linux下检测ip冲突

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 

 /* vi: set sw=4 ts=4: */
 /*
  * arpping.c
  *
  * Mostly stolen from: dhcpcd - DHCP client daemon
  * by Yoichi Hariguchi <yoichi@fore.com>
  */

 #include <netinet/if_ether.h>
 #include <net/if_arp.h>

 #include "common.h"
 #include "dhcpd.h"

 //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.
 struct arpMsg {
     /* Ethernet header */
     uint8_t  h_dest[6];     /* 00 destination ether addr */
     uint8_t  h_source[6];   /* 06 source ether addr */
     uint16_t h_proto;       /* 0c packet type ID field */

     /* ARP packet */
     uint16_t htype;         /* 0e hardware type (must be ARPHRD_ETHER) */
     uint16_t ptype;         /* 10 protocol type (must be ETH_P_IP) */
     uint8_t  hlen;          /* 12 hardware address length (must be 6) */
     uint8_t  plen;          /* 13 protocol address length (must be 4) */
     uint16_t operation;     /* 14 ARP opcode */
     uint8_t  sHaddr[6];     /* 16 sender's hardware address */
     uint8_t  sInaddr[4];    /* 1c sender's IP address */
     uint8_t  tHaddr[6];     /* 20 target's hardware address */
     uint8_t  tInaddr[4];    /* 26 target's IP address */
     uint8_t  pad[18];       /* 2a pad for min. ethernet payload (60 bytes) */
 } PACKED;

 enum {
     ARP_MSG_SIZE = 0x2a
 };


 /* Returns 1 if no reply received */

 //主程序，如果返回1说明此ip可用
 int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *interface)
 {

     int timeout_ms;
 //这里使用poll来检测句柄。
     struct pollfd pfd[1];
 #define s (pfd[0].fd)           /* socket */
     int rv = 1;             /* "no reply received" yet */
     struct sockaddr addr;   /* for interface name */
     struct arpMsg arp;

 //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.  //这里是我转这篇文章的原因

     s = socket(PF_PACKET, SOCK_PACKET, htons(ETH_P_ARP));
     if (s == -1) {
         bb_perror_msg(bb_msg_can_not_create_raw_socket);
         return -1;
     }

     if (setsockopt_broadcast(s) == -1) {
         bb_perror_msg("cannot enable bcast on raw socket");
         goto ret;
     }
 //进行组包，由于是要广播，因此目的mac地址为全0.
     /* send arp request */
     memset(&arp, 0, sizeof(arp));
     memset(arp.h_dest, 0xff, 6);                    /* MAC DA */
     memcpy(arp.h_source, from_mac, 6);              /* MAC SA */
     arp.h_proto = htons(ETH_P_ARP);                 /* protocol type (Ethernet) */
     arp.htype = htons(ARPHRD_ETHER);                /* hardware type */
     arp.ptype = htons(ETH_P_IP);                    /* protocol type (ARP message) */
     arp.hlen = 6;                                   /* hardware address length */
     arp.plen = 4;                                   /* protocol address length */
     arp.operation = htons(ARPOP_REQUEST);           /* ARP op code */
     memcpy(arp.sHaddr, from_mac, 6);                /* source hardware address */
     memcpy(arp.sInaddr, &from_ip, sizeof(from_ip)); /* source IP address */
     /* tHaddr is zero-fiiled */                     /* target hardware address */
     memcpy(arp.tInaddr, &test_ip, sizeof(test_ip)); /* target IP address */

     memset(&addr, 0, sizeof(addr));
     safe_strncpy(addr.sa_data, interface, sizeof(addr.sa_data));
 //广播arp包.
     if (sendto(s, &arp, sizeof(arp), 0, &addr, sizeof(addr)) < 0) {
         // TODO: error message? caller didn't expect us to fail,
         // just returning 1 "no reply received" misleads it.
         goto ret;
     }

     /* wait for arp reply, and check it */
 //等待时间，超时则认为此ip地址可用
     timeout_ms = 2000;
     do {
         int r;
         unsigned prevTime = monotonic_us();

         pfd[0].events = POLLIN;
 //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。
         r = safe_poll(pfd, 1, timeout_ms);
         if (r < 0)
             break;
         if (r) {
 //读取返回数据.
             r = read(s, &arp, sizeof(arp));
             if (r < 0)
                 break;
 //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。
             if (r >= ARP_MSG_SIZE
              && arp.operation == htons(ARPOP_REPLY)
              /* don't check it: Linux doesn't return proper tHaddr (fixed in 2.6.24?) */
              /* && memcmp(arp.tHaddr, from_mac, 6) == 0 */
              && *((uint32_t *) arp.sInaddr) == test_ip
             ) {
 //说明ip地址已被使用
                 rv = 0;
                 break;
             }
         }
         timeout_ms -= ((unsigned)monotonic_us() - prevTime) / 1000;
     } while (timeout_ms > 0);

  ret:
     close(s);
     DEBUG("%srp reply received for this address", rv ? "No a" : "A");
     return rv;
 }