linuxIP冲突检测.doc

原理其实很简单，那就是广播一个arp包，然后recv，如果没有数据(这里要设置延时)，那么说明这个ip是可用的，否则就检测这个数据是否为回复我们发出的arp的应答包.如果是则证明ip已被使用，否则继续等待. 这里可以看下busybox的dhcp中的检测程序。 networking/udhcp/arpping.c C代码 1./* vi: set sw=4 ts=4: */ 2./* 3. * arpping.c 4. * 5. * Mostly stolen from: dhcpcd - DHCP client daemon 6. * by Yoichi Hariguchi 7. */ 8. 9.#include 10.#include 11. 12.#include common.h 13.#include dhcpd.h 14. 15./这里是arp包的格式,其中的数据格式都是宏了，比如uint_8_t为无符char. 16.struct arpMsg 17. /* Ethernet header */ 18. uint8_t h_dest6; /* 00 destination ether addr */ 19. uint8_t h_source6; /* 06 source ether addr */ 20. uint16_t h_proto; /* 0c packet type ID field */ 21. 22. /* ARP packet */ 23. uint16_t htype; /* 0e hardware type (must be ARPHRD_ETHER) */ 24. uint16_t ptype; /* 10 protocol type (must be ETH_P_IP) */ 25. uint8_t hlen; /* 12 hardware address length (must be 6) */ 26. uint8_t plen; /* 13 protocol address length (must be 4) */ 27. uint16_t operation; /* 14 ARP opcode */ 28. uint8_t sHaddr6; /* 16 senders hardware address */ 29. uint8_t sInaddr4; /* 1c senders IP address */ 30. uint8_t tHaddr6; /* 20 targets hardware address */ 31. uint8_t tInaddr4; /* 26 targets IP address */ 32. uint8_t pad18; /* 2a pad for min. ethernet payload (60 bytes) */ 33. PACKED; 34. 35.enum 36. ARP_MSG_SIZE = 0x2a 37.; 38. 39. 40./* Returns 1 if no reply received */ 41. 42./主程序，如果返回1说明此ip可用 43.int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *interface) 44. 45. 46. int timeout_ms; 47./这里使用poll来检测句柄。 48. struct pollfd pfd1; 49.#define s (pfd0.fd) /* socket */ 50. int rv = 1; /* no reply received yet */ 51. struct sockaddr addr; /* for interface name */ 52. struct arpMsg arp; 53. 54./建立scoket.由于我们是要直接访问访问链路层并自己组arp包.因此我们使用PF_PACKET协议簇.socket类型为SOCK_PACKET. 55. 56. s = socket(PF_PACKET, SOCK_PACKET, htons(ETH_P_ARP); 57. if (s = -1) 58. bb_perror_msg(bb_msg_can_not_create_raw_socket); 59. return -1; 60. 61. 62. if (setsockopt_broadcast(s) = -1) 63. bb_perror_msg(cannot enable bcast on raw socket); 64. goto ret; 65. 66./进行组包，由于是要广播，因此目的mac地址为全0. 67. /* send arp request */ 68. memset(&arp, 0, sizeof(arp); 69. memset(arp.h_dest, 0xff, 6); /* MAC DA */ 70. memcpy(arp.h_source, from_mac, 6); /* MAC SA */ 71. arp.h_proto = htons(ETH_P_ARP); /* protocol type (Ethernet) */ 72. arp.htype = htons(ARPHRD_ETHER); /* hardware type */ 73. arp.ptype = htons(ETH_P_IP); /* protocol type (ARP message) */ 74. arp.hlen = 6; /* hardware address length */ 75. arp.plen = 4; /* protocol address length */ 76. arp.operation = htons(ARPOP_REQUEST); /* ARP op code */ 77. memcpy(arp.sHaddr, from_mac, 6); /* source hardware address */ 78. memcpy(arp.sInaddr, &from_ip, sizeof(from_ip); /* source IP address */ 79. /* tHaddr is zero-fiiled */ /* target hardware address */ 80. memcpy(arp.tInaddr, &test_ip, sizeof(test_ip); /* target IP address */ 81. 82. memset(&addr, 0, sizeof(addr); 83. safe_strncpy(addr.sa_data, interface, sizeof(addr.sa_data); 84./广播arp包. 85. if (sendto(s, &arp, sizeof(arp), 0, &addr, sizeof(addr) 0) 86. / TODO: error message? caller didnt expect us to fail, 87. / just returning 1 no reply received misleads it. 88. goto ret; 89. 90. 91. /* wait for arp reply, and check it */ 92./等待时间，超时则认为此ip地址可用 93. timeout_ms = 2000; 94. do 95. int r; 96. unsigned prevTime = monotonic_us(); 97. 98. pfd0.events = POLLIN; 99./这边他是害怕poll被信号打断，因此加了层循环，其实这边我们还可以使用ppoll的，就可以了。 100. r = safe_poll(pfd, 1, timeout_ms); 101. if (r 0) 102. break; 103. if (r) 104./读取返回数据. 105. r = read(s, &arp, sizeof(arp); 106. if (r = ARP_MSG_SIZE 110. & arp.operation = htons(ARPOP_REPLY) 111. /* dont check it: Linux doesnt return proper tHaddr (fixed in 2.6.24?) */ 112. /* & memcmp(arp.tHaddr, from_mac, 6) = 0 */ 113. & *(uint32_t *) arp.sInaddr) = test_ip 114. ) 115./说明ip地址已被使用 116. rv = 0; 117. break; 118. 119. 120. timeout_ms -= (unsigned)monotonic_us() - prevTime) / 1000; 121. while (timeout_