C套接字编程：将ip header的id设置为0？

Question

I want to send raw IP packets, which works perfectly fine, except for one field in the IP header.

I need to set the IP id to 0. I tried it the following way:

    struct iphdr ip;

ip.version = 4;                               // Ipv4
ip.frag_off = 0;                              // IP Fragmentation
ip.tos     = 0;                               // Type of Service - We don't need this
ip.ttl = 64;                                  // Maxmimum value
ip.protocol = IPPROTO_UDP;                    // DHCP uses UDP
ip.check = 0;                                 // The checksum needs to be 0 before being calculated

// We don't yet have an IP Address
if((ip.saddr = inet_addr("0.0.0.0")) == -1) {
    perror("Failed to convert IP address");
    exit(1);
}

ip.daddr = inet_addr("255.255.255.255");      // we have to do a broadcast
ip.id = 0;                                    //  
ip.ihl = 5;                                   // Header length - 5 means no additional options are being sent in the IP header
ip.tot_len = sizeof(struct packet);           // The total length of the Packet

I then create a RAW_SOCKET with the IP_HDRINCL option set:

      if ((sockd = socket(AF_INET, SOCK_RAW, IPPROTO_UDP)) == -1) {
      perror("Failed to create socket");
      exit(1);
  }
int hdrincl = 1;
if (setsockopt(sockd, IPPROTO_IP, IP_HDRINCL, &hdrincl, sizeof(hdrincl)) == -1) {
    perror("Failed to set IP_HDRINCL");
    exit(1);
}

I then create a buffer, containing my IP header, UDP header and the payload and send it out.

However, the network stack keeps changing my id in the IP header. I need to include the IP header, but I also need to disable fragmentation and / or set the IP id to 0. How do I achieve this?

EDIT: I am on Linux

Answer 1

On linux, you can use AF_PACKET with SOCK_DGRAM sockets instead of AF_INET with SOCK_RAW and fill an sockaddr_ll .

Something like that:

socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))

However, to make you program work with Windows too, you should use libpcap .

Keep in mind that you should resolve the destination's physical address by yourself (On linux, you can try using the rtnetlink interface for that).

See more on packet(7) and rtnetlink(7) . If your project is Open Source, you can use my old project that doing some of these things (for more information, contact me).