c - 如何正确填充套接字地址？

Question

我在 iOS 中通过套接字发送 ICMP 数据包:

struct sockaddr         hostAddress;

self->hostAddress.sa_family = AF_INET;
inet_pton(AF_INET, "173.194.67.93", &self->hostAddress.sa_data);

然后，我打开套接字并发送数据包:

bytesSent = sendto(
                   CFSocketGetNative(self->_socket),
                   [packet bytes],
                   [packet length],
                   0,
                   &self->hostAddress,
                   sizeof(self->hostAddress)
                   );

当我在 WireShark 中看到数据包时，它被发送到“67.93.0.0”，而不是“173.194.67.93”。

问题出在哪里？

Answer 1

inet_pton(AF_INET, ...);

应该将 struct in_addr 作为第三个参数(参见 docs)。

你给它sa_data，来自

struct sockaddr {
    unsigned short    sa_family;    // address family, AF_xxx
    char              sa_data[14];  // 14 bytes of protocol address
};

真正被视为:

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
};

因此，前 16 位进入端口，IPV4 地址的后 16 位实际上进入 sin_addr，其余似乎被零初始化。

---

你应该这样做(为了清楚起见，我会加入大量的转换)

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);