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SIOCGARP to display the entire ARP table.

Programming




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SIOCGARP to display the entire ARP table..
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Old Unix and Linux 08-25-2010
semash! semash! is offline
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Yeah... Here's part of the function i was talking about, just for sharing and general knowledge...

Code:
struct nlist nl[] = {
#define    X_ARPTAB    0
    { "_arptab" },
#define    X_ARPTAB_SIZE    1
    { "_arptab_size" },
#define    N_SYSMAP    2
    { "_Sysmap" },
#define    N_SYSSIZE    3
    { "_Syssize" },
    { "" },
};

static struct pte *Sysmap;

/*
 * Dump the entire arp table
 */
dump(kernel, mem)
    char *kernel, *mem;
{
    extern int h_errno;
    struct arptab *at;
    struct hostent *hp;
    int bynumber, mf, arptab_size, sz;
    char *host, *malloc();
    off_t lseek();

    if (nlist(kernel, nl) < 0 || nl[X_ARPTAB_SIZE].n_type == 0) {
        fprintf(stderr, "arp: %s: bad namelist\n", kernel);
        exit(1);
    }
    mf = open(mem, O_RDONLY);
    if (mf < 0) {
        fprintf(stderr, "arp: cannot open %s\n", mem);
        exit(1);
    }
    if (kflag) {
        off_t off;

        Sysmap = (struct pte *)
           malloc((u_int)(nl[N_SYSSIZE].n_value * sizeof(struct pte)));
        if (!Sysmap) {
            fputs("arp: can't get memory for Sysmap.\n", stderr);
            exit(1);
        }
        off = nl[N_SYSMAP].n_value & ~KERNBASE;
        (void)lseek(mf, off, L_SET);
        (void)read(mf, (char *)Sysmap,
            (int)(nl[N_SYSSIZE].n_value * sizeof(struct pte)));
    }
    klseek(mf, (long)nl[X_ARPTAB_SIZE].n_value, L_SET);
    read(mf, &arptab_size, sizeof arptab_size);
    if (arptab_size <= 0 || arptab_size > 1000) {
        fprintf(stderr, "arp: %s: namelist wrong\n", kernel);
        exit(1);
    }
    sz = arptab_size * sizeof (struct arptab);
    at = (struct arptab *)malloc((u_int)sz);
    if (at == NULL) {
        fputs("arp: can't get memory for arptab.\n", stderr);
        exit(1);
    }
    klseek(mf, (long)nl[X_ARPTAB].n_value, L_SET);
    if (read(mf, (char *)at, sz) != sz) {
        perror("arp: error reading arptab");
        exit(1);
    }
    close(mf);
    for (bynumber = 0; arptab_size-- > 0; at++) {
        if (at->at_iaddr.s_addr == 0 || at->at_flags == 0)
            continue;
        if (bynumber == 0)
            hp = gethostbyaddr((caddr_t)&at->at_iaddr,
                sizeof at->at_iaddr, AF_INET);
        else
            hp = 0;
        if (hp)
            host = hp->h_name;
        else {
            host = "?";
            if (h_errno == TRY_AGAIN)
                bynumber = 1;
        }
        printf("%s (%s) at ", host, inet_ntoa(at->at_iaddr));
        if (at->at_flags & ATF_COM)
            ether_print(at->at_enaddr);
        else
            printf("(incomplete)");
        if (at->at_flags & ATF_PERM)
            printf(" permanent");
        if (at->at_flags & ATF_PUBL)
            printf(" published");
        if (at->at_flags & ATF_USETRAILERS)
            printf(" trailers");
        printf("\n");
    }
}



/*
 * Seek into the kernel for a value.
 */
klseek(fd, base, off)
    int fd, off;
    off_t base;
{
    off_t lseek();

    if (kflag) {    /* get kernel pte */
        base &= ~KERNBASE;
        base = ctob(Sysmap[btop(base)].pg_pfnum) + (base & PGOFSET);
    }
    (void)lseek(fd, base, off);
}

Linux

You see? It's incomprehensible.