#include <stdlib.h> 
#include <stdio.h> 
#include <unistd.h> 
#include <sys/time.h> 
#include <sys/types.h> 
#include <string.h> 
#include <signal.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <arpa/inet.h>
#include <errno.h> 

static int forward_port; 

#undef max 
#define max(x,y) ((x) > (y) ? (x) : (y)) 

static int listen_socket (int listen_port) 
{ 
    struct sockaddr_in a; 
    int s; 
    int yes; 
    if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) 
	{ 
        perror ("socket"); 
        return -1; 
    } 
    yes = 1; 
    if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *) &yes, sizeof (yes)) < 0) 
	{ 
        perror ("setsockopt"); 
        close (s); 
        return -1; 
    } 
    memset (&a, 0, sizeof (a)); 

    a.sin_port = htons (listen_port); 
    a.sin_family = AF_INET; 
    if (bind(s, (struct sockaddr *) &a, sizeof (a)) < 0) 
	{ 
        perror ("bind"); 
        close (s); 
        return -1; 
    } 
    printf ("accepting connections on port %d\n", (int) listen_port); 
    listen (s, 10); 
    return s; 
} 

static int connect_socket (int connect_port, char *address) 
{ 
    struct sockaddr_in a; 
    int s; 
    if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) 
	{ 
        perror ("socket"); 
        close (s); 
        return -1; 
    } 

    memset (&a, 0, sizeof (a)); 
    a.sin_port = htons (connect_port); 
    a.sin_family = AF_INET; 

    if (!inet_aton(address, (struct in_addr *) &a.sin_addr.s_addr)) 
	{ 
        perror ("bad IP address format"); 
        close (s); 
        return -1; 
    } 

    if (connect(s, (struct sockaddr *) &a, sizeof (a)) < 0) 
	{ 
        perror ("connect()"); 
        shutdown (s, SHUT_RDWR); 
        close (s); 
        return -1; 
    } 
    return s; 
} 

#define SHUT_FD1 { \ 
    if (fd1 >= 0) {   \ 
        shutdown (fd1, SHUT_RDWR);  \ 
        close (fd1);  \ 
        fd1 = -1;     \ 
    }   \ 
} 

#define SHUT_FD2 { \ 
    if (fd2 >= 0) {   \ 
        shutdown (fd2, SHUT_RDWR);  \ 
        close (fd2);  \ 
        fd2 = -1;     \ 
    }   \ 
} 

#define BUF_SIZE 1024 

int main (int argc, char **argv)
{ 
    int h; 
    int fd1 = -1, fd2 = -1; 
    char buf1[BUF_SIZE], buf2[BUF_SIZE]; 
    int buf1_avail, buf1_written; 
    int buf2_avail, buf2_written; 

    if (argc != 4) 
	{ 
        fprintf (stderr, "Usage\n\tfwd   \n"); 
        exit (1); 
    } 

    signal (SIGPIPE, SIG_IGN); 

    forward_port = atoi (argv[2]); 

    /*建立監聽socket*/ 
    h = listen_socket (atoi (argv[1])); 
    if (h < 0)
	{
		exit (1); 
	}

    for (;;) 
	{ 
        int r, nfds = 0; 
        fd_set rd, wr, er; 
        FD_ZERO (&rd); 
        FD_ZERO (&wr); 
        FD_ZERO (&er); 
        FD_SET (h, &rd); 

        /*把監聽socket和可讀socket三個一起放入select的可讀句柄列表里*/ 

        nfds = max (nfds, h); 
        if (fd1 > 0 && buf1_avail < BUF_SIZE)
		{ 
            FD_SET (fd1, &rd); 
            nfds = max (nfds, fd1); 
        } 
        if (fd2 > 0 && buf2_avail < BUF_SIZE) 
		{ 
            FD_SET (fd2, &rd); 
            nfds = max (nfds, fd2); 
        } 

        /*把可寫socket兩個一起放入select的可寫句柄列表里*/ 中國網管論壇 
        if (fd1 > 0 && buf2_avail - buf2_written > 0) 
		{ 
            FD_SET (fd1, &wr); 
            nfds = max (nfds, fd1); 
        } 
        if (fd2 > 0 && buf1_avail - buf1_written > 0) 
		{ 
            FD_SET (fd2, &wr); 
            nfds = max (nfds, fd2); 
        } 

        /*把有異常數據的socket兩個一起放入select的異常句柄列表里*/ 
        if (fd1 > 0) 
		{ 
            FD_SET (fd1, &er); 
            nfds = max (nfds, fd1); 
        } 
        if (fd2 > 0) 
		{ 
            FD_SET (fd2, &er); 
            nfds = max (nfds, fd2); 
        } 

        /*開始select*/ 
        r = select (nfds + 1, &rd, &wr, &er, NULL); 

        if (r == -1 && errno == EINTR)
		{
			continue; 
		}

        if (r < 0) 
		{ 
            perror ("select()"); 
            exit (1); 
        } 

        /*處理新連接*/ 
        if (FD_ISSET (h, &rd))
		{ 
            unsigned int l; 
            struct sockaddr_in client_address; 
            memset (&client_address, 0, l = sizeof (client_address)); 
            r = accept (h, (struct sockaddr *)&client_address, &l); 

            if (r < 0) 
			{ 
                perror ("accept()"); 
            } 
			else
			{ 
                /*關閉原有連接，把新連接作為fd1，同時連接新的目標fd2*/ 
                SHUT_FD1; 
                SHUT_FD2; 
                buf1_avail = buf1_written = 0; 
                buf2_avail = buf2_written = 0; 

                fd1 = r; 
                fd2 = connect_socket (forward_port, argv[3]); 
                if (fd2 < 0)
				{ 
                    SHUT_FD1; 
                }
				else 
				{
					printf ("connect from %s\n", inet_ntoa(client_address.sin_addr)); 
				}
            } 
        } 

        /* NB: read oob data before normal reads */ 
        if (fd1 > 0) 
		{
			if (FD_ISSET (fd1, &er)) 
			{ 
				char c; 
				errno = 0; 
				r = recv (fd1, &c, 1, MSG_OOB); 
				if (r < 1) 
				{ 
					SHUT_FD1; 
				} 
				else 
				{
					send (fd2, &c, 1, MSG_OOB); 
				}
			} 
		}

        if (fd2 > 0) 
		{
			if (FD_ISSET (fd2, &er)) 
			{ 
				char c; 
				errno = 0; 
				r = recv (fd2, &c, 1, MSG_OOB); 
				if (r < 1) 
				{ 
					SHUT_FD1; 
				} 
				else 
				{
					send (fd1, &c, 1, MSG_OOB); 
				}
			} 
		}

        /* NB: read data from fd1 */ 
        if (fd1 > 0) 
		{
			if (FD_ISSET (fd1, &rd)) 
			{ 
				r = read (fd1, buf1 + buf1_avail, BUF_SIZE - buf1_avail); 
				if (r < 1)
				{ 
					SHUT_FD1; 
				} 
				else 
				{
					buf1_avail += r; 
				}
			} 
		}

        /* NB: read data from fd2 */ 
        if (fd2 > 0) 
		{
			if (FD_ISSET (fd2, &rd)) 
			{ 
				r = read (fd2, buf2 + buf2_avail, BUF_SIZE - buf2_avail); 
				if (r < 1) 
				{ 
					SHUT_FD2; 
				} 
				else 
				{
					buf2_avail += r; 
				}
			} 
		}

        /* NB: write data to fd1 */ bitscn.com 
        if (fd1 > 0) 
		{
			if (FD_ISSET (fd1, &wr))
			{ 
				r = write (fd1, buf2 + buf2_written, buf2_avail - buf2_written); 
				if (r < 1) 
				{ 
					SHUT_FD1; 
				} 
				else 
				{
					buf2_written += r; 
				}
			} 
		}

        /* NB: write data to fd1 */ 
        if (fd2 > 0) 
		{
			if (FD_ISSET (fd2, &wr)) 
			{ 
				r = write (fd2, buf1 + buf1_written, buf1_avail - buf1_written); 
				if (r < 1) 
				{ 
					SHUT_FD2; 
				} 
				else 
				{
					buf1_written += r; 
				}
			} 
		}

        /* check if write data has caught read data */ 
        if (buf1_written == buf1_avail)
		{
			buf1_written = buf1_avail = 0; 
		}

        if (buf2_written == buf2_avail)
		{
			buf2_written = buf2_avail = 0; 
		}
        
        /* one side has closed the connection, keep writing to the other side until empty */ 
        if (fd1 < 0 && buf1_avail - buf1_written == 0)
		{ 
            SHUT_FD2; 
        } 
        if (fd2 < 0 && buf2_avail - buf2_written == 0)
		{ 
            SHUT_FD1; 
        } 
    } 
    return 0; 
}