Simple TCP pair

This page describes step-by-step how to play with TCP dumping and restoring.

Compile and run simple TCP client and server

The below program is a simple echo server and a client, that pings server with increasing numbers once a second. In this howto we will dump and restore the client part.

tcp-howto.c:

#include <sys/socket.h>
#include <linux/types.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>

static int serve_new_conn(int sk)
{
	int rd, wr;
	char buf[1024];

	printf("New connection\n");

	while (1) {
		rd = read(sk, buf, sizeof(buf));
		if (!rd)
			break;

		if (rd < 0) {
			perror("Can't read socket");
			return 1;
		}

		wr = 0;
		while (wr < rd) {
			int w;

			w = write(sk, buf + wr, rd - wr);
			if (w <= 0) {
				perror("Can't write socket");
				return 1;
			}

			wr += w;
		}
	}

	printf("Done\n");
	return 0;
}

static int main_srv(int argc, char **argv)
{
	int sk, port, ret;
	struct sockaddr_in addr;

	/*
	 * Let kids die themselves
	 */

	signal(SIGCHLD, SIG_IGN);

	sk = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (sk < 0) {
		perror("Can't create socket");
		return -1;
	}

	port = atoi(argv[1]);
	memset(&addr, 0, sizeof(addr));
	addr.sin_family = AF_INET;
	addr.sin_addr.s_addr = htonl(INADDR_ANY);
	addr.sin_port = htons(port);

	printf("Binding to port %d\n", port);

	ret = bind(sk, (struct sockaddr *)&addr, sizeof(addr));
	if (ret < 0) {
		perror("Can't bind socket");
		return -1;
	}

	ret = listen(sk, 16);
	if (ret < 0) {
		perror("Can't put sock to listen");
		return -1;
	}

	printf("Waiting for connections\n");
	while (1) {
		int ask, pid;

		ask = accept(sk, NULL, NULL);
		if (ask < 0) {
			perror("Can't accept new conn");
			return -1;
		}

		pid = fork();
		if (pid < 0) {
			perror("Can't fork");
			return -1;
		}

		if (pid > 0)
			close(ask);
		else {
			close(sk);
			ret = serve_new_conn(ask);
			exit(ret);
		}
	}
}

static int main_cl(int argc, char **argv)
{
	int sk, port, ret, val = 1, rval;
	struct sockaddr_in addr;

	sk = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (sk < 0) {
		perror("Can't create socket");
		return -1;
	}

	port = atoi(argv[2]);
	printf("Connecting to %s:%d\n", argv[1], port);
	memset(&addr, 0, sizeof(addr));
	addr.sin_family = AF_INET;
	ret = inet_aton(argv[1], &addr.sin_addr);
	if (ret < 0) {
		perror("Can't convert addr");
		return -1;
	}
	addr.sin_port = htons(port);

	ret = connect(sk, (struct sockaddr *)&addr, sizeof(addr));
	if (ret < 0) {
		perror("Can't connect");
		return -1;
	}

	while (1) {
		write(sk, &val, sizeof(val));
		rval = -1;
		read(sk, &rval, sizeof(rval));
		printf("PP %d -> %d\n", val, rval);
		sleep(2);
		val++;
	}
}

int main(int argc, char **argv)
{
	if (argc == 2)
		return main_srv(argc, argv);
	else if (argc == 3)
		return main_cl(argc, argv);

	printf("Bad usage\n");
	return 1;
}

So compile it and run server

# ./tcp-howto <some-port>

and on another terminal (for better output readability) -- the client

# ./tcp-howto 127.0.0.1 <some-port>

Try to dump the client

Create a directory for images (img-dir/ below) and dump the client

# crtools dump --tree <tcp-howto-client-pid> --images-dir img-dir/ -v 4 -o dump.log --shell-job --tcp-established

The --tcp-established option is a must, since client have active TCP connection and we should explicitly inform crtools about it.

The --shell-job option is also required, as the client was launched from shell and uses shell's session and terminal.

Then restore the client

It's done like this

# crtools restore --tree <tcp-howto-client-pid> --images-dir img-dir/ -v 4 -o rst.log --shell-job --tcp-established

That's it. After this in the terminal you launched crtools from you should see the contunuing output of the tcp client.