cong@msi:/work/test/tcpip/busy/arp$ cat arp.c

#include "utils.h"

#include <net/if_arp.h>

#include <linux/sockios.h>

//usage: arp 192.168.4.111 78:6a:89:18:31:0c

#define ETH_ALEN 6

int INET_resolve(const char *name, struct sockaddr_in *s_in)

{

    struct hostent *hp;

    /* Grmpf. -FvK */

    s_in->sin_family = AF_INET;

    s_in->sin_port = 0;

    /* Look to see if it's a dotted quad. */

    if (inet_aton(name, &s_in->sin_addr)) {

        return 0;

    }

    hp = gethostbyname(name);

    if (hp == NULL) {

        return -1;

    }

    memcpy(&s_in->sin_addr, hp->h_addr_list[0], sizeof(struct in_addr));

    return 0;

}

/* Convert Ethernet address from "XX[:]XX[:]XX[:]XX[:]XX[:]XX" to sockaddr.

 * Return nonzero on error.

 */

int in_ether(const char *bufp, struct sockaddr *sap)

{

    char *ptr;

    int i, j;

    unsigned char val;

    unsigned char c;

    dbmsg();

    sap->sa_family = ARPHRD_ETHER;

    ptr = (char *) sap->sa_data;

    i = ETH_ALEN;

    goto first;

    do {

        /* We might get a semicolon here */

        if (*bufp == ':')

            bufp++;

 first:

        j = val = 0;

        do {

            c = *bufp;

            if (((unsigned char)(c - '0')) <= 9) {

                c -= '0';

            } else if ((unsigned char)((c|0x20) - 'a') <= 5) {

                c = (unsigned char)((c|0x20) - 'a') + 10;

            } else {

                if (j && (c == ':' || c == '\0'))

                    /* One-digit byte: __:X:__ */

                    break;

                return -1;

            }

            ++bufp;

            val <<= 4;

            val += c;

            j ^= 1;

        } while (j);

        *ptr++ = val;

    } while (--i);

    /* Error if we aren't at end of string */

    return *bufp;

}

int main ( int argc, char *argv[] )

{

    struct arpreq req;

    struct sockaddr sa;

    int ret;

    if(argc != 3)

    {

        dbmsg("usage: ./arp ");

        exit(0);

    }

    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    if(sockfd < 0)

        return -1;

    memset(&req, 0, sizeof(req));

    //a. put host IP to arp_p

    INET_resolve(argv[1], (struct sockaddr_in*)&sa);

    memcpy(&req.arp_pa, &sa, sizeof(struct sockaddr));

    //b. put host MAC to arp_ha

    in_ether(argv[2], &req.arp_ha);

    //c. set flag

    req.arp_flags = ATF_PERM | ATF_COM;

    //d. invoke ioctl

    dbmsg("next invoke ioctl");

    ret = ioctl(sockfd, SIOCSARP, &req);

    if(ret < 0)

    {

        dbmsg("error %s", strerror(errno));

        exit(-1);

    }

    return EXIT_SUCCESS;

}

cong@msi:/work/test/tcpip/busy/arp$ cat /proc/net/arp | grep "192.168.4.111"   //查看arp缓存中的192.168.4.111的MAC地址

192.168.4.111 0x1 0x2 78:6a:89:18:31:0b * eth2    -->结尾是0b

cong@msi:/work/test/tcpip/busy/arp$ ping 192.168.4.111   //可以ping通的

PING 192.168.4.111 (192.168.4.111) 56(84) bytes of data.

64 bytes from 192.168.4.111: icmp_seq=1 ttl=64 time=93.9 ms

cong@msi:/work/test/tcpip/busy/arp$ sudo ./arp 192.168.4.111 78:6a:89:18:31:0c  //修改arp缓存中的192.168.4.111的MAC地址

arp.c:in_ether[38]:

arp.c:main[101]: next invoke ioctl

cong@msi:/work/test/tcpip/busy/arp$ ping 192.168.4.111     //再次ping发现不通了

PING 192.168.4.111 (192.168.4.111) 56(84) bytes of data.

From 192.168.4.62: icmp_seq=1 Redirect Network(New nexthop: 192.168.4.62)

From 192.168.4.62: icmp_seq=2 Redirect Network(New nexthop: 192.168.4.62)

From 192.168.4.62: icmp_seq=3 Redirect Network(New nexthop: 192.168.4.62)

cong@msi:/work/test/tcpip/busy/arp$ cat /proc/net/arp | grep "192.168.4.111"     //查看arp缓存中的192.168.4.111的MAC地址

192.168.4.111 0x1 0x6 78:6a:89:18:31:0c * eth2  -->将结尾的0b改成了0c