Protostar/Net 0

If you get stuck, you can watch the solution and explanation here:

We will learn how to daemonize a process and see how a server handles connections

This level takes a look at converting strings to little endian integers.

This level is at /opt/protostar/bin/net0


#include "../common/common.c"

#define NAME "net0"
#define UID 999
#define GID 999
#define PORT 2999

void run()
  unsigned int i;
  unsigned int wanted;

  wanted = random();

  printf("Please send '%d' as a little endian 32bit int\n", wanted);

  if(fread(&i, sizeof(i), 1, stdin) == NULL) {
      errx(1, ":(\n");

  if(i == wanted) {
      printf("Thank you sir/madam\n");
  } else {
      printf("I'm sorry, you sent %d instead\n", i);

int main(int argc, char **argv, char **envp)
  int fd;
  char *username;

  /* Run the process as a daemon */
  background_process(NAME, UID, GID); 
  /* Wait for socket activity and return */
  fd = serve_forever(PORT);

  /* Set the client socket to STDIN, STDOUT, and STDERR */

  /* Don't do this :> */


See also

Before we continue with the network challenges, let's first introduce the TCP protocol properly. Using netcat and wireshark to understand it.

This is a mirror. Copyright and original can be found here: