Question

I’m trying to make a little client-server script like many others that I’ve done in the past.
But in this one I have a problem. It is better if I post the code and the output it give me.
Code:

#include <mysql.h> //not important now
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>

//constant definition
#define SERVER_PORT 2121
#define LINESIZE 21

//global var definition
char victim_ip[LINESIZE], file_write[LINESIZE], hacker_ip[LINESIZE];

//function
void leggi (int); //not use now for debugging purpose
//void scriviDB (); //not important now

main () {

int sock, client_len, fd;

struct sockaddr_in server, client;

// transport end point
if((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
  perror("system call socket fail");
  exit(1);
}

server.sin_family = AF_INET;
server.sin_addr.s_addr = inet_addr("10.10.10.1");
server.sin_port = htons(SERVER_PORT);

// binding address at transport end point
if (bind(sock, (struct sockaddr *)&server, sizeof server) == -1) {
  perror("system call bind fail");
  exit(1);
}

//fprintf(stderr, "Server open: listening.\n");
listen(sock, 5);

/* managae client connection */
while (1) {
  client_len = sizeof(client);
  if ((fd = accept(sock, (struct sockaddr *)&client, &client_len)) < 0) 
     { perror("accepting connection"); exit(1);  }

  strcpy(hacker_ip, inet_ntoa(client.sin_addr));
  printf("1 %s\n", hacker_ip); //debugging purpose
  //leggi(fd);

//////////////////////////
//receive client 
  recv(fd, victim_ip, LINESIZE, 0);
  victim_ip[sizeof(victim_ip)] = '\0';
  printf("2 %s\n", hacker_ip); //debugging purpose
  recv(fd, file_write, LINESIZE, 0);
  file_write[sizeof(file_write)] = '\0';
  printf("3 %s\n", hacker_ip); //debugging purpose
  printf("%s@%s for %s\n", file_write, victim_ip, hacker_ip);

  //send to client
  send(fd, hacker_ip, 40, 0); //now is hacker_ip for debug

/////////////////////////

  close(fd);

}//end while

exit(0);
} //end main

Client send string: ./send -i 10.10.10.4 -f filename.ext
so the script send -i (IP) and -f (FILE) at the server.
Here’s my output server side:

1 10.10.10.6
2 10.10.10.6
3
filename.ext@10.10.10.4 for

As you can see the printf(3) and the printf(ip,file,ip) fail.

I don’t know how and where but someone overwrite my hacker_ip string.
Thanks for your help! 🙂

Answer 1

TCP provides a stream, not packets. Thus, you are not guaranteed that the data you send with
1 send() call takes 1 recv() call to receive. It could take several recv() calls to receive what one send() call sent, or it could take 1 recv() call to receive what several send() calls sent – somehow you have to handle that.

In particular, you should check the return value of recv() to know how many bytes you received, maybe this can be a start so you are atleast not printing garbage in your strings.

ssize_t bytes = recv(fd, victim_ip, LINESIZE, 0);
 if(bytes == 0) {
   //remote closed the connection, handle it
 } else if (bytes < 0) {
    //handle error
 } else {
   victim_ip[bytes] = '\0';
   printf("%s\n", victim_ip); 
}