Translate MAC address to IP address

[Nick Barnes]

At 2003-08-15 11:13:57+0000, I wrote:
> Next week I may spend the time to extend my "ethercount" program,
> using the "pingall" code and the guts of "arp -a", to report using IP
> addresses instead of MACs.

FYI, here are the relevant guts of "arp -na":

/* Usual BSD copyright notice goes here */

#include <sys/file.h>
#include <sys/socket.h>
#include <sys/sysctl.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <netinet/if_ether.h>
#include <arpa/inet.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>

#define ROUNDUP(a) \
        ((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))
int
main(int argc, char *argv[])
{
        int mib[6];
        size_t needed;
        char *lim, *buf, *next;
        if (argc != 1) {
                (void)fprintf(stderr, "Usage: %s\n", argv[0]);
                exit(1);
        }

        mib[0] = CTL_NET;
        mib[1] = PF_ROUTE;
        mib[2] = 0;
        mib[3] = AF_INET;
        mib[4] = NET_RT_FLAGS;
        mib[5] = RTF_LLINFO;
        if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0)
                errx(1, "route-sysctl-estimate");
        if ((buf = malloc(needed)) == NULL)
                errx(1, "malloc");
        if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0)
                errx(1, "actual retrieval of routing table");

        lim = buf + needed;
        next = buf;
        while (next < lim) {
                struct rt_msghdr *rtm = (struct rt_msghdr *)next;
                struct sockaddr_inarp *sinarp = (struct sockaddr_inarp *)(rtm + 1);
                struct sockaddr_dl *sdl =
                        (struct sockaddr_dl *)((char *)sinarp + ROUNDUP(sinarp->sin_len));
                if (sdl->sdl_alen) { /* complete ARP entry */
                        printf("%s at ", inet_ntoa(sinarp->sin_addr));
                        printf("%s", ether_ntoa((struct ether_addr *)LLADDR(sdl)));
                        printf("\n");
                }
                next += rtm->rtm_msglen;
        }
        free(buf);
        return(0);
}