Socket Programming PF Packet Socket

Question

I am writing a code to communicate via the SOCK_RAW sockets with a process on another computer.

I know the IP address of the other machine.

I am aware that filling up the sockaddr_ll.sll_addr values one byte at a time will solve the problem ie something like

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;

But I don't know how to do the same thing when I have character array of 6 bytes having the hexadecimal address of the other machine.

I am able to print the hex address in ':' format using

printf("%.2x",*ptr++ & 0xff);

where ptr is an array to the character array.

But how to use these values to fill the sll_addr bytes?

Answer 1

You could use the sscanf() function to do this, like so:

#include <stdio.h>
#include <linux/if_packet.h>

const char sMac[] = "01:02:03:04:05:ff";

int main()
{
  struct sockaddr_ll sa = {0};

  sscanf(sMac, "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx",
    &sa.sll_addr[0],
    &sa.sll_addr[1],
    &sa.sll_addr[2],
    &sa.sll_addr[3],
    &sa.sll_addr[4],
    &sa.sll_addr[5]
  );

  return 0; 
}

The magic is the format string passed to sscanf() .

It tells the scanner where to find what in which range.

• x tells it to expect hexadecimal notation
• hh specifies 8bit values.