发送原始数据包时，sendto函数不使用struct sockaddr_ll中提供的MAC地址

Question

I am trying to send an OAM ethernet frame using raw socket. I was successful in doing so.

The send function I have written is:

    int send_frame(sock_info *info,char *buf,int length)
    {
       struct sockaddr_ll dest_addr;
       memset(&dest_addr,0,sizeof(struct sockaddr_ll));
       dest_addr.sll_family = PF_PACKET;
       dest_addr.sll_protocol = htons(8902);
       dest_addr.sll_ifindex = info->if_index;
       dest_addr.sll_halen = ETH_MAC_ADDR_LEN;
       dest_addr.sll_pkttype = PACKET_OTHERHOST;
       dest_addr.sll_hatype   = ARPHRD_ETHER;
       memset(dest_addr.sll_addr,0,8);

       dest_addr.sll_addr[0] = 0x00;
       dest_addr.sll_addr[1] = 0xE0;
       dest_addr.sll_addr[2] = 0x0C;
       dest_addr.sll_addr[3] = 0x00;
       dest_addr.sll_addr[4] = 0x95;
       dest_addr.sll_addr[5] = 0x02;

       return sendto(info->sock_fd, buf, length, 0, (struct sockaddr*) &dest_addr, sizeof(struct sockaddr_ll));
    }

I was unable to capture the packet using wireshark. After tryiing too many things, I found out that buffer used to send should have all ethernet frame fields (starting from destination address). When I added the destination and source address and other ethernet fields into the buffer, I was able to capture the packet using wireshark. So the send function doesn't use the MAC address stored in dest_addr.sll_addr .

My question is, Then what's the need of sll_addr field in the struct sockaddr_ll ? Manuals say that it is the destination MAC address.

Answer 1

To me it sounds like it works as the manual page describes it ( man 7 packet ):

SOCK_RAW packets are passed to and from the device driver without any changes in the packet data. When receiving a packet, the address is still parsed and passed in a standard sockaddr_ll address structure. When transmitting a packet, the user supplied buffer should contain the physical layer header . That packet is then queued unmodified to the network driver of the interface defined by the destination address. Some device drivers always add other headers. SOCK_RAW is similar to but not compatible with the obsolete PF_INET/SOCK_PACKET of Linux 2.0.

The buffer here refers to the 2nd parameter of sendto() . So, the stuct sockaddr_ll is only used to return data to the caller, not to format the RAW packet. Maybe you want to user SOCK_DGRAM or libpcap instead?