#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/wait.h>

#define RCV_PORT 8642
#define SND_PORT 8080
#define SND_HOST "192.168.1.23"

#define BUF_SIZE 262144

// function declarations
void connection (int newsock);
void retrieve(char * host, char * request, char * reply);

// global variables
time_t tLastUpd = (time_t) 0;

void sigchld_handler(int signo)
{
  while (waitpid(-1, NULL, WNOHANG) > 0);
}

// code entry point
int main (int argc, char *argv[])
{
    struct sockaddr_in sAddr;
    int listensock;
    int newsock;
    int result;
    int pid;
    int val;
    
    listensock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    val = 1;
    result = setsockopt(listensock, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));
    if (result < 0) {
        perror("proxy error - main setsockopt");
        return 0;
    }

    sAddr.sin_family = AF_INET;
    sAddr.sin_port = htons(RCV_PORT);
    sAddr.sin_addr.s_addr = INADDR_ANY;
    result = bind(listensock, (struct sockaddr *) &sAddr, sizeof(sAddr));
    if (result < 0) {
        perror("proxy error - main bind");
        return 0;
    }

    result = listen(listensock, 10);
    if (result < 0) {
        perror("proxy error - main listen");
        return 0;
    }

    signal(SIGCHLD, sigchld_handler);

    while (1) {
        newsock = accept(listensock, NULL ,NULL);
        if ((pid = fork()) == 0) {
            close (listensock);
            connection (newsock);
            exit(0);
        }
        close(newsock);
    }
}

void connection (int newsock)
{
    FILE *fp;

    char req_buf[BUF_SIZE];
    char rpl_buf[BUF_SIZE];
    char *p;
    int n, nleft;
    
    memset (req_buf, 0, BUF_SIZE);
    p = req_buf;
    nleft = BUF_SIZE;
    while (1)
    {
        n = recv (newsock, p, nleft, 0);
        p += n;
        nleft -= n;

	if ((n == 0) || (nleft <= 0) ||
            (strstr(req_buf, "\n\n") != 0) ||
            (strstr(req_buf, "\r\n\r\n") != 0))
            break;
    }

    // modify some of the http request headers
    if ((p = strstr(req_buf, "RCV_PORT")) != NULL)
        strncpy (p, "SND_PORT", 4);

    if ((p = strstr(req_buf, "If-Modified-Since:")) != NULL)
        strncpy (p, "XXXXXX", 6);

    if ((p = strstr(req_buf, "Accept-Encoding:")) != NULL)
        strncpy (p, "XXXXXX", 6);

    if ((p = strstr(req_buf, "Keep-Alive")) != NULL)
        strncpy (p, "XXXXXX", 6);
    if ((p = strstr(req_buf, "Connection: keep-alive")) != NULL)
        strncpy (p + 12, "close     ", 10);
    if ((p = strstr(req_buf, "Connection: Keep-Alive")) != NULL)
        strncpy (p + 12, "Close     ", 10);

    retrieve (SND_HOST, req_buf, rpl_buf);

    // modify URLs in the result before sending back
    while ((p = strstr(rpl_buf, "http://192.168.1.23:SND_PORT/roller/folder/")) != NULL)
                    strncpy (p, "                    http://www.i-am.ws/", 39);
    while ((p = strstr(rpl_buf, "http://192.168.1.23:RCV_PORT/roller/folder/")) != NULL)
                    strncpy (p, "                    http://www.i-am.ws/", 39);

    // send the page back to the client (via the DMZ)
    send (newsock, rpl_buf, strlen(rpl_buf), 0);
 
    close (newsock);
    return;
}

void retrieve(char * host, char * request, char * reply)
{
    int sock;
    struct sockaddr_in sAddr;
    struct hostent *hent;
    unsigned int a, b, c, d;
    char addr[5];
    char buffer[BUF_SIZE];
    int n, nleft;
    char *p;

    sAddr.sin_family = AF_INET;
    sAddr.sin_addr.s_addr = INADDR_ANY;
    sAddr.sin_port = 0;
    
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    bind(sock, (const struct sockaddr *) &sAddr, sizeof(sAddr));

    if ((hent = gethostbyname(host)) == NULL) {
        n = sscanf(host, "%u.%u.%u.%u", &a,&b,&c,&d);
        addr[0] = a; addr[1] = b; addr[2] = c; addr[3] = d;
	hent = gethostbyaddr(addr, 4, AF_INET);
    }    
    memcpy (&sAddr.sin_addr, hent->h_addr, hent->h_length);
    sAddr.sin_port = htons(SND_PORT);

    if (connect(sock, (const struct sockaddr *) &sAddr, sizeof(sAddr)) != 0) 
    {
      perror("proxy error - retrieve connect");
      return;
    }

    // send the socket request
    n = send (sock, request, strlen(request), 0);

    // now get the reply (piece by piece)
    memset (reply, 0, BUF_SIZE);
    p = reply;
    nleft = BUF_SIZE;
    while (1)
    {
        n = recv (sock, p, nleft, 0);
        p += n;
        nleft -= n;
        if ((n == 0) || (nleft <= 0))
             break;
    }
    close(sock);

    return;
}

// end of source