From: ftp://ftp.measurement-factory.com/pub/simple-tcp-proxy/simple-tcp-proxy.c

simple-tcp-proxy.c

/*
 * $Id: simple-tcp-proxy.c,v 1.6 2002/11/27 00:40:31 wessels Exp $
 */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <netdb.h>
#include <string.h>
#include <signal.h>
#include <assert.h>
#include <syslog.h>


#include <sys/types.h>
#include <sys/select.h>
#include <sys/file.h>
#include <sys/ioctl.h>
#include <sys/param.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/wait.h>

#include <netinet/in.h>

#include <arpa/ftp.h>
#include <arpa/inet.h>
#include <arpa/telnet.h>

#define BUF_SIZE 4096

extern int sys_nerr, errno;

char client_hostname[64];

void
cleanup(int sig)
{
    syslog(LOG_INFO, "Cleaning up...");
    exit(0);
}

void
sigreap(int sig)
{
    int status;
    pid_t p;
    while ((p = waitpid(-1, &status, WNOHANG)) > 0) {
	syslog(LOG_INFO, "sigreap: pid=%d, status=%d\n", (int) p, status);
    }
    /* doh! */
    signal(SIGCHLD, sigreap);
}

void
set_nonblock(int fd)
{
    int fl;
    int x;
    x = fcntl(fd, F_GETFL, &fl);
    if (x < 0) {
	syslog(LOG_ERR, "fcntl F_GETFL: FD %d: %s", fd, strerror(errno));
	exit(1);
    }
    fl |= O_NONBLOCK;
    x = fcntl(fd, F_SETFL, &fl);
    if (x < 0) {
	syslog(LOG_ERR, "fcntl F_SETFL: FD %d: %s", fd, strerror(errno));
	exit(1);
    }
}


int
create_server_sock(char *addr, int port)
{
    int addrlen, s, on = 1, x;
    static struct sockaddr_in client_addr;

    s = socket(AF_INET, SOCK_STREAM, 0);
    if (s < 0)
	perror("socket"), exit(1);

    addrlen = sizeof(client_addr);
    memset(&client_addr, '\0', addrlen);
    client_addr.sin_family = AF_INET;
    client_addr.sin_addr.s_addr = inet_addr(addr);
    client_addr.sin_port = htons(port);
    setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, 4);
    x = bind(s, (struct sockaddr *) &client_addr, addrlen);
    if (x < 0)
	perror("bind"), exit(1);

    x = listen(s, 5);
    if (x < 0)
	perror("listen"), exit(1);

    return s;
}

int
open_remote_host(char *host, int port)
{
    struct sockaddr_in rem_addr;
    int len, s, x;
    struct hostent *H;
    int on = 1;

    H = gethostbyname(host);
    if (!H)
	return (-2);

    len = sizeof(rem_addr);

    s = socket(AF_INET, SOCK_STREAM, 0);
    if (s < 0)
	return s;

    setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, 4);

    len = sizeof(rem_addr);
    memset(&rem_addr, '\0', len);
    rem_addr.sin_family = AF_INET;
    memcpy(&rem_addr.sin_addr, H->h_addr, H->h_length);
    rem_addr.sin_port = htons(port);
    x = connect(s, (struct sockaddr *) &rem_addr, len);
    if (x < 0) {
	close(s);
	return x;
    }
    set_nonblock(s);
    return s;
}

int
get_hinfo_from_sockaddr(struct sockaddr_in addr, int len, char *fqdn)
{
    struct hostent *hostinfo;

    hostinfo = gethostbyaddr((char *) &addr.sin_addr.s_addr, len, AF_INET);
    if (!hostinfo) {
	sprintf(fqdn, "%s", inet_ntoa(addr.sin_addr));
	return 0;
    }
    if (hostinfo && fqdn)
	sprintf(fqdn, "%s [%s]", hostinfo->h_name, inet_ntoa(addr.sin_addr));
    return 0;
}


int
wait_for_connection(int s)
{
    static int newsock, len;
    static struct sockaddr_in peer;

    len = sizeof(struct sockaddr);
    newsock = accept(s, (struct sockaddr *) &peer, &len);
    /* dump_sockaddr (peer, len); */
    if (newsock < 0) {
	if (errno != EINTR)
	    perror("accept");
    }
    get_hinfo_from_sockaddr(peer, len, client_hostname);
    set_nonblock(newsock);
    return (newsock);
}

int
mywrite(int fd, char *buf, int *len)
{
	int x = write(fd, buf, *len);
	if (x < 0)
		return x;
	if (x == 0)
		return x;
	if (x != *len)
		memmove(buf, buf+x, (*len)-x);
	*len -= x;
	return x;
}

void
service_client(int cfd, int sfd)
{
    int maxfd;
    char *sbuf;
    char *cbuf;
    int x, n;
    int cbo = 0;
    int sbo = 0;
    fd_set R;

    sbuf = malloc(BUF_SIZE);
    cbuf = malloc(BUF_SIZE);
    maxfd = cfd > sfd ? cfd : sfd;
    maxfd++;

    while (1) {
	struct timeval to;
	if (cbo) {
		if (mywrite(sfd, cbuf, &cbo) < 0 && errno != EWOULDBLOCK) {
			syslog(LOG_ERR, "write %d: %s", sfd, strerror(errno));
				exit(1);
		}
	}
	if (sbo) {
		if (mywrite(cfd, sbuf, &sbo) < 0 && errno != EWOULDBLOCK) {
			syslog(LOG_ERR, "write %d: %s", cfd, strerror(errno));
				exit(1);
		}
	}
	FD_ZERO(&R);
	if (cbo < BUF_SIZE)
		FD_SET(cfd, &R);
	if (sbo < BUF_SIZE)
		FD_SET(sfd, &R);
	to.tv_sec = 0;
	to.tv_usec = 1000;
	x = select(maxfd+1, &R, 0, 0, &to);
	if (x > 0) {
	    if (FD_ISSET(cfd, &R)) {
		n = read(cfd, cbuf+cbo, BUF_SIZE-cbo);
		syslog(LOG_INFO, "read %d bytes from CLIENT (%d)", n, cfd);
		if (n > 0) {
		    cbo += n;
		} else {
		    close(cfd);
		    close(sfd);
		    _exit(0);
		}
	    }
	    if (FD_ISSET(sfd, &R)) {
		n = read(sfd, sbuf+sbo, BUF_SIZE-sbo);
		syslog(LOG_INFO, "read %d bytes from SERVER (%d)\n", n, sfd);
		if (n > 0) {
		    sbo += n;
		} else {
		    close(sfd);
		    close(cfd);
		    _exit(0);
		}
	    }
	} else if (x < 0 && errno != EINTR) {
	    close(sfd);
	    close(cfd);
	    _exit(0);
	}
    }
}


int
main(int argc, char *argv[])
{
    char *localaddr = strdup(argv[1]);
    int localport = atoi(argv[2]);
    char *remoteaddr = strdup(argv[3]);
    int remoteport = atoi(argv[4]);
    int client, server;
    int master_sock;

    if (5 != argc) {
	fprintf(stderr, "usage: %s laddr lport rhost rport\n", argv[0]);
	exit(1);
    }
    assert(localaddr);
    assert(localport > 0);
    assert(remoteaddr);
    assert(remoteport > 0);

    openlog(argv[0], LOG_PID, LOG_LOCAL4);

    signal(SIGINT, cleanup);
    signal(SIGCHLD, sigreap);

    master_sock = create_server_sock(localaddr, localport);
    for (;;) {
	if ((client = wait_for_connection(master_sock)) < 0)
	    continue;
	if ((server = open_remote_host(remoteaddr, remoteport)) < 0)
	    continue;
	if (!fork()) {
	    syslog(LOG_NOTICE, "connection from %s fd=%d\n", client_hostname, client);
	    syslog(LOG_INFO, "connected to %s:%d fd=%d\n", remoteaddr, remoteport, server);
	    service_client(client, server);
	}
	close(client);
	close(server);
    }

}