import socket
import struct

# use with
# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 2000

SO_ORIGINAL_DST = 80  # from linux/netfilter_ipv4.h

def get_original_dst(conn):
    """Return (ip, port) of the original destination before REDIRECT."""
    # Get raw sockaddr_in (16 bytes)
    odest = conn.getsockopt(socket.SOL_IP, SO_ORIGINAL_DST, 16)
    port, raw_ip = struct.unpack_from('!2xH4s', odest)
    ip = socket.inet_ntoa(raw_ip)
    return ip, port

def main():
    HOST = '0.0.0.0'
    PORT = 2000

    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    s.bind((HOST, PORT))
    s.listen(10)

    print(f"Listening on {HOST}:{PORT}")

    while True:
        conn, addr = s.accept()
        try:
            orig_ip, orig_port = get_original_dst(conn)
            print(f"Connection from {addr}, originally to {orig_ip}:{orig_port}")
        except OSError as e:
            print(f"Could not get original destination: {e}")

        with conn:
            while True:
                data = conn.recv(1024)
                if not data:
                    break
                conn.sendall(data)

if __name__ == "__main__":
    main()