How can I send a datagram using an Ethernet trailer?

Question

How can I send a datagram using an Ethernet trailer? If I use SocketType.Raw, I will have to send the entire IP header, and I do not know how to do this.

Answer 1

Is something like this possible?

  #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #include <unistd.h>

 #include <sys / socket.h>
 #include <linux / if_packet.h>
 #include <linux / if_ether.h>
 #include <linux / if_arp.h>

 #include <sys / ioctl.h>

 int s;

 unsigned char buffer [513];

 struct sockaddr_ll socket_address;

 int main (void)
 {
     unsigned char seq;
     unsigned int ra;
     int length;
     struct ifreq ifr;

     s = socket (AF_PACKET, SOCK_RAW, htons (ETH_P_ALL));
     if (s == -1)
     {
         printf ("error creating socket \ n");
         return (1);
     }

     memset (& ifr, 0, sizeof (struct ifreq));
     strncpy (ifr.ifr_name, "eth0", IFNAMSIZ);
     if (ioctl (s, SIOCGIFINDEX, & ifr) <0)
     {
         perror ("ioctl SIOCGIFINDEX");
         exit (1);
     }

     printf ("index% d \ n", ifr.ifr_ifindex);


     printf ("socket created \ n");

     memset (& socket_address, 0, sizeof (socket_address));

     socket_address.sll_family = PF_PACKET;
     socket_address.sll_protocol = htons (ETH_P_ALL);
     socket_address.sll_ifindex = ifr.ifr_ifindex;

     if (bind (s, (struct sockaddr *) (& socket_address), sizeof (socket_address)) <0)
     {
         perror ("bind error");
         exit (1);
     }

     printf ("bound \ n");

     length = 27;

     memset (buffer, 0, sizeof (buffer));
 // destination
     buffer [0] = 0xFF;
     buffer [1] = 0xFF;
     buffer [2] = 0xFF;
     buffer [3] = 0xFF;
     buffer [4] = 0xFF;
     buffer [5] = 0xFF;
 // source
     buffer [6] = 0x00;
     buffer [7] = 0x19;
     buffer [8] = 0xd1;
     buffer [9] = 0x02;
     buffer [10] = 0xdc;
     buffer [11] = 0xb3;
 // length
     buffer [12] = ((length-14) >> 8) & 0xFF;
     buffer [13] = ((length-14) >> 0) & 0xFF;
 // payload
     buffer [14] = 0x12;
     buffer [15] = 0x34;

     for (ra = 0; ra <20; ra ++)
     {
         buffer [16] = ra;
         if (send (s, buffer, length, 0) <0)
         {
             printf ("sendto failed \ n");
             break;
         }
         else
         {
             printf ("sent \ n");
         }
     }

     close (s);
     return (1);

 }

This should give a raw package that you can see on wirehark. if you want to have ip eader or make it udp or something like that, you can use this method and create a header yourself (this is a trivial look at rfcs or just use wirehark to look at a bunch of other packet headers) that for udp you do not need to calculate the checksum. 0x0000 is the actual checksum that must pass through.

If all you need is an udp package with zeros at the end, which is the same, maybe simpler, let me know.

Answer 2

This trailer is used to accommodate Ethernet frames with a minimum length (46 bytes of payload). So send a small UDP packet - less than 18 bytes (since IP + UDP is usually 28 bytes)