How do I populate socket address properly? - c

Question

I'm sending a ICMP packet through a socket in iOS:
struct sockaddr         hostAddress;
self->hostAddress.sa_family = AF_INET;
inet_pton(AF_INET, "173.194.67.93", &self->hostAddress.sa_data);
Then, I open the socket and I send the packet:
bytesSent = sendto(
CFSocketGetNative(self->_socket),
[packet bytes],
[packet length],
0,
&self->hostAddress,
sizeof(self->hostAddress)
);
When I see the packet in WireShark, it's being sent to "67.93.0.0", instead to "173.194.67.93".
Where could be the problem?

Answer 1

inet_pton(AF_INET, ...);
should take a struct in_addr as the 3rd argument (see docs).
You're giving it sa_data, from
struct sockaddr {
unsigned short sa_family; // address family, AF_xxx
char sa_data[14]; // 14 bytes of protocol address
};
which is really treated as:
struct sockaddr_in {
short sin_family; // e.g. AF_INET, AF_INET6
unsigned short sin_port; // e.g. htons(3490)
struct in_addr sin_addr; // see struct in_addr, below
char sin_zero[8]; // zero this if you want to
};
So, the first 16 bits are going in the port, the second 16 bits of the IPV4 address are actually making it into sin_addr, and the rest appears to be zero-initialized.
You should be doing this (I'll put loads of casts in for clarity)
struct sockaddr *addr = &self->hostAddress; // generic address
struct sockaddr_in *ipv4 = (struct sockaddr_in *)addr; // downcast to ipv4
inet_pton(AF_INET, "173.194.67.93", &ipv4->sin_addr);