Giscard Wépiwé wrote:
>
> Hallo everybody,
> i write a small program to read from the arp table. It
> functions but the output is corrupted.
> I would want to know why?
>
> The output of my program:
> [root@heghta fleetnet]# ./arp
> 00 ffffffe0 2b 6f 1c 00
>
> The output of the well-know routine arp:
> [root@heghta fleetnet]# arp -a
> pern.fokus.gmd.de (193.175.135.3) at 08:00:20:8F:7C:50
> [ether] on eth0
> buran.fokus.gmd.de (193.175.135.1) at
> 00:E0:2B:6F:1C:00 [ether] on eth0
>
> The program:
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> #include <sys/socket.h>
> #include <linux/if_arp.h>
> #include <sys/ioctl.h>
> #include <netinet/in.h>
> #include <arpa/inet.h>
>
> int main(int argc, char **argv)
> {
> struct sockaddr_in gw_addr;
> struct arpreq myreq;
> int sockfd, i;
> FILE *logfd;
>
> logfd = fopen("logfile", "w+");
> memset(&gw_addr, '\0', sizeof(gw_addr));
> gw_addr.sin_family = AF_INET;
> gw_addr.sin_addr.s_addr =
> inet_addr("193.175.135.1");
>
> memset(&myreq, '\0', sizeof(myreq));
> memcpy(&myreq.arp_pa, &gw_addr, sizeof(gw_addr));
> memcpy(&myreq.arp_dev, "eth0",
> sizeof(myreq.arp_dev));
>
> if ((sockfd = socket(AF_INET,SOCK_DGRAM, 0)) < 0) {
> perror("socket");
> exit(-1);
> }
>
> if (ioctl(sockfd,SIOCGARP,(char *)&myreq) < 0) {
> perror("ioctl");
> exit(-1);
> }
>
> for ( i = 0; i < 6; i++)
> fprintf(stdout,"%02x%s",myreq.arp_ha.sa_data[i], i
> == 5 ?"\n": " ");
>
> return 0;
> }
>
> Please try it on your machine and tell me if it
> functions well.
> Thanks.
>
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Hi,
What you are getting is correct (at least according to your code).
You should replace myreq.arp_ha.sa_data[i] with
(myreq.arp_ha.sa_data[i] & 0xff).
myreq.arp_ha.sa_data[i] is a char and is implicitely
converted to an int when passed to fprintf. This is
normal C behavior.
Regards
--
Joseph Bueno
NetClub/Trader.com
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