service.c

#include <stdio.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netdb.h>
#include <unistd.h>
#include <stdlib.h>
#include <service.h>

int compute(char part[12], int base) {

	char* p = part;
	int v = base;

	while ((*p) && (p < part + 8))
		v = v + *(p++);

	return v;
}


int validate (char* sn) {

	int value1, value2;
	char buf[8];

	sn[8] = 0;
	strcpy(buf, sn);
	value1 = compute(buf, 0);

	strcpy(buf, (sn + 9));
	value2 = compute(buf, 0x41414141);

	return value1 == value2;
}


void session(int cliSd)
{
	char buff[28];
	int recv_size, end_ptr = 0, done = 0;

	struct sockaddr_in cliAddr;
	int cliLen;
	cliLen = sizeof(cliAddr);
	getpeername(cliSd, (struct sockaddr *)&cliAddr, &cliLen);
	printf("%s\t: Connected\n", inet_ntoa(cliAddr.sin_addr));

	send (cliSd, "Please enter your serial number:\r\n", 35, 0);
	while (!done) {
		recv_size = recv (cliSd, (char *) buff + end_ptr, sizeof (buff) - end_ptr, 0);

		if (recv_size == 0)	break;
		if (recv_size == -1)	continue;

		end_ptr += recv_size;
		if ((buff[end_ptr - 1] == 0) && end_ptr > 0)	end_ptr--;

		if ((buff[end_ptr - 1] == '\n') || (buff[end_ptr - 1] == '\r')) {
			buff[end_ptr - 1] = 0;
			if (validate(buff)) {
				done = 1;
				printf("%s\t: Succeeded!! <------------------\n", inet_ntoa(cliAddr.sin_addr));
				send (cliSd, "Thanks you for your registration.\r\n", 36, 0);
				send (cliSd, "The confirmation number is " CONFIRM_CODE "\r\n", 30 + sizeof(CONFIRM_CODE), 0);
			} else {
				send (cliSd, "Sorry, wrong serial number.\r\n", 29, 0);
				end_ptr = 0;
			}
		}
	}

	printf("%s\t: Disconnected or bad overflow\n", inet_ntoa(cliAddr.sin_addr));
	close(cliSd);
}


int main () {
  struct sockaddr_in servAddr;
  int servSd, cliSd;
  int one = 1;

  servSd = socket (PF_INET, SOCK_STREAM, 0);
  setsockopt (servSd, SOL_SOCKET, SO_REUSEADDR, (char *) &one, sizeof (one));
  servAddr.sin_family = AF_INET;
  servAddr.sin_port = htons (12345);
  servAddr.sin_addr.s_addr = htonl (INADDR_ANY);

  if (bind (servSd, (struct sockaddr *) &servAddr, sizeof (servAddr)) < 0){ perror ("Can't bind\n"); return 1;}
  if (listen (servSd, 3) < 0) { perror ("Can't listen\n"); return 1;}

  while (1){
      printf ("Server is ready!\n");
      cliSd = accept (servSd, NULL, NULL);
      if (fork()==0){
      	session(cliSd);
	return 0;
      }
  }

  return 0;
}

service.h =

#define CONFIRM_CODE "local_confirmation_code"

To compile

gcc -I./ -fno-stack-protector service.c -o serviceExecute -m32

To test:

./serviceExecute

# and for the other
nc localhost 12345

Goal get The confirmation number is xxxxxxx!