linux IP 冲突检测

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
 */

#include
#include

#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.

    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;
}