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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <time.h>
#if defined(__WIN32__) || defined(WIN32) || defined(_WIN32)
#define WIN32_LEAN_AND_MEAN
#include <winsock.h>
#define bzero(p, l) memset(p, 0, l)
#define bcopy(s, t, l) memmove(t, s, l)
#else
#include <sys/time.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <strings.h>
#define recv(x,y,z,a) read(x,y,z)
#define send(x,y,z,a) write(x,y,z)
#define closesocket(s) close(s)
typedef int SOCKET;
#endif

#ifndef INADDR_NONE
#define INADDR_NONE 0xffffffff
#endif


struct client_t
{
int inuse;
SOCKET csock, osock;
time_t activity;
};

#define MAXCLIENTS 20
#define IDLETIMEOUT 300


const char ident[] = "$Id: datapipe.c,v 1.8 1999/01/29 01:21:54 jlawson Exp $";

int main(int argc, char *argv[])
{
SOCKET lsock;
char buf[4096];
struct sockaddr_in laddr, oaddr;
int i;
struct client_t clients[MAXCLIENTS];


#if defined(__WIN32__) || defined(WIN32) || defined(_WIN32)
/* Winsock needs additional startup activities */
WSADATA wsadata;
WSAStartup(MAKEWORD(1,1), &wsadata);
#endif


/* check number of command line arguments */
if (argc != 5) {
fprintf(stderr,"Usage: %s localhost localport remotehost remoteport\n",argv[0]);
return 30;
}


/* reset all of the client structures */
for (i = 0; i < MAXCLIENTS; i++)
clients[i].inuse = 0;


/* determine the listener address and port */
bzero(&laddr, sizeof(struct sockaddr_in));
laddr.sin_family = AF_INET;
laddr.sin_port = htons((unsigned short) atol(argv[2]));
laddr.sin_addr.s_addr = inet_addr(argv[1]);
if (!laddr.sin_port) {
fprintf(stderr, "invalid listener port\n");
return 20;
}
if (laddr.sin_addr.s_addr == INADDR_NONE) {
struct hostent *n;
if ((n = gethostbyname(argv[1])) == NULL) {
perror("gethostbyname");
return 20;
}
bcopy(n->h_addr, (char *) &laddr.sin_addr, n->h_length);
}


/* determine the outgoing address and port */
bzero(&oaddr, sizeof(struct sockaddr_in));
oaddr.sin_family = AF_INET;
oaddr.sin_port = htons((unsigned short) atol(argv[4]));
if (!oaddr.sin_port) {
fprintf(stderr, "invalid target port\n");
return 25;
}
oaddr.sin_addr.s_addr = inet_addr(argv[3]);
if (oaddr.sin_addr.s_addr == INADDR_NONE) {
struct hostent *n;
if ((n = gethostbyname(argv[3])) == NULL) {
perror("gethostbyname");
return 25;
}
bcopy(n->h_addr, (char *) &oaddr.sin_addr, n->h_length);
}


/* create the listener socket */
if ((lsock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
return 20;
}
if (bind(lsock, (struct sockaddr *)&laddr, sizeof(laddr))) {
perror("bind");
return 20;
}
if (listen(lsock, 5)) {
perror("listen");
return 20;
}


/* change the port in the listener struct to zero, since we will
* use it for binding to outgoing local sockets in the future. */
laddr.sin_port = htons(0);


/* fork off into the background. */
#if !defined(__WIN32__) && !defined(WIN32) && !defined(_WIN32)
if ((i = fork()) == -1) {
perror("fork");
return 20;
}
if (i > 0)
return 0;
setsid();
#endif


/* main polling loop. */
while (1)
{
fd_set fdsr;
int maxsock;
struct timeval tv = {1,0};
time_t now = time(NULL);

/* build the list of sockets to check. */
FD_ZERO(&fdsr);
FD_SET(lsock, &fdsr);
maxsock = (int) lsock;
for (i = 0; i < MAXCLIENTS; i++)
if (clients[i].inuse) {
FD_SET(clients[i].csock, &fdsr);
if ((int) clients[i].csock > maxsock)
maxsock = (int) clients[i].csock;
FD_SET(clients[i].osock, &fdsr);
if ((int) clients[i].osock > maxsock)
maxsock = (int) clients[i].osock;
}
if (select(maxsock + 1, &fdsr, NULL, NULL, &tv) < 0) {
return 30;
}


/* check if there are new connections to accept. */
if (FD_ISSET(lsock, &fdsr))
{
SOCKET csock = accept(lsock, NULL, 0);

for (i = 0; i < MAXCLIENTS; i++)
if (!clients[i].inuse) break;
if (i < MAXCLIENTS)
{
/* connect a socket to the outgoing host/port */
SOCKET osock;
if ((osock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
closesocket(csock);
}
else if (bind(osock, (struct sockaddr *)&laddr, sizeof(laddr))) {
perror("bind");
closesocket(csock);
closesocket(osock);
}
else if (connect(osock, (struct sockaddr *)&oaddr, sizeof(oaddr))) {
perror("connect");
closesocket(csock);
closesocket(osock);
}
else {
clients[i].osock = osock;
clients[i].csock = csock;
clients[i].activity = now;
clients[i].inuse = 1;
}
} else {
fprintf(stderr, "too many clients\n");
closesocket(csock);
}
}


/* service any client connections that have waiting data. */
for (i = 0; i < MAXCLIENTS; i++)
{
int nbyt, closeneeded = 0;
if (!clients[i].inuse) {
continue;
} else if (FD_ISSET(clients[i].csock, &fdsr)) {
if ((nbyt = recv(clients[i].csock, buf, sizeof(buf), 0)) <= 0 ||
send(clients[i].osock, buf, nbyt, 0) <= 0) closeneeded = 1;
else clients[i].activity = now;
} else if (FD_ISSET(clients[i].osock, &fdsr)) {
if ((nbyt = recv(clients[i].osock, buf, sizeof(buf), 0)) <= 0 ||
send(clients[i].csock, buf, nbyt, 0) <= 0) closeneeded = 1;
else clients[i].activity = now;
} else if (now - clients[i].activity > IDLETIMEOUT) {
closeneeded = 1;
}
if (closeneeded) {
closesocket(clients[i].csock);
closesocket(clients[i].osock);
clients[i].inuse = 0;
}
}

}
return 0;
}


---------------------------------------------
 * Compile with:
*     cc -O -o datapipe datapipe.c
* On Solaris/SunOS, compile with:
*     gcc -Wall datapipe.c -lsocket -lnsl -o datapipe
* On Windows compile with:
*     bcc32 /w datapipe.c                (Borland C++)
*     cl /W3 datapipe.c wsock32.lib      (Microsoft Visual C++)
*
* Run as:
*   datapipe localhost localport remoteport remotehost