Why doesn´t my program send ARP requests (c++)? - c++

Question

I am learning low level sockets with c++. I have done a simple program that shall send an ARP request. The socket seems to send the packet but I cannot catch it with Wireshark. I have another small program that also sends ARP packets and those packets are captured by Wireshark (my program below is inspired by that program).
Have I done something wrong?
Removed code
EDIT
Removed code
EDIT 2
It seems that I also need to include ethernet header data in the packet, so I now make a packet containing ethernet header and ARP header data. Now the packet goes away and is captured by Wireshark. But Wireshark says it is gratuitous. As you can see, nor IP or MAC address of sender and receiver seem to have been set properly.
36 13.318179 Cimsys_33:44:55 Broadcast ARP 42 Gratuitous ARP for <No address> (Request)
EDIT 3
/*Fill arp header data*/
p.arp.ea_hdr.ar_hrd = htons(ARPHRD_ETHER);
p.arp.ea_hdr.ar_pro = htons(ETH_P_IP);
p.arp.ea_hdr.ar_hln = ETH_ALEN; // Must be pure INTEGER, not called with htons(), as I did
p.arp.ea_hdr.ar_pln = 4; // Must be pure INTEGER, not called with htons(), as I did
p.arp.ea_hdr.ar_op = htons(ETH_P_ARP);

Answer 1

This code does not look quite right:
struct in_addr *s_in_addr = (in_addr*)malloc(sizeof(struct in_addr));
struct in_addr *t_in_addr = (in_addr*)malloc(sizeof(struct in_addr));
s_in_addr->s_addr = inet_addr("192.168.1.5"); // source ip
t_in_addr->s_addr = inet_addr("192.168.1.6"); // target ip
memcpy(arp->arp_spa, &s_in_addr, 6);
memcpy(arp->arp_tpa, &t_in_addr, 6);
In the memcpy You care copying 6 bytes out. However, you are taking an address of a pointer type, which makes it a pointer to a pointer type. I think you meant to just pass in s_in_addr and t_in_addr.
Edit: Alan Curry notes that you are copying 6 bytes from and to objects that are only 4 bytes long.
However, it doesn't seem like the dynamic allocation is doing your code any good, you should just create the the s_in_addr and t_in_addr variables off the stack. Than, you would not need to change your memcpy code.
struct in_addr s_in_addr;
struct in_addr t_in_addr;
s_in_addr.s_addr = inet_addr("192.168.1.5"); // source ip
t_in_addr.s_addr = inet_addr("192.168.1.6"); // target ip
memcpy(arp->arp_spa, &s_in_addr, sizeof(arg->arp_spa));
memcpy(arp->arp_tpa, &t_in_addr, sizeof(arg->arg_tpa));
There is a similar problem with your arp packet itself. So you should allocate it off the stack. To prevent myself from making a lot of code changes, I'll illustrate it slightly differently:
struct ether_arp arp_packet;
struct ether_arp *arp = &arp_packet;
//...
for(int i = 0; i < 10; i++) {
if (sendto(sock, arp, sizeof(arp_packet), 0,
(struct sockaddr *)&sending_socket,
sizeof(sending_socket)) < 0) {
std::cout << "Could not send!" << std::endl;
}
}

Answer 2

#user315052 say that you should use memcpy(arp->arp_spa, &s_in_addr, sizeof(arg->arp_spa)); but this code just copy first 4 bytes of s_in_addr to arp->arp_spa that absolutely do nothing!
so just try this:
* (int32_t*) arp->arp_spa = inet_addr("192.168.1.1")
* (int32_t*) arp->arp_tpa = inet_addr("192.168.1.2")