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; }

Compile it and run the server:

# ./tcp-howto <some-port>

On another terminal (for better output readability) run 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 --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.

See also

• TCP connection