Build A Sample Custom Packet [Embedded Systems]

This code snippet was my submission for embedded systems assignment from the embedded system black book by Dr. K.V.K.K. Prasad. It is in no way a real packet and is not meant to represent the IP layer.


Question: Write a C program that takes the filename as input and generates packets of 100 bytes. Develop a simple packet format of your own.

Compilation:

gcc -Wall -lm -o custom_packet custom_packet.c

//custom_packet.c
//      
//eg: ./custom_packet /home/samar/Desktop/cs_rules.txt
//Compilation: gcc -Wall -lm -o custom_packet custom_packet.c
//Custom Packet: Header -> 20 bytes and Data -> 80 bytes
//Find me on http://www.techgaun.com

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

typedef struct
{
        int8_t fragment_offset;
        int8_t ttl;
        int32_t source_ip;
        int32_t dest_ip;
} custom_packet_header;

typedef struct
{
        custom_packet_header header;
        char data[80];
} custom_packet;

long int get_file_size(char fname[])
{
        int fd;
        int count;
        if ((fd = open(fname, O_RDONLY)) == -1)
        {
                perror("Error reading the file\n");
                exit(EXIT_FAILURE);
        }
        
        struct stat buf;
    fstat(fd, &buf);
    count = buf.st_size;
        close(fd);
        return count;
}

int decimalip2numeric(int a, int b, int c, int d)
{
        return (a * 16777216 + b * 65536 + c * 256 + d);
}

/*char * numericip2decimal(int num)
{
        char strs[4];
        strs[0] = (char *) num / 1677;
}*/

int main(int argc, char **argv)
{
        FILE *fp;
        //char fname[256];      //255 bytes is the limit of filename in extN filesystems
        custom_packet * packets;
        long int fsize;
        int num_of_packet, i;
        if (argc != 2)
        {
                printf("Usage: %s filename\n", argv[0]);
                exit(1);
        }
        
        fsize = get_file_size(argv[1]);
        num_of_packet = ceil((double)fsize / 80.0);
        printf("%ld => %d",fsize, num_of_packet);
        
        if ((fp = fopen(argv[1], "rb")) == NULL)
        {
                perror("Error opening the file");
                exit(1);
        }
        
        packets = (custom_packet *) malloc(sizeof(custom_packet) * num_of_packet);
        for (i = 0; i < num_of_packet; i++)
        {               
                packets[i].header.source_ip = decimalip2numeric(127, 0, 0, 1); //storing source ip as 127.0.0.1 for now
                packets[i].header.dest_ip = decimalip2numeric(127, 0, 0, 1); //storing dest ip as 127.0.0.1 for now
                packets[i].header.ttl = 127;
                packets[i].header.fragment_offset = i;
        }
        i = 0;
        while (!feof(fp))
        {
                fread((void *)packets[i].data, 80, 1, fp);
                i++;
        }
        
        fclose(fp);
        
        printf("\n\n----- Printing all the crafted packets -----\n\n");
        for (i = 0; i < num_of_packet; i++)
        {
                printf("[---- Packet Fragment no. %d ----", packets[i].header.fragment_offset);
                printf("\nSource IP -> %d\nDestination IP -> %d\nTime to live -> %d\n", packets[i].header.source_ip, packets[i].header.dest_ip, packets[i].header.ttl);
                printf("Packet data -> %s", packets[i].data);
                printf("\n---- End of Packet no. %d ----]\n\n", packets[i].header.fragment_offset);
        }
        
        return 0;
}