Running home-made proxy program takes screenshots

I ran into a bit of a problem while trying out this python 2.7 code featured in the book called 'Black Hat Python'. It is supposed to emulate a proxy, but upon running the following command the cursor changes into a cross and take abour 5 screenshots:

sudo ./proxy.py 127.0.0.1 21 ftp.target.ca 21 True

Here is the code:

import sys
import socket
import threading

def server_loop(local_host, local_port, remote_host, remote_port, receive_first):
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    try:
        server.bind((local_host, local_port))

    except:
        print "[!!] Failed to listen on %s:%d" % (local_host, local_port)
        print "[!!] Check for other listening sockets or correct permissions."

        sys.exit(0)

    print "[*] Listening on %s:%d" % (local_host, local_port)

    server.listen(5)

    while True:
        client_socket, addr = server.accept()

        # print out the local connection information
        print "[==>] Received incoming connection from %s:%d" % (addr[0], addr[1])

        # start a thread to talk to the remote host
        proxy_thread = threading.Thread(target=proxy_handler, args=(client_socket, remote_host, remote_port, receive_first))

        proxy_thread.start()

def proxy_handler(client_socket, remote_host, remote_port, receive_first):
    # connect to the remote host
    remote_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    remote_socket.connect((remote_host, remote_port))

    # receive data from the remote end if necessary
    if receive_first:
        remote_buffer = receive_from(remote_socket)

        hexdump(remote_buffer)

        # send it to our response handler
        remote_buffer = response_handler(remote_buffer)

        # if we have data to send to our local client, send it
        if len(remote_buffer):
            print "[<==] Sending %d bytes to local host." % len(remote_buffer)

            client_socket.send(remote_buffer)

    # now lets loop and read from local,
    # send to remote, send to local
    # rinse, wash, repeat
    while True:
        # read from local host
        local_buffer = receive_from(client_socket)

        if len(local_buffer):
            print "[==>] Received %d bytes from local host." % len(local_buffer)

            hexdump(local_buffer)

            # send it to our request handler
            local_buffer = request_handler(local_buffer)

            # send off the data to the remote host
            remote_socket.send(local_buffer)

            print "[==>] Sent to remote."

        # receive back the response
        remote_buffer = receive_from(remote_socket)

        if len(remote_buffer):
            print "[<==] Received %d bytes from remote." % len(remote_buffer)

            hexdump(remote_buffer)

            # send to our response handler
            remote_buffer = response_handler(remote_buffer)

            # send the response to the local socket
            client_socket.send(remote_buffer)

            print "[<==] Sent to local host."

        # if no more data on either side, close the connections
        if not len(local_buffer) or not len(remote_buffer):
            client_socket.close()
            remote_socket.close()

            print "[*] No more data. Closing connections."

            break

def hexdump(src, length=16):
    result = []

    digits = 4 if isinstance(src, unicode) else 2

    for i in xrange(0, len(src), length):
        s = src[i:i+length]

        hexa = b' '.join(["%0*X" % (digits, ord(x)) for x in s])

        text = b''.join([x if 0x20 <= ord(x) < 0x7F else b'.' for x in s])

        result.append(b"%04X %-*s %s" % (i, length*(digits + 1), hexa, text))

    print b'\n'.join(result)

def receive_from(connection):
    buffer = ""

    # We set a 2 second timeout; depending on your
    # target, this may need to be adjusted
    connection.settimeout(2)

    try:
        # keep reading into the buffer until
        # there's no more data
        # or we time out
        while True:
            data = connection.recv(4096)

            if not data:
                break

            buffer += data

    except:
        pass

    return buffer

# modify any requests destined for the remote host
def request_handler(buffer):
    # perform packet modifications
    return buffer

# modify any responses destined for the local host
def response_handler(buffer):
    # perform packet modifications
    return buffer

def main():
    # no fancy command-line parsing here
    if len(sys.argv[1:]) != 5:
        print "Usage: ./proxy.py [local_host] [local_port] [remote_host] [remote_port] [receive_first]"
        print "Example: ./proxy.py 127.0.0.1 9000 10.12.132.1 9000 True"

        sys.exit(0)

    # setup local listening parameters
    local_host = sys.argv[1]
    local_port = int(sys.argv[2])

    # setup remote target
    remote_host = sys.argv[3]
    remote_port = int(sys.argv[4])

    # this tells our proxy to connect and receive data
    # before sending to the remote host
    receive_first = sys.argv[5]

    if "True" in receive_first:
        receive_first = True

    else:
        receive_first = False

    # now spin up our listening socket
    server_loop(local_host, local_port, remote_host, remote_port, receive_first)

main()

I am currently running a Kali Linux (not VM).