (metric) of an interface. Default value is 10.

	<tag><label id="ospf-stub-iface">stub <M>switch</M></tag>
	If set to interface it does not listen to any packet and does not send
	any hello. Default value is no.

	<tag><label id="ospf-hello">hello <M>num</M></tag>
	Specifies interval in seconds between sending of Hello messages. Beware,
	all routers on the same network need to have the same hello interval.
	Default value is 10.

	<tag><label id="ospf-poll">poll <M>num</M></tag>
	Specifies interval in seconds between sending of Hello messages for some
	neighbors on NBMA network. Default value is 20.

	<tag><label id="ospf-retransmit">retransmit <M>num</M></tag>
	Specifies interval in seconds between retransmissions of unacknowledged
	updates. Default value is 5.

	<tag><label id="ospf-priority">priority <M>num</M></tag>
	On every multiple access network (e.g., the Ethernet) Designated Router
	and Backup Designated router are elected. These routers have some special
	functions in the flooding process. Higher priority increases preferences
	in this election. Routers with priority 0 are not eligible. Default
	value is 1.

	<tag><label id="ospf-wait">wait <M>num</M></tag>
	After start, router waits for the specified number of seconds between
	starting election and building adjacency. Default value is 4*<m/hello/.

	<tag><label id="ospf-dead-count">dead count <M>num</M></tag>
	When the router does not receive any messages from a neighbor in
	<m/dead count/*<m/hello/ seconds, it will consider the neighbor down.

	<tag><label id="ospf-dead">dead <M>num</M></tag>
	When the router does not receive any messages from a neighbor in
	<m/dead/ seconds, it will consider the neighbor down. If both directives
	<cf/dead count/ and <cf/dead/ are used, <cf/dead/ has precedence.

	<tag><label id="ospf-secondary">secondary <M>switch</M></tag>
	On BSD systems, older versions of BIRD supported OSPFv2 only for the
	primary IP address of an interface, other IP ranges on the interface
	were handled as stub networks. Since v1.4.1, regular operation on
	secondary IP addresses is supported, but disabled by default for
	compatibility. This option allows to enable it. The option is a
	transitional measure, will be removed in the next major release as the
	behavior will be changed. On Linux systems, the option is irrelevant, as
	operation on non-primary addresses is already the regular behavior.

	<tag><label id="ospf-rx-buffer">rx buffer <M>num</M></tag>
	This option allows to specify the size of buffers used for packet
	processing. The buffer size should be bigger than maximal size of any
	packets. By default, buffers are dynamically resized as needed, but a
	fixed value could be specified. Value <cf/large/ means maximal allowed
	packet size - 65535.

	<tag><label id="ospf-tx-length">tx length <M>num</M></tag>
	Transmitted OSPF messages that contain large amount of information are
	segmented to separate OSPF packets to avoid IP fragmentation. This
	option specifies the soft ceiling for the length of generated OSPF
	packets. Default value is the MTU of the network interface. Note that
	larger OSPF packets may still be generated if underlying OSPF messages
	cannot be splitted (e.g. when one large LSA is propagated).

	<tag><label id="ospf-type-bcast">type broadcast|bcast</tag>
	BIRD detects a type of a connected network automatically, but sometimes
	it's convenient to force use of a different type manually. On broadcast
	networks (like ethernet), flooding and Hello messages are sent using
	multicasts (a single packet for all the neighbors). A designated router
	is elected and it is responsible for synchronizing the link-state
	databases and originating network LSAs. This network type cannot be used
	on physically NBMA networks and on unnumbered networks (networks without
	proper IP prefix).

	<tag><label id="ospf-type-ptp">type pointopoint|ptp</tag>
	Point-to-point networks connect just 2 routers together. No election is
	performed and no network LSA is originated, which makes it simpler and
	faster to establish. This network type is useful not only for physically
	PtP ifaces (like PPP or tunnels), but also for broadcast networks used
	as PtP links. This network type cannot be used on physically NBMA
	networks.

	<tag><label id="ospf-type-nbma">type nonbroadcast|nbma</tag>
	On NBMA networks, the packets are sent to each neighbor separately
	because of lack of multicast capabilities. Like on broadcast networks,
	a designated router is elected, which plays a central role in propagation
	of LSAs. This network type cannot be used on unnumbered networks.

	<tag><label id="ospf-type-ptmp">type pointomultipoint|ptmp</tag>
	This is another network type designed to handle NBMA networks. In this
	case the NBMA network is treated as a collection of PtP links. This is
	useful if not every pair of routers on the NBMA network has direct
	communication, or if the NBMA network is used as an (possibly
	unnumbered) PtP link.

	<tag><label id="ospf-link-lsa-suppression">link lsa suppression <m/switch/</tag>
	In OSPFv3, link LSAs are generated for each link, announcing link-local
	IPv6 address of the router to its local neighbors. These are useless on
	PtP or PtMP networks and this option allows to suppress the link LSA
	origination for such interfaces. The option is ignored on other than PtP
	or PtMP interfaces. Default value is no.

	<tag><label id="ospf-strict-nonbroadcast">strict nonbroadcast <m/switch/</tag>
	If set, don't send hello to any undefined neighbor. This switch is
	ignored on other than NBMA or PtMP interfaces. Default value is no.

	<tag><label id="ospf-real-broadcast">real broadcast <m/switch/</tag>
	In <cf/type broadcast/ or <cf/type ptp/ network configuration, OSPF
	packets are sent as IP multicast packets. This option changes the
	behavior to using old-fashioned IP broadcast packets. This may be useful
	as a workaround if IP multicast for some reason does not work or does
	not work reliably. This is a non-standard option and probably is not
	interoperable with other OSPF implementations. Default value is no.

	<tag><label id="ospf-ptp-netmask">ptp netmask <m/switch/</tag>
	In <cf/type ptp/ network configurations, OSPFv2 implementations should
	ignore received netmask field in hello packets and should send hello
	packets with zero netmask field on unnumbered PtP links. But some OSPFv2
	implementations perform netmask checking even for PtP links. This option
	specifies whether real netmask will be used in hello packets on <cf/type
 	ptp/ interfaces. You should ignore this option unless you meet some
	compatibility problems related to this issue. Default value is no for
	unnumbered PtP links, yes otherwise.

	<tag><label id="ospf-check-link">check link <M>switch</M></tag>
	If set, a hardware link state (reported by OS) is taken into consideration.
	When a link disappears (e.g. an ethernet cable is unplugged), neighbors
	are immediately considered unreachable and only the address of the iface
	(instead of whole network prefix) is propagated. It is possible that
	some hardware drivers or platforms do not implement this feature.
	Default value is no.

	<tag><label id="ospf-bfd">bfd <M>switch</M></tag>
	OSPF could use BFD protocol as an advisory mechanism for neighbor
	liveness and failure detection. If enabled, BIRD setups a BFD session
	for each OSPF neighbor and tracks its liveness by it. This has an
	advantage of an order of magnitude lower detection times in case of
	failure. Note that BFD protocol also has to be configured, see
	<ref id="bfd" name="BFD"> section for details. Default value is no.

	<tag><label id="ospf-ttl-security">ttl security [<m/switch/ | tx only]</tag>
	TTL security is a feature that protects routing protocols from remote
	spoofed packets by using TTL 255 instead of TTL 1 for protocol packets
	destined to neighbors. Because TTL is decremented when packets are
	forwarded, it is non-trivial to spoof packets with TTL 255 from remote
	locations. Note that this option would interfere with OSPF virtual
	links.

	If this option is enabled, the router will send OSPF packets with TTL
	255 and drop received packets with TTL less than 255. If this option si
	set to <cf/tx only/, TTL 255 is used for sent packets, but is not
	checked for received packets. Default value is no.

	<tag><label id="ospf-tx-class">tx class|dscp|priority <m/num/</tag>
	These options specify the ToS/DiffServ/Traffic class/Priority of the
	outgoing OSPF packets. See <ref id="proto-tx-class" name="tx class"> common
	option for detailed description.

	<tag><label id="ospf-ecmp-weight">ecmp weight <M>num</M></tag>
	When ECMP (multipath) routes are allowed, this value specifies a
	relative weight used for nexthops going through the iface. Allowed
	values are 1-256. Default value is 1.

	<tag><label id="ospf-auth-none">authentication none</tag>
	No passwords are sent in OSPF packets. This is the default value.

	<tag><label id="ospf-auth-simple">authentication simple</tag>
	Every packet carries 8 bytes of password. Received packets lacking this
	password are ignored. This authentication mechanism is very weak.
	This option is not available in OSPFv3.

	<tag><label id="ospf-auth-cryptographic">authentication cryptographic</tag>
	An authentication code is appended to every packet. The specific
	cryptographic algorithm is selected by option <cf/algorithm/ for each
	key. The default cryptographic algorithm for OSPFv2 keys is Keyed-MD5
	and for OSPFv3 keys is HMAC-SHA-256. Passwords are not sent open via
	network, so this mechanism is quite secure. Packets can still be read by
	an attacker.

	<tag><label id="ospf-pass">password "<M>text</M>"</tag>
	Specifies a password used for authentication. See
	<ref id="proto-pass" name="password"> common option for detailed
	description.

	<tag><label id="ospf-neighbors">neighbors { <m/set/ } </tag>
	A set of neighbors to which Hello messages on NBMA or PtMP networks are
	to be sent. For NBMA networks, some of them could be marked as eligible.
	In OSPFv3, link-local addresses should be used, using global ones is
	possible, but it is nonstandard and might be problematic. And definitely,
	link-local and global addresses should not be mixed.
</descrip>

<sect1>Attributes
<label id="ospf-attr">

<p>OSPF defines four route attributes. Each internal route has a <cf/metric/.

<p>Metric is ranging from 1 to infinity (65535). External routes use
<cf/metric type 1/ or <cf/metric type 2/. A <cf/metric of type 1/ is comparable
with internal <cf/metric/, a <cf/metric of type 2/ is always longer than any
<cf/metric of type 1/ or any <cf/internal metric/. <cf/Internal metric/ or
<cf/metric of type 1/ is stored in attribute <cf/ospf_metric1/, <cf/metric type
2/ is stored in attribute <cf/ospf_metric2/. If you specify both metrics only
metric1 is used.

<p>Each external route can also carry attribute <cf/ospf_tag/ which is a 32-bit
integer which is used when exporting routes to other protocols; otherwise, it
doesn't affect routing inside the OSPF domain at all. The fourth attribute
<cf/ospf_router_id/ is a router ID of the router advertising that route /
network. This attribute is read-only. Default is <cf/ospf_metric2 = 10000/ and
<cf/ospf_tag = 0/.

<sect1>Example
<label id="ospf-exam">

<p><code>
protocol ospf MyOSPF {
	rfc1583compat yes;
	tick 2;
	export filter {
		if source = RTS_BGP then {
			ospf_metric1 = 100;
			accept;
		}
		reject;
	};
	area 0.0.0.0 {
		interface "eth*" {
			cost 11;
			hello 15;
			priority 100;
			retransmit 7;
			authentication simple;
			password "aaa";
		};
		interface "ppp*" {
			cost 100;
			authentication cryptographic;
			password "abc" {
				id 1;
				generate to "22-04-2003 11:00:06";
				accept from "17-01-2001 12:01:05";
				algorithm hmac sha384;
			};
			password "def" {
				id 2;
				generate to "22-07-2005 17:03:21";
				accept from "22-02-2001 11:34:06";
				algorithm hmac sha512;
			};
		};
		interface "arc0" {
			cost 10;
			stub yes;
		};
		interface "arc1";
	};
	area 120 {
		stub yes;
		networks {
			172.16.1.0/24;
			172.16.2.0/24 hidden;
		}
		interface "-arc0" , "arc*" {
			type nonbroadcast;
			authentication none;
			strict nonbroadcast yes;
			wait 120;
			poll 40;
			dead count 8;
			neighbors {
				192.168.120.1 eligible;
				192.168.120.2;
				192.168.120.10;
			};
		};
	};
}
</code>


<sect>Pipe
<label id="pipe">

<sect1>Introduction
<label id="pipe-intro">

<p>The Pipe protocol serves as a link between two routing tables, allowing
routes to be passed from a table declared as primary (i.e., the one the pipe is
connected to using the <cf/table/ configuration keyword) to the secondary one
(declared using <cf/peer table/) and vice versa, depending on what's allowed by
the filters. Export filters control export of routes from the primary table to
the secondary one, import filters control the opposite direction.

<p>The Pipe protocol may work in the transparent mode mode or in the opaque
mode. In the transparent mode, the Pipe protocol retransmits all routes from
one table to the other table, retaining their original source and attributes.
If import and export filters are set to accept, then both tables would have
the same content. The transparent mode is the default mode.

<p>In the opaque mode, the Pipe protocol retransmits optimal route from one
table to the other table in a similar way like other protocols send and receive
routes. Retransmitted route will have the source set to the Pipe protocol, which
may limit access to protocol specific route attributes. This mode is mainly for
compatibility, it is not suggested for new configs. The mode can be changed by
<tt/mode/ option.

<p>The primary use of multiple routing tables and the Pipe protocol is for
policy routing, where handling of a single packet doesn't depend only on its
destination address, but also on its source address, source interface, protocol
type and other similar parameters. In many systems (Linux being a good example),
the kernel allows to enforce routing policies by defining routing rules which
choose one of several routing tables to be used for a packet according to its
parameters. Setting of these rules is outside the scope of BIRD's work (on
Linux, you can use the <tt/ip/ command), but you can create several routing
tables in BIRD, connect them to the kernel ones, use filters to control which
routes appear in which tables and also you can employ the Pipe protocol for
exporting a selected subset of one table to another one.

<sect1>Configuration
<label id="pipe-config">

<p><descrip>
	<tag><label id="pipe-peer-table">peer table <m/table/</tag>
	Defines secondary routing table to connect to. The primary one is
	selected by the <cf/table/ keyword.

	<tag><label id="pipe-mode">mode opaque|transparent</tag>
	Specifies the mode for the pipe to work in. Default is transparent.
</descrip>

<sect1>Attributes
<label id="pipe-attr">

<p>The Pipe protocol doesn't define any route attributes.

<sect1>Example
<label id="pipe-exam">

<p>Let's consider a router which serves as a boundary router of two different
autonomous systems, each of them connected to a subset of interfaces of the
router, having its own exterior connectivity and wishing to use the other AS as
a backup connectivity in case of outage of its own exterior line.

<p>Probably the simplest solution to this situation is to use two routing tables
(we'll call them <cf/as1/ and <cf/as2/) and set up kernel routing rules, so that
packets having arrived from interfaces belonging to the first AS will be routed
according to <cf/as1/ and similarly for the second AS. Thus we have split our
router to two logical routers, each one acting on its own routing table, having
its own routing protocols on its own interfaces. In order to use the other AS's
routes for backup purposes, we can pass the routes between the tables through a
Pipe protocol while decreasing their preferences and correcting their BGP paths
to reflect the AS boundary crossing.

<code>
table as1;				# Define the tables
table as2;

protocol kernel kern1 {			# Synchronize them with the kernel
	table as1;
	kernel table 1;
}

protocol kernel kern2 {
	table as2;
	kernel table 2;
}

protocol bgp bgp1 {			# The outside connections
	table as1;
	local as 1;
	neighbor 192.168.0.1 as 1001;
	export all;
	import all;
}

protocol bgp bgp2 {
	table as2;
	local as 2;
	neighbor 10.0.0.1 as 1002;
	export all;
	import all;
}

protocol pipe {				# The Pipe
	table as1;
	peer table as2;
	export filter {
		if net ~ [ 1.0.0.0/8+] then {	# Only AS1 networks
			if preference>10 then preference = preference-10;
			if source=RTS_BGP then bgp_path.prepend(1);
			accept;
		}
		reject;
	};
	import filter {
		if net ~ [ 2.0.0.0/8+] then {	# Only AS2 networks
			if preference>10 then preference = preference-10;
			if source=RTS_BGP then bgp_path.prepend(2);
			accept;
		}
		reject;
	};
}
</code>


<sect>RAdv
<label id="radv">

<sect1>Introduction
<label id="radv-intro">

<p>The RAdv protocol is an implementation of Router Advertisements, which are
used in the IPv6 stateless autoconfiguration. IPv6 routers send (in irregular
time intervals or as an answer to a request) advertisement packets to connected
networks. These packets contain basic information about a local network (e.g. a
list of network prefixes), which allows network hosts to autoconfigure network
addresses and choose a default route. BIRD implements router behavior as defined
in <rfc id="4861">, router preferences and specific routes (<rfc id="4191">),
and DNS extensions (<rfc id="6106">).

<sect1>Configuration
<label id="radv-config">

<p>There are several classes of definitions in RAdv configuration -- interface
definitions, prefix definitions and DNS definitions:

<descrip>
	<tag><label id="radv-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
	Interface definitions specify a set of interfaces on which the
	protocol is activated and contain interface specific options.
	See <ref id="proto-iface" name="interface"> common options for
	detailed description.

	<tag><label id="radv-prefix">prefix <m/prefix/ { <m/options/ }</tag>
	Prefix definitions allow to modify a list of advertised prefixes. By
	default, the advertised prefixes are the same as the network prefixes
	assigned to the interface. For each network prefix, the matching prefix
	definition is found and its options are used. If no matching prefix
	definition is found, the prefix is used with default options.

	Prefix definitions can be either global or interface-specific. The
	second ones are part of interface options. The prefix definition
	matching is done in the first-match style, when interface-specific
	definitions are processed before global definitions. As expected, the
	prefix definition is matching if the network prefix is a subnet of the
	prefix in prefix definition.

	<tag><label id="radv-rdnss">rdnss { <m/options/ }</tag>
	RDNSS definitions allow to specify a list of advertised recursive DNS
	servers together with their options. As options are seldom necessary,
	there is also a short variant <cf>rdnss <m/address/</cf> that just
	specifies one DNS server. Multiple definitions are cumulative. RDNSS
	definitions may also be interface-specific when used inside interface
	options. By default, interface uses both global and interface-specific
	options, but that can be changed by <cf/rdnss local/ option.

	<tag><label id="radv-dnssl">dnssl { <m/options/ }</tag>
	DNSSL definitions allow to specify a list of advertised DNS search
	domains together with their options. Like <cf/rdnss/ above, multiple
	definitions are cumulative, they can be used also as interface-specific
	options and there is a short variant <cf>dnssl <m/domain/</cf> that just
	specifies one DNS search domain.

	<tag><label id="radv-trigger">trigger <m/prefix/</tag>
	RAdv protocol could be configured to change its behavior based on
	availability of routes. When this option is used, the protocol waits in
	suppressed state until a <it/trigger route/ (for the specified network)
	is exported to the protocol, the protocol also returnsd to suppressed
	state if the <it/trigger route/ disappears. Note that route export
	depends on specified export filter, as usual. This option could be used,
	e.g., for handling failover in multihoming scenarios.

	During suppressed state, router advertisements are generated, but with
	some fields zeroed. Exact behavior depends on which fields are zeroed,
	this can be configured by <cf/sensitive/ option for appropriate
	fields. By default, just <cf/default lifetime/ (also called <cf/router
	lifetime/) is zeroed, which means hosts cannot use the router as a
	default router. <cf/preferred lifetime/ and <cf/valid lifetime/ could
	also be configured as <cf/sensitive/ for a prefix, which would cause
	autoconfigured IPs to be deprecated or even removed.

	<tag><label id="radv-propagate-routes">propagate routes <m/switch/</tag>
	This option controls propagation of more specific routes, as defined in
	<rfc id="4191">. If enabled, all routes exported to the RAdv protocol,
	with the exception of the trigger prefix, are added to advertisments as
	additional options. The lifetime and preference of advertised routes can
	be set individually by <cf/ra_lifetime/ and <cf/ra_preference/ route
	attributes, or per interface by <cf/route lifetime/ and
	<cf/route preference/ options. Default: disabled.

	Note that the RFC discourages from sending more than 17 routes and
	recommends the routes to be configured manually.
</descrip>

<p>Interface specific options:

<descrip>
	<tag><label id="radv-iface-max-ra-interval">max ra interval <m/expr/</tag>
	Unsolicited router advertisements are sent in irregular time intervals.
	This option specifies the maximum length of these intervals, in seconds.
	Valid values are 4-1800. Default: 600

	<tag><label id="radv-iface-min-ra-interval">min ra interval <m/expr/</tag>
	This option specifies the minimum length of that intervals, in seconds.
	Must be at least 3 and at most 3/4 * <cf/max ra interval/. Default:
	about 1/3 * <cf/max ra interval/.

	<tag><label id="radv-iface-min-delay">min delay <m/expr/</tag>
	The minimum delay between two consecutive router advertisements, in
	seconds. Default: 3

	<tag><label id="radv-iface-managed">managed <m/switch/</tag>
	This option specifies whether hosts should use DHCPv6 for IP address
	configuration. Default: no

	<tag><label id="radv-iface-other-config">other config <m/switch/</tag>
	This option specifies whether hosts should use DHCPv6 to receive other
	configuration information. Default: no

	<tag><label id="radv-iface-link-mtu">link mtu <m/expr/</tag>
	This option specifies which value of MTU should be used by hosts. 0
	means unspecified. Default: 0

	<tag><label id="radv-iface-reachable-time">reachable time <m/expr/</tag>
	This option specifies the time (in milliseconds) how long hosts should
	assume a neighbor is reachable (from the last confirmation). Maximum is
	3600000, 0 means unspecified. Default 0.

	<tag><label id="radv-iface-retrans-timer">retrans timer <m/expr/</tag>
	This option specifies the time (in milliseconds) how long hosts should
	wait before retransmitting Neighbor Solicitation messages. 0 means
	unspecified. Default 0.

	<tag><label id="radv-iface-current-hop-limit">current hop limit <m/expr/</tag>
	This option specifies which value of Hop Limit should be used by
	hosts. Valid values are 0-255, 0 means unspecified. Default: 64

	<tag><label id="radv-iface-default-lifetime">default lifetime <m/expr/ [sensitive <m/switch/]</tag>
	This option specifies the time (in seconds) how long (since the receipt
	of RA) hosts may use the router as a default router. 0 means do not use
	as a default router. For <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
	Default: 3 * <cf/max ra	interval/, <cf/sensitive/ yes.

	<tag><label id="radv-iface-default-preference">default preference low|medium|high</tag>
	This option specifies the Default Router Preference value to advertise
	to hosts. Default: medium.

	<tag><label id="radv-iface-route-lifetime">route lifetime <m/expr/ [sensitive <m/switch/]</tag>
	This option specifies the default value of advertised lifetime for
	specific routes; i.e., the time (in seconds) for how long (since the
	receipt of RA) hosts should consider these routes valid. A special value
	0xffffffff represents infinity. The lifetime can be overriden on a per
	route basis by the <ref id="rta-ra-lifetime" name="ra_lifetime"> route
	attribute. Default: 3 * <cf/max ra interval/, <cf/sensitive/ no.

	For the <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
	If <cf/sensitive/ is enabled, even the routes with the <cf/ra_lifetime/
	attribute become sensitive to the trigger.

	<tag><label id="radv-iface-route-preference">route preference low|medium|high</tag>
	This option specifies the default value of advertised route preference
	for specific routes. The value can be overriden on a per route basis by
	the <ref id="rta-ra-preference" name="ra_preference"> route attribute.
	Default: medium.

	<tag><label id="radv-prefix-linger-time">prefix linger time <m/expr/</tag>
	When a prefix or a route disappears, it is advertised for some time with
	zero lifetime, to inform clients it is no longer valid. This option
	specifies the time (in seconds) for how long prefixes are advertised
	that way. Default: 3 * <cf/max ra interval/.

	<tag><label id="radv-route-linger-time">route linger time <m/expr/</tag>
	When a prefix or a route disappears, it is advertised for some time with
	zero lifetime, to inform clients it is no longer valid. This option
	specifies the time (in seconds) for how long routes are advertised
	that way. Default: 3 * <cf/max ra interval/.

	<tag><label id="radv-iface-rdnss-local">rdnss local <m/switch/</tag>
	Use only local (interface-specific) RDNSS definitions for this
	interface. Otherwise, both global and local definitions are used. Could
	also be used to disable RDNSS for given interface if no local definitons
	are specified. Default: no.

	<tag><label id="radv-iface-dnssl-local">dnssl local <m/switch/</tag>
	Use only local DNSSL definitions for this interface. See <cf/rdnss local/
	option above. Default: no.
</descrip>


<p>Prefix specific options

<descrip>
	<tag><label id="radv-prefix-skip">skip <m/switch/</tag>
	This option allows to specify that given prefix should not be
	advertised. This is useful for making exceptions from a default policy
	of advertising all prefixes. Note that for withdrawing an already
	advertised prefix it is more useful to advertise it with zero valid
	lifetime. Default: no

	<tag><label id="radv-prefix-onlink">onlink <m/switch/</tag>
	This option specifies whether hosts may use the advertised prefix for
	onlink determination. Default: yes

	<tag><label id="radv-prefix-autonomous">autonomous <m/switch/</tag>
	This option specifies whether hosts may use the advertised prefix for
	stateless autoconfiguration. Default: yes

	<tag><label id="radv-prefix-valid-lifetime">valid lifetime <m/expr/ [sensitive <m/switch/]</tag>
	This option specifies the time (in seconds) how long (after the
	receipt of RA) the prefix information is valid, i.e., autoconfigured
	IP addresses can be assigned and hosts with that IP addresses are
	considered directly reachable. 0 means the prefix is no longer
	valid. For <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
	Default: 86400 (1 day), <cf/sensitive/ no.

	<tag><label id="radv-prefix-preferred-lifetime">preferred lifetime <m/expr/ [sensitive <m/switch/]</tag>
	This option specifies the time (in seconds) how long (after the
	receipt of RA) IP addresses generated from the prefix using stateless
	autoconfiguration remain preferred. For <cf/sensitive/ option,
	see <ref id="radv-trigger" name="trigger">. Default: 14400 (4 hours),
	<cf/sensitive/ no.
</descrip>

<p>RDNSS specific options:

<descrip>
	<tag><label id="radv-rdnss-ns">ns <m/address/</tag>
	This option specifies one recursive DNS server. Can be used multiple
	times for multiple servers. It is mandatory to have at least one
	<cf/ns/ option in <cf/rdnss/ definition.

	<tag><label id="radv-rdnss-lifetime">lifetime [mult] <m/expr/</tag>
	This option specifies the time how long the RDNSS information may be
	used by clients after the receipt of RA. It is expressed either in
	seconds or (when <cf/mult/ is used) in multiples of <cf/max ra
	interval/. Note that RDNSS information is also invalidated when
	<cf/default lifetime/ expires. 0 means these addresses are no longer
	valid DNS servers. Default: 3 * <cf/max ra interval/.
</descrip>


<p>DNSSL specific options:

<descrip>
	<tag><label id="radv-dnssl-domain">domain <m/address/</tag>
	This option specifies one DNS search domain. Can be used multiple times
	for multiple domains. It is mandatory to have at least one <cf/domain/
	option in <cf/dnssl/ definition.

	<tag><label id="radv-dnssl-lifetime">lifetime [mult] <m/expr/</tag>
	This option specifies the time how long the DNSSL information may be
	used by clients after the receipt of RA. Details are the same as for
	RDNSS <cf/lifetime/ option above. Default: 3 * <cf/max ra interval/.
</descrip>

<sect1>Attributes
<label id="radv-attr">

<p>RAdv defines two route attributes:

<descrip>
	<tag><label id="rta-ra-preference">enum ra_preference</tag>
	The preference of the route. The value can be <it/RA_PREF_LOW/,
	<it/RA_PREF_MEDIUM/ or <it/RA_PREF_HIGH/. If the attribute is not set,
	the <ref id="radv-iface-route-preference" name="route preference">
	option is used.

	<tag><label id="rta-ra-lifetime">int ra_lifetime</tag>
	The advertised lifetime of the route, in seconds. The special value of
	0xffffffff represents infinity. If the attribute is not set, the
	<ref id="radv-iface-route-lifetime" name="route lifetime">
	option is used.
</descrip>

<sect1>Example
<label id="radv-exam">

<p><code>
table radv_routes;			# Manually configured routes go here

protocol static {
	table radv_routes;

	route 2001:0DB8:4000::/48 unreachable;
	route 2001:0DB8:4010::/48 unreachable;

	route 2001:0DB8:4020::/48 unreachable {
		ra_preference = RA_PREF_HIGH;
		ra_lifetime = 3600;
	};
}

protocol radv {
	propagate routes yes;		# Propagate the routes from the radv_routes table
	table radv_routes;
	export all;

	interface "eth2" {
		max ra interval 5;	# Fast failover with more routers
		managed yes;		# Using DHCPv6 on eth2
		prefix ::/0 {
			autonomous off;	# So do not autoconfigure any IP
		};
	};

	interface "eth*";		# No need for any other options

	prefix 2001:0DB8:1234::/48 {
		preferred lifetime 0;	# Deprecated address range
	};

	prefix 2001:0DB8:2000::/48 {
		autonomous off;		# Do not autoconfigure
	};

	rdnss 2001:0DB8:1234::10;	# Short form of RDNSS

	rdnss {
		lifetime mult 10;
		ns 2001:0DB8:1234::11;
		ns 2001:0DB8:1234::12;
	};

	dnssl {
		lifetime 3600;
		domain "abc.com";
		domain "xyz.com";
	};
}
</code>


<sect>RIP
<label id="rip">

<sect1>Introduction
<label id="rip-intro">

<p>The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol,
where each router broadcasts (to all its neighbors) distances to all networks it
can reach. When a router hears distance to another network, it increments it and
broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some
network goes unreachable, routers keep telling each other that its distance is
the original distance plus 1 (actually, plus interface metric, which is usually
one). After some time, the distance reaches infinity (that's 15 in RIP) and all
routers know that network is unreachable. RIP tries to minimize situations where
counting to infinity is necessary, because it is slow. Due to infinity being 16,
you can't use RIP on networks where maximal distance is higher than 15
hosts.

<p>BIRD supports RIPv1 (<rfc id="1058">), RIPv2 (<rfc id="2453">), RIPng (<rfc
id="2080">), and RIP cryptographic authentication (<rfc id="4822">).

<p>RIP is a very simple protocol, and it has a lot of shortcomings. Slow
convergence, big network load and inability to handle larger networks makes it
pretty much obsolete. It is still usable on very small networks.

<sect1>Configuration
<label id="rip-config">

<p>RIP configuration consists mainly of common protocol options and interface
definitions, most RIP options are interface specific.

<code>
protocol rip [&lt;name&gt;] {
	infinity &lt;number&gt;;
	ecmp &lt;switch&gt; [limit &lt;number&gt;];
	interface &lt;interface pattern&gt; {
		metric &lt;number&gt;;
		mode multicast|broadcast;
		passive &lt;switch&gt;;
		address &lt;ip&gt;;
		port &lt;number&gt;;
		version 1|2;
		split horizon &lt;switch&gt;;
		poison reverse &lt;switch&gt;;
		check zero &lt;switch&gt;;
		update time &lt;number&gt;;
		timeout time &lt;number&gt;;
		garbage time &lt;number&gt;;
		ecmp weight &lt;number&gt;;
		ttl security &lt;switch&gt;; | tx only;
		tx class|dscp &lt;number&gt;;
		tx priority &lt;number&gt;;
		rx buffer &lt;number&gt;;
		tx length &lt;number&gt;;
		check link &lt;switch&gt;;
		authentication none|plaintext|cryptographic;
		password "&lt;text&gt;";
		password "&lt;text&gt;" {
			id &lt;num&gt;;
			generate from "&lt;date&gt;";
			generate to "&lt;date&gt;";
			accept from "&lt;date&gt;";
			accept to "&lt;date&gt;";
			from "&lt;date&gt;";
			to "&lt;date&gt;";
			algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
		};
	};
}
</code>

<descrip>
	<tag><label id="rip-infinity">infinity <M>number</M></tag>
	Selects the distance of infinity. Bigger values will make
	protocol convergence even slower. The default value is 16.

	<tag><label id="rip-ecmp">ecmp <M>switch</M> [limit <M>number</M>]</tag>
	This option specifies whether RIP is allowed to generate ECMP
	(equal-cost multipath) routes. Such routes are used when there are
	several directions to the destination, each with the same (computed)
	cost. This option also allows to specify a limit on maximum number of
	nexthops in one route. By default, ECMP is disabled. If enabled,
	default	value of the limit is 16.

	<tag><label id="rip-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
	Interface definitions specify a set of interfaces on which the
	protocol is activated and contain interface specific options.
	See <ref id="proto-iface" name="interface"> common options for
	detailed description.
</descrip>

<p>Interface specific options:

<descrip>
	<tag><label id="rip-iface-metric">metric <m/num/</tag>
	This option specifies the metric of the interface. When a route is
	received from the interface, its metric is increased by this value
	before further processing. Valid values are 1-255, but values higher
	than infinity has no further meaning. Default: 1.

	<tag><label id="rip-iface-mode">mode multicast|broadcast</tag>
	This option selects the mode for RIP to use on the interface. The
	default is multicast mode for RIPv2 and broadcast mode for RIPv1.
	RIPng always uses the multicast mode.

	<tag><label id="rip-iface-passive">passive <m/switch/</tag>
	Passive interfaces receive routing updates but do not transmit any
	messages. Default: no.

	<tag><label id="rip-iface-address">address <m/ip/</tag>
	This option specifies a destination address used for multicast or
	broadcast messages, the default is the official RIP (224.0.0.9) or RIPng
	(ff02::9) multicast address, or an appropriate broadcast address in the
	broadcast mode.

	<tag><label id="rip-iface-port">port <m/number/</tag>
	This option selects an UDP port to operate on, the default is the
	official RIP (520) or RIPng (521) port.

	<tag><label id="rip-iface-version">version 1|2</tag>
	This option selects the version of RIP used on the interface. For RIPv1,
	automatic subnet aggregation is not implemented, only classful network
	routes and host routes are propagated. Note that BIRD allows RIPv1 to be
	configured with features that are defined for RIPv2 only, like
	authentication or using multicast sockets. The default is RIPv2 for IPv4
	RIP, the option is not supported for RIPng, as no further versions are
	defined.

	<tag><label id="rip-iface-version-only">version only <m/switch/</tag>
	Regardless of RIP version configured for the interface, BIRD accepts
	incoming packets of any RIP version. This option restrict accepted
	packets to the configured version. Default: no.

	<tag><label id="rip-iface-split-horizon">split horizon <m/switch/</tag>
	Split horizon is a scheme for preventing routing loops. When split
	horizon is active, routes are not regularly propagated back to the
	interface from which they were received. They are either not propagated
	back at all (plain split horizon) or propagated back with an infinity
	metric (split horizon with poisoned reverse). Therefore, other routers
	on the interface will not consider the router as a part of an
	independent path to the destination of the route. Default: yes.

	<tag><label id="rip-iface-poison-reverse">poison reverse <m/switch/</tag>
	When split horizon is active, this option specifies whether the poisoned
	reverse variant (propagating routes back with an infinity metric) is
	used. The poisoned reverse has some advantages in faster convergence,
	but uses more network traffic. Default: yes.

	<tag><label id="rip-iface-check-zero">check zero <m/switch/</tag>
	Received RIPv1 packets with non-zero values in reserved fields should
	be discarded. This option specifies whether the check is performed or
	such packets are just processed as usual. Default: yes.

	<tag><label id="rip-iface-update-time">update time <m/number/</tag>
	Specifies the number of seconds between periodic updates. A lower number
	will mean faster convergence but bigger network load. Default: 30.

	<tag><label id="rip-iface-timeout-time">timeout time <m/number/</tag>
	Specifies the time interval (in seconds) between the last received route
	announcement and the route expiration. After that, the network is
	considered unreachable, but still is propagated with infinity distance.
	Default: 180.

	<tag><label id="rip-iface-garbage-time">garbage time <m/number/</tag>
	Specifies the time interval (in seconds) between the route expiration
	and the removal of the unreachable network entry. The garbage interval,
	when a route with infinity metric is propagated, is used for both
	internal (after expiration) and external (after withdrawal) routes.
	Default: 120.

	<tag><label id="rip-iface-ecmp-weight">ecmp weight <m/number/</tag>
	When ECMP (multipath) routes are allowed, this value specifies a
	relative weight used for nexthops going through the iface. Valid
	values are 1-256. Default value is 1.

	<tag><label id="rip-iface-auth">authentication none|plaintext|cryptographic</tag>
	Selects authentication method to be used. <cf/none/ means that packets
	are not authenticated at all, <cf/plaintext/ means that a plaintext
	password is embedded into each packet, and <cf/cryptographic/ means that
	packets are authenticated using some cryptographic hash function
	selected by option <cf/algorithm/ for each key. The default
	cryptographic algorithm for RIP keys is Keyed-MD5. If you set
	authentication to not-none, it is a good idea to add <cf>password</cf>
	section. Default: none.

	<tag><label id="rip-iface-pass">password "<m/text/"</tag>
	Specifies a password used for authentication. See <ref id="proto-pass"
	name="password"> common option for detailed description.

	<tag><label id="rip-iface-ttl-security">ttl security [<m/switch/ | tx only]</tag>
	TTL security is a feature that protects routing protocols from remote
	spoofed packets by using TTL 255 instead of TTL 1 for protocol packets
	destined to neighbors. Because TTL is decremented when packets are
	forwarded, it is non-trivial to spoof packets with TTL 255 from remote
	locations.

	If this option is enabled, the router will send RIP packets with TTL 255
	and drop received packets with TTL less than 255. If this option si set
	to <cf/tx only/, TTL 255 is used for sent packets, but is not checked
	for received packets. Such setting does not offer protection, but offers
	compatibility with neighbors regardless of whether they use ttl
	security.

	For RIPng, TTL security is a standard behavior (required by <rfc
	id="2080">) and therefore default value is yes. For IPv4 RIP, default
	value is no.

	<tag><label id="rip-iface-tx-class">tx class|dscp|priority <m/number/</tag>
	These options specify the ToS/DiffServ/Traffic class/Priority of the
	outgoing RIP packets. See <ref id="proto-tx-class" name="tx class"> common
	option for detailed description.

	<tag><label id="rip-iface-rx-buffer">rx buffer <m/number/</tag>
	This option specifies the size of buffers used for packet processing.
	The buffer size should be bigger than maximal size of received packets.
	The default value is 532 for IPv4 RIP and interface MTU value for RIPng.

	<tag><label id="rip-iface-tx-length">tx length <m/number/</tag>
	This option specifies the maximum length of generated RIP packets. To
	avoid IP fragmentation, it should not exceed the interface MTU value.
	The default value is 532 for IPv4 RIP and interface MTU value for RIPng.

	<tag><label id="rip-iface-check-link">check link <m/switch/</tag>
	If set, the hardware link state (as reported by OS) is taken into
	consideration. When the link disappears (e.g. an ethernet cable is
	unplugged), neighbors are immediately considered unreachable and all
	routes received from them are withdrawn. It is possible that some
	hardware drivers or platforms do not implement this feature.
	Default: no.
</descrip>

<sect1>Attributes
<label id="rip-attr">

<p>RIP defines two route attributes:

<descrip>
	<tag><label id="rta-rip-metric">int rip_metric</tag>
	RIP metric of the route (ranging from 0 to <cf/infinity/).  When routes
	from different RIP instances are available and all of them have the same
	preference, BIRD prefers the route with lowest <cf/rip_metric/. When a
	non-RIP route is exported to RIP, the default metric is 1.

	<tag><label id="rta-rip-tag">int rip_tag</tag>
	RIP route tag: a 16-bit number which can be used to carry additional
	information with the route (for example, an originating AS number in
	case of external routes). When a non-RIP route is exported to RIP, the
	default tag is 0.
</descrip>

<sect1>Example
<label id="rip-exam">

<p><code>
protocol rip {
	import all;
	export all;
	interface "eth*" {
		metric 2;
		port 1520;
		mode multicast;
		update time 12;
		timeout time 60;
		authentication cryptographic;
		password "secret" { algorithm hmac sha256; };
	};
}
</code>


<sect>Static
<label id="static">

<p>The Static protocol doesn't communicate with other routers in the network,
but instead it allows you to define routes manually. This is often used for
specifying how to forward packets to parts of the network which don't use
dynamic routing at all and also for defining sink routes (i.e., those telling to
return packets as undeliverable if they are in your IP block, you don't have any
specific destination for them and you don't want to send them out through the
default route to prevent routing loops).

<p>There are five types of static routes: `classical' routes telling to forward
packets to a neighboring router, multipath routes specifying several (possibly
weighted) neighboring routers, device routes specifying forwarding to hosts on a
directly connected network, recursive routes computing their nexthops by doing
route table lookups for a given IP, and special routes (sink, blackhole etc.)
which specify a special action to be done instead of forwarding the packet.

<p>When the particular destination is not available (the interface is down or
the next hop of the route is not a neighbor at the moment), Static just
uninstalls the route from the table it is connected to and adds it again as soon
as the destination becomes adjacent again.

<p>There are three classes of definitions in Static protocol configuration --
global options, static route definitions, and per-route options. Usually, the
definition of the protocol contains mainly a list of static routes.

<p>Global options:

<descrip>
	<tag><label id="static-check-link">check link <m/switch/</tag>
	If set, hardware link states of network interfaces are taken into
	consideration.  When link disappears (e.g. ethernet cable is unplugged),
	static routes directing to that interface are removed. It is possible
	that some hardware drivers or platforms do not implement this feature.
	Default: off.

	<tag><label id="static-igp-table">igp table <m/name/</tag>
	Specifies a table that is used for route table lookups of recursive
	routes. Default: the same table as the protocol is connected to.
</descrip>

<p>Route definitions (each may also contain a block of per-route options):

<descrip>
	<tag><label id="static-route-via-ip">route <m/prefix/ via <m/ip/</tag>
	Static route through a neighboring router. For link-local next hops,
	interface can be specified as a part of the address (e.g.,
	<cf/via fe80::1234%eth0/).

	<tag><label id="static-route-via-mpath">route <m/prefix/ multipath via <m/ip/ [weight <m/num/] [bfd <m/switch/] [via <m/.../]</tag>
	Static multipath route. Contains several nexthops (gateways), possibly
	with their weights.

	<tag><label id="static-route-via-iface">route <m/prefix/ via <m/"interface"/</tag>
	Static device route through an interface to hosts on a directly
	connected network.

	<tag><label id="static-route-recursive">route <m/prefix/ recursive <m/ip/</tag>
	Static recursive route, its nexthop depends on a route table lookup for
	given IP address.

	<tag><label id="static-route-drop">route <m/prefix/ blackhole|unreachable|prohibit</tag>
	Special routes specifying to silently drop the packet, return it as
	unreachable or return it as administratively prohibited. First two
	targets are also known as <cf/drop/ and <cf/reject/.
</descrip>

<p>Per-route options:

<descrip>
	<tag><label id="static-route-bfd">bfd <m/switch/</tag>
	The Static protocol could use BFD protocol for next hop liveness
	detection. If enabled, a BFD session to the route next hop is created
	and the static route is BFD-controlled -- the static route is announced
	only if the next hop liveness is confirmed by BFD. If the BFD session
	fails, the static route is removed. Note that this is a bit different
	compared to other protocols, which may use BFD as an advisory mechanism
	for fast failure detection but ignores it if a BFD session is not even
	established.

	This option can be used for static routes with a direct next hop, or
	also for for individual next hops in a static multipath route (see
	above). Note that BFD protocol also has to be configured, see
	<ref id="bfd" name="BFD"> section for details. Default value is no.

	<tag><label id="static-route-filter"><m/filter expression/</tag>
	This is a special option that allows filter expressions to be configured
	on per-route basis. Can be used multiple times. These expressions are
	evaluated when the route is originated, similarly to the import filter
	of the static protocol. This is especially useful for configuring route
	attributes, e.g., <cf/ospf_metric1 = 100;/ for a route that will be
	exported to the OSPF protocol.
</descrip>

<p>Static routes have no specific attributes.

<p>Example static config might look like this:

<p><code>
protocol static {
	table testable;			# Connect to a non-default routing table
	check link;			# Advertise routes only if link is up
	route 0.0.0.0/0 via 198.51.100.130; # Default route
	route 10.0.0.0/8 multipath	# Multipath route
		via 198.51.100.10 weight 2
		via 198.51.100.20 bfd	# BFD-controlled next hop
		via 192.0.2.1;
	route 203.0.113.0/24 unreachable; # Sink route
	route 10.2.0.0/24 via "arc0";	# Secondary network
	route 192.168.10.0/24 via 198.51.100.100 {
		ospf_metric1 = 20;	# Set extended attribute
	}
	route 192.168.10.0/24 via 198.51.100.100 {
		ospf_metric2 = 100;	# Set extended attribute
		ospf_tag = 2;		# Set extended attribute
		bfd;			# BFD-controlled route
	}
}
</code>


<chapt>Conclusions
<label id="conclusion">

<sect>Future work
<label id="future-work">

<p>Although BIRD supports all the commonly used routing protocols, there are
still some features which would surely deserve to be implemented in future
versions of BIRD:

<itemize>
<item>Opaque LSA's
<item>Route aggregation and flap dampening
<item>Multipath routes
<item>Multicast routing protocols
<item>Ports to other systems
</itemize>


<sect>Getting more help
<label id="help">

<p>If you use BIRD, you're welcome to join the bird-users mailing list
(<HTMLURL URL="mailto:bird-users@network.cz" name="bird-users@network.cz">)
where you can share your experiences with the other users and consult
your problems with the authors. To subscribe to the list, visit
<HTMLURL URL="http://bird.network.cz/?m_list" name="http://bird.network.cz/?m_list">.
The home page of BIRD can be found at <HTMLURL URL="http://bird.network.cz/" name="http://bird.network.cz/">.

<p>BIRD is a relatively young system and it probably contains some bugs. You can
report any problems to the bird-users list and the authors will be glad to solve
them, but before you do so, please make sure you have read the available
documentation and that you are running the latest version (available at
<HTMLURL URL="ftp://bird.network.cz/pub/bird" name="bird.network.cz:/pub/bird">).
(Of course, a patch which fixes the bug is always welcome as an attachment.)

<p>If you want to understand what is going inside, Internet standards are a good
and interesting reading. You can get them from
<HTMLURL URL="ftp://ftp.rfc-editor.org/" name="ftp.rfc-editor.org"> (or a
nicely sorted version from <HTMLURL URL="ftp://atrey.karlin.mff.cuni.cz/pub/rfc"
name="atrey.karlin.mff.cuni.cz:/pub/rfc">).

<p><it/Good luck!/

</book>

<!--
LocalWords:  GPL IPv GateD BGPv RIPv OSPFv Linux sgml html dvi sgmltools Pavel
LocalWords:  linuxdoc dtd descrip config conf syslog stderr auth ospf bgp Mbps
LocalWords:  router's eval expr num birdc ctl UNIX if's enums bool int ip GCC
LocalWords:  len ipaddress pxlen netmask enum bgppath bgpmask clist gw md eth
LocalWords:  RTS printn quitbird iBGP AS'es eBGP RFC multiprotocol IGP Machek
LocalWords:  EGP misconfigurations keepalive pref aggr aggregator BIRD's RTC
LocalWords:  OS'es AS's multicast nolisten misconfigured UID blackhole MRTD MTU
LocalWords:  uninstalls ethernets IP binutils ANYCAST anycast dest RTD ICMP rfc
LocalWords:  compat multicasts nonbroadcast pointopoint loopback sym stats
LocalWords:  Perl SIGHUP dd mm yy HH MM SS EXT IA UNICAST multihop Discriminator txt
LocalWords:  proto wildcard Ondrej Filip
-->