c - 使用原始套接字时，为什么需要给出两次 MAC 地址？

Question

我想在 Linux 中使用 raw_socket 发送以太网帧，我使用下面的代码。它有效，但我不明白既然我已经在帧(缓冲区)中给出了 MAC 地址，为什么我必须在 struct sockaddr_ll 中再次给出它？

s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
/*socket_address*/
struct sockaddr_ll socket_address;      
socket_address.sll_addr[0]  = 0x00;     
socket_address.sll_addr[1]  = 0x04;     
socket_address.sll_addr[2]  = 0x75;
socket_address.sll_addr[3]  = 0xC8;
socket_address.sll_addr[4]  = 0x28;
socket_address.sll_addr[5]  = 0xE5;
/*frame*/
unsigned char src_mac[6] = {0x00, 0x01, 0x02, 0xFA, 0x70, 0xAA};
unsigned char dest_mac[6] = {0x00, 0x04, 0x75, 0xC8, 0x28, 0xE5};
memcpy((void*)buffer, (void*)dest_mac, ETH_ALEN);
memcpy((void*)(buffer+ETH_ALEN), (void*)src_mac, ETH_ALEN);

send_result = sendto(s, buffer, ETH_FRAME_LEN, 0, 
          (struct sockaddr*)&socket_address, sizeof(socket_address));

Answer 1

如果你输入 man 2 sendto 你会得到以下信息:

 ssize_t
 send(int socket, const void *buffer, size_t length, int flags);

 ssize_t
 sendmsg(int socket, const struct msghdr *buffer, int flags);

 ssize_t
 sendto(int socket, const void *buffer, size_t length, int flags,
     const struct sockaddr *dest_addr, socklen_t dest_len);

也许你应该使用 send 或 sendmsg