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.

Simple TCP pair

Contents

Compile and run simple TCP client and server

Try to dump the client

Then restore the client

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