/* version 2026-02-12 */
/* compilation: gcc -Wall -o central central.c */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <sys/wait.h>
#include <poll.h>

int fullread(int fd, void *_buf, int count)
{
  char *buf = _buf;
  int ret = 0;
  int l;
  while (count) {
    l = read(fd, buf, count);
    if (l <= 0) return -1;
    count -= l;
    buf += l;
    ret += l;
  }
  return ret;
}

int fullwrite(int fd, void *_buf, int count)
{
  char *buf = _buf;
  int ret = 0;
  int l;
  while (count) {
    l = write(fd, buf, count);
    if (l <= 0) return -1;
    count -= l;
    buf += l;
    ret += l;
  }
  return ret;
}

void central(int sock, int remote_port)
{
  int s = fork();
  if (s == -1) abort();
  if (s) {
    waitpid(s, 0, 0);
    close(sock);
    return;
  }

  /* son - detach from parent */
  s = fork();
  if (s == -1) abort();
  if (s) exit(0);

  /* wait for remote on port 'remote_port' */
  s = socket(AF_INET, SOCK_STREAM, 0);
  int t;
  if (s == -1) abort();

  int v = 1;
  if (setsockopt(s, IPPROTO_TCP, TCP_NODELAY, &v, sizeof(v)) == -1)
    { perror("setsockopt(NODELAY)"); exit(1); }
  v = 1;
  if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &v, sizeof(int))) {
    perror("setsockopt");
    exit(1);
  }

  struct sockaddr_in a;
  a.sin_family = AF_INET;
  a.sin_port   = htons(remote_port);
  a.sin_addr.s_addr = inet_addr("0.0.0.0");
  if (bind(s, (struct sockaddr *)&a, sizeof(a))) abort();

  if (listen(s, 5)) abort();

  socklen_t alen = sizeof(a);
  if ((t = accept(s, (struct sockaddr *)&a, &alen)) == -1) abort();
  printf("connect from %s\n", inet_ntoa(a.sin_addr));

  v = 1;
  if (setsockopt(t, IPPROTO_TCP, TCP_NODELAY, &v, sizeof(v)) == -1)
    { perror("setsockopt(NODELAY)"); exit(1); }

  close(s);

  /* get key from client */
  char key[512];
  if (fullread(sock, key, 512) != 512) abort();

  /* get key from remote */
  char remote_key[512];
  if (fullread(t, remote_key, 512) != 512) abort();

  /* key not equal -> bad */
  if (memcmp(key, remote_key, 512)) { printf("bad remote/client keys\n"); exit(1); }

  printf("good connection, go\n");

  /* loop */
  while (1) {
    struct pollfd fds[2];
    fds[0].fd = sock;
    fds[0].events = POLLIN;
    fds[0].revents = 0;
    fds[1].fd = t;
    fds[1].events = POLLIN;
    fds[1].revents = 0;

    int r = poll(fds, 2, -1);
    if (r == -1) abort();
    if (r == 0) continue;

    char b[512];

    if (fds[0].revents) {
      /* from sock to t */
      int count = read(sock, b, 512);
      if (count <= 0) {
        printf("fullwrite fails\n");
        exit(1);
      }
      if (fullwrite(t, b, count) != count) {
        printf("ERROR: connection_send_rx failed, dropping\n");
        exit(1);
      }
    }
    if (fds[1].revents) {
      /* from t to sock */
      int count = read(t, b, 512);
      if (count <= 0) {
        printf("fullwrite fails\n");
        exit(1);
      }
      if (fullwrite(sock, b, count) != count) {
        printf("ERROR: connection_send_rx failed, dropping\n");
        exit(1);
      }
    }
  }
}

int main(int n, char **v)
{
  int port = 8001;
  int remote_port = 8000;

  int s = socket(AF_INET, SOCK_STREAM, 0);
  int t;
  if (s == -1) abort();

  int vv = 1;
  if (setsockopt(s, IPPROTO_TCP, TCP_NODELAY, &vv, sizeof(v)) == -1)
    { perror("setsockopt(NODELAY)"); exit(1); }
  vv = 1;
  if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &vv, sizeof(int))) {
    perror("setsockopt");
    exit(1);
  }

  struct sockaddr_in a;
  a.sin_family = AF_INET;
  a.sin_port   = htons(port);
  a.sin_addr.s_addr = inet_addr("0.0.0.0");
  if (bind(s, (struct sockaddr *)&a, sizeof(a))) abort();

  if (listen(s, 5)) abort();

  while (1) {
    socklen_t alen = sizeof(a);
    if ((t = accept(s, (struct sockaddr *)&a, &alen)) == -1) abort();
    printf("connect from %s\n", inet_ntoa(a.sin_addr));

    vv = 1;
    if (setsockopt(t, IPPROTO_TCP, TCP_NODELAY, &vv, sizeof(vv)) == -1)
      { perror("setsockopt(NODELAY)"); exit(1); }

    central(t, remote_port);
  }

  return 0;
}