【转】busybox分析——arp设置ARP缓存表中的mac地址

转自：http://blog.chinaunix.net/uid-26009923-id-5098083.html

1. 将arp缓存表中某一IP的MAC地址修改

[email protected]:/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;
}

3. 执行结果

[email protected]:/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

[email protected]:/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

[email protected]:/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

[email protected]:/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)

[email protected]:/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