wolflion

判断IP是否冲突的方法

  1. 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
  4. /* vi: set sw=4 ts=4: */  
  5. /*
  6. * arpping.c
  7. *
  8. * Mostly stolen from: dhcpcd - DHCP client daemon
  9. * by Yoichi Hariguchi <yoichi@fore.com>
  10. */  
  11.   
  12. #include <netinet/if_ether.h>   
  13. #include <net/if_arp.h>   
  14.   
  15. #include "common.h"   
  16. #include "dhcpd.h"   
  17.   
  18. //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.   
  19. struct arpMsg {   
  20.     /* Ethernet header */  
  21.      uint8_t   h_dest[6];     /* 00 destination ether addr */  
  22.      uint8_t   h_source[6];   /* 06 source ether addr */  
  23.      uint16_t h_proto;       /* 0c packet type ID field */  
  24.   
  25.     /* ARP packet */  
  26.      uint16_t htype;         /* 0e hardware type (must be ARPHRD_ETHER) */  
  27.      uint16_t ptype;         /* 10 protocol type (must be ETH_P_IP) */  
  28.      uint8_t   hlen;          /* 12 hardware address length (must be 6) */  
  29.      uint8_t   plen;          /* 13 protocol address length (must be 4) */  
  30.      uint16_t operation;     /* 14 ARP opcode */  
  31.      uint8_t   sHaddr[6];     /* 16 sender's hardware address */  
  32.      uint8_t   sInaddr[4];    /* 1c sender's IP address */  
  33.      uint8_t   tHaddr[6];     /* 20 target's hardware address */  
  34.      uint8_t   tInaddr[4];    /* 26 target's IP address */  
  35.      uint8_t   pad[18];       /* 2a pad for min. ethernet payload (60 bytes) */  
  36. } PACKED;   
  37.   
  38. enum {   
  39.      ARP_MSG_SIZE = 0x2a   
  40. };   
  41.   
  42.   
  43. /* Returns 1 if no reply received */  
  44.   
  45. //主程序，如果返回1说明此ip可用   
  46. int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *interface)   
  47. {   
  48.   
  49.     int timeout_ms;   
  50. //这里使用poll来检测句柄。   
  51.     struct pollfd pfd[1];   
  52. #define s (pfd[0].fd)            /* socket */   
  53.     int rv = 1;             /* "no reply received" yet */  
  54.     struct sockaddr addr;   /* for interface name */  
  55.     struct arpMsg arp;   
  56.   
  57. //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.   
  58.   
  59.      s = socket(PF_PACKET, SOCK_PACKET, htons(ETH_P_ARP));   
  60.     if (s == -1) {   
  61.          bb_perror_msg(bb_msg_can_not_create_raw_socket);   
  62.         return -1;   
  63.      }   
  64.   
  65.     if (setsockopt_broadcast(s) == -1) {   
  66.          bb_perror_msg("cannot enable bcast on raw socket");   
  67.         goto ret;   
  68.      }   
  69. //进行组包，由于是要广播，因此目的mac地址为全0.   
  70.     /* send arp request */  
  71.      memset(&arp, 0, sizeof(arp));   
  72.      memset(arp.h_dest, 0xff, 6);                    /* MAC DA */  
  73.      memcpy(arp.h_source, from_mac, 6);              /* MAC SA */  
  74.     arp.h_proto = htons(ETH_P_ARP);                 /* protocol type (Ethernet) */  
  75.     arp.htype = htons(ARPHRD_ETHER);                /* hardware type */  
  76.     arp.ptype = htons(ETH_P_IP);                    /* protocol type (ARP message) */  
  77.     arp.hlen = 6;                                   /* hardware address length */  
  78.     arp.plen = 4;                                   /* protocol address length */  
  79.     arp.operation = htons(ARPOP_REQUEST);           /* ARP op code */  
  80.      memcpy(arp.sHaddr, from_mac, 6);                /* source hardware address */  
  81.      memcpy(arp.sInaddr, &from_ip, sizeof(from_ip)); /* source IP address */  
  82.     /* tHaddr is zero-fiiled */                     /* target hardware address */  
  83.      memcpy(arp.tInaddr, &test_ip, sizeof(test_ip)); /* target IP address */  
  84.   
  85.      memset(&addr, 0, sizeof(addr));   
  86.      safe_strncpy(addr.sa_data, interface, sizeof(addr.sa_data));   
  87. //广播arp包.   
  88.     if (sendto(s, &arp, sizeof(arp), 0, &addr, sizeof(addr)) < 0) {   
  89.         // TODO: error message? caller didn't expect us to fail,   
  90.         // just returning 1 "no reply received" misleads it.   
  91.         goto ret;   
  92.      }   
  93.   
  94.     /* wait for arp reply, and check it */  
  95. //等待时间，超时则认为此ip地址可用   
  96.      timeout_ms = 2000;   
  97.     do {   
  98.         int r;   
  99.          unsigned prevTime = monotonic_us();   
  100.   
  101.          pfd[0].events = POLLIN;   
  102. //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。   
  103.          r = safe_poll(pfd, 1, timeout_ms);   
  104.         if (r < 0)   
  105.             break;   
  106.         if (r) {   
  107. //读取返回数据.   
  108.              r = read(s, &arp, sizeof(arp));   
  109.             if (r < 0)   
  110.                 break;   
  111. //检测是否为应打包，发送ip是否为我们所请求的ip,这里是为了防止其他的数据包干扰我们检测。   
  112.             if (r >= ARP_MSG_SIZE   
  113.               && arp.operation == htons(ARPOP_REPLY)   
  114.              /* don't check it: Linux doesn't return proper tHaddr (fixed in 2.6.24?) */  
  115.              /* && memcmp(arp.tHaddr, from_mac, 6) == 0 */  
  116.               && *((uint32_t *) arp.sInaddr) == test_ip   
  117.              ) {   
  118. //说明ip地址已被使用   
  119.                  rv = 0;   
  120.                 break;   
  121.              }   
  122.          }   
  123.          timeout_ms -= ((unsigned)monotonic_us() - prevTime) / 1000;   
  124.      } while (timeout_ms > 0);   
  125.   
  126. ret:   
  127.      close(s);   
  128.      DEBUG("%srp reply received for this address", rv ? "No a" : "A");   
  129.     return rv;   
  130. }  

posted on 2010-04-23 03:50 lionel 阅读(823) 评论(0)  编辑 收藏 引用


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