Playing with DNS packets in C

CDNS.c

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

#define BUFFER_SIZE 4096

void usage(void);


int main(int argc, char **argv)
{
    WSADATA w;							/* Used to open windows connection */
    unsigned short port_number;			/* Port number to use */
    int octet1, octet2, octet3, octet4;	/* Components of address in xxx.xxx.xxx.xxx form */
    int client_length;					/* Length of client struct */
    int bytes_received;					/* Bytes received from client */
    SOCKET sd;							/* Socket descriptor of server */
    struct sockaddr_in server;			/* Information about the server */
    struct sockaddr_in client;			/* Information about the client */
    char buffer[BUFFER_SIZE];			/* Where to store received data */
    struct hostent *hp;					/* Information about this computer */
    char host_name[256];				/* Name of the server */

    // Need to use a better option than this.  getopt
    /* Interpret command line */
    if (argc == 2)
    {
        /* Use local address */
        if (sscanf(argv[1], "%u", &port_number) != 1)
        {
            usage();
        }
    }
    else if (argc >= 3) //allow
    {
        /* Copy address */
        if (sscanf(argv[1], "%d.%d.%d.%d", &octet1, &octet2, &octet3, &octet4) != 4)
        {
            usage();
        }
        if (sscanf(argv[2], "%u", &port_number) != 1)
        {
            usage();
        }
    }
    else
    {
        usage();
    }


    /* Open winsock connection */
    if (WSAStartup(0x0101, &w) != 0)
    {
        fprintf(stderr, "Could not open winsock connection.\n");
        exit(0);
    }

    /* Open a datagram socket */
    sd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sd == INVALID_SOCKET)
    {
        fprintf(stderr, "Could not create socket.\n");
        WSACleanup();
        exit(0);
    }

    /* Clear out server struct */
    memset((void *)&server, '\0', sizeof(struct sockaddr_in));

    /* Set family and port */
    server.sin_family = AF_INET;
    server.sin_port = htons(port_number);

    /* Set address automatically if desired */
    if (argc == 2)
    {
        /* Get host name of this computer */
        gethostname(host_name, sizeof(host_name));
        hp = gethostbyname(host_name);

        /* Check for NULL pointer */
        if (hp == NULL)
        {
            fprintf(stderr, "Could not get host name.\n");
            closesocket(sd);
            WSACleanup();
            exit(0);
        }

        /* Assign the address */
        server.sin_addr.S_un.S_un_b.s_b1 = hp->h_addr_list[0][0];
        server.sin_addr.S_un.S_un_b.s_b2 = hp->h_addr_list[0][1];
        server.sin_addr.S_un.S_un_b.s_b3 = hp->h_addr_list[0][2];
        server.sin_addr.S_un.S_un_b.s_b4 = hp->h_addr_list[0][3];
    }
    /* Otherwise assign it manually */
    else
    {
        server.sin_addr.S_un.S_un_b.s_b1 = (unsigned char)octet1;
        server.sin_addr.S_un.S_un_b.s_b2 = (unsigned char)octet2;
        server.sin_addr.S_un.S_un_b.s_b3 = (unsigned char)octet3;
        server.sin_addr.S_un.S_un_b.s_b4 = (unsigned char)octet4;
    }

    /* Bind address to socket */
    if (bind(sd, (struct sockaddr *)&server, sizeof(struct sockaddr_in)) == -1)
    {
        fprintf(stderr, "Could not bind name to socket.\n");
        closesocket(sd);
        WSACleanup();
        exit(0);
    }

    /* Print out server information */
    printf("Server running on %u.%u.%u.%u\n", (unsigned char)server.sin_addr.S_un.S_un_b.s_b1,
           (unsigned char)server.sin_addr.S_un.S_un_b.s_b2,
           (unsigned char)server.sin_addr.S_un.S_un_b.s_b3,
           (unsigned char)server.sin_addr.S_un.S_un_b.s_b4);
    printf("Press CTRL + C to quit\n");

    /* Loop and get data from clients */
    while (1)
    {
        client_length = (int)sizeof(struct sockaddr_in);

        /* Receive bytes from client */
        bytes_received = recvfrom(sd, buffer, BUFFER_SIZE, 0, (struct sockaddr *)&client, &client_length);
        if (bytes_received < 0)
        {
            fprintf(stderr, "Could not receive datagram.\n");
            closesocket(sd);
            WSACleanup();
            exit(0);
        }

        //We have a socket...
        int STARTQ = 0;
        int STOPQ = bytes_received - 5;
        char Q[STOPQ - 13];
        Q[STOPQ - 13] = '\0';
        printf("------\n");
        int x  = 0;
        for(STARTQ = 13; STARTQ < STOPQ; STARTQ++)
        {

            if(buffer[STARTQ] == 3 || buffer[STARTQ] == 7)
                buffer[STARTQ] = 46;

            Q[x] = buffer[STARTQ];
            x++;
            if(argc == 4)
            {
                if(strcmp(argv[3], "PRINTBYTES") == 0)
                    printf("%i : %c\n",buffer[STARTQ],buffer[STARTQ]);
            }

        }
        printf("%s\n", Q);

    }
    closesocket(sd);
    WSACleanup();

    return 0;
}

void usage(void)
{
    fprintf(stderr, "CDNS [server_address] port \nCDNS [server_address] port [PRINTBYTES]\n");
    exit(0);
}



/* http://imgur.com/RWkmflK */