Why does this code produce an incomplete IP address?

Question

I am reading a buffer from a socket (AF_PACKET, SOCK_DGRAM, htons(ETH_P_ARP)) with the following code. I am using an arp_frame struct to access the component parts of the contained ARP reply. The inet_ntoa() returns the correct first octet of the IP but the other octets are 0 producing 172.0.0.0.

Question 1 is why might this happen? Question 2 is how can I print r bytes of the msg buffer as hex in host byte order to debug the packet?

    unsigned char msg[65535];
    struct ether_arp *arp_frame = (struct ether_arp *)msg;

    while ((r = recv(sock, msg, sizeof(msg), 0))) {
            // skip it's not an ARP REPLY
            if (ntohs(arp_frame->arp_op) != ARPOP_REPLY)
                    continue;

            for (i = 0; i < SONOS_PREFIX_NUM; i++) {
                    if (!memcmp(sonos_prefixes[i], arp_frame->arp_sha, 3)) {
                            struct in_addr addr;
                            addr.s_addr = *arp_frame->arp_spa;

                            printf("Blah: %lu\n", ntohl(*arp_frame->arp_spa));
                            printf("Sonos found at %s\n", inet_ntoa(addr));
                    }
            }
    }

Answer 1

struct ether_arp looks like this:

struct  ether_arp {
    struct  arphdr ea_hdr;          /* fixed-size header */
    u_int8_t arp_sha[ETH_ALEN];     /* sender hardware address */
    u_int8_t arp_spa[4];            /* sender protocol address */
    u_int8_t arp_tha[ETH_ALEN];     /* target hardware address */
    u_int8_t arp_tpa[4];            /* target protocol address */
};

With that in mind, I think that your addr.s_addr = *arp_frame->arp_spa; looks a little fishy. arp_frame->arp_spa yields a u_int8_t[4], which you then dereference as a pointer. I think memcpy() might be more appropriate there.

Answer 2

If you think the API is broken, print out the bytes.

"Methinks, Brutus, the fault is not in the stars."