1. A Python TFTP server Bernie Network engineer
Needed a GUI driven TFTP server
Wrote it several years ago when learning Python
Was a god awful mess, but recently updated

2. TFTP -why? Trivial File Transfer Protocol
A way of passing files to and from a server in an easy fashion.
Used to store and get Router and switch config files.
No passwords, just an ip address of a server and:
Get filename
Put filename

3. GUI – why? No password access, so insecure
Network engineer needs a simple GUI driven server for his laptop, only enabled when needed
Easy to install (standard library, pure python, no dependencies, put it anywhere and run)
Easy to uninstall – delete directory and files
Python 2 and Python 3 versions, Linux and Windows

4. TFTP - what? Trivial File Transfer Protocol TFTP - uses UDP port 69
TFTP Protocol (revision 2) RFC 1350
TFTP Option Extension RFC 2347
TFTP Blocksize Option RFC 2348
TFTP Timeout Interval and Transfer Size Options RFC 2349

5. Summary Client calls with a request to get or put a file
Server acknowledges
Files transferred with acknowledgements, the acks contain an incrementing block count.
UDP not TCP - connectionless
Up to the application to create a 'connection' with acknowledgements, timeouts and error packets.

6. Uniquely identify the connection: 192.168.1.50/2002 - 192.168.1.1/69
Client
Client
15037
2002
26004 69 69 69
Server
Uniquely identify the connection:
/ /69
/ /69
/ /69

7. The problem A 'connection' is defined by:
Server address/port + Client address/port
TFTP server listens on 69, but can reply on any port, creating a connection suitable for a threaded application.
But not Firwall friendly
If server replies with port 69, must implement multiplexing for simultaneous connections.

8. Asyncore UDP listenning socket Create a dispatcher class
Use __init__ to bind a socket
Whenever asyncore.poll() is called:
Dispatcher handle_read method called when data is available
Dispatcher handle_write method called to send data

9. import asyncore, socket
class MyServer(asyncore.dispatcher):
def __init__(self, local_addr, local_port):
asyncore.dispatcher.__init__(self)
self.create_socket(socket.AF_INET,
socket.SOCK_DGRAM)
self.bind((local_addr, local_port))
def handle_read(self):
rx_data, (addr,port) = self.recvfrom(1024)
def handle_write(self):
sent = self.sendto(tx_data, (addr,port))
def handle_connect(self):
pass
def handle_error(self):

10. myserver = MyServer(addr, port)
While True:
asyncore.poll()
myserver.close()

11. Read command line options and configuration file
Create server
GUI? NO
YES
GUI
Loop
New Thread
Server
Loop
EXIT

12. ServerState class Has an asyncore dispatcher class as an attribute
Maintains a dictionary of connections
Each dictionary key is the remote client (address, port) tuple, and value is a 'Connection' object.
Has a poll() method to call asyncore.poll() and also to check each connection for timeouts
Maintains a text attribute, which can be read by a GUI and produces logs

13. Checks 'serving' attribute value and turns on and off dispatcher
ServerState
Dispatcher
Dictionary of
Connections
poll() method
Checks 'serving' attribute value
and turns on and off dispatcher
So this can be changed by the GUI
If serving:
Calls asyncore.poll()
Calls each connection poll() for
each connection in the dictionary

14. Connection class
On initialising, parses requested filename, sets options such as blksize, keeps blksize counter
Has a poll method to check timeouts and re-tries
Has a senddata method, called from the dispatcher handle_write method, to send any data in the tx_data attribute
Acts as a parent to two child classes:
SendData – for sending a file
ReceiveData – for receiving a file

15. sends any data in tx_data poll() method checks timers
Connection class
Parse filename
Parse blksize option
Keep blkcount
send_data method
sends any data in tx_data
poll() method checks timers
SendData class
incoming_data method
checks acknowledgements
if ok reads file and
puts data in tx_data
ReceiveData class
incoming_data method
writes to file,
creates acknowledgement
and puts it in tx_data

16. SendData class Inherits from the Connection class
Opens the requested file for reading
Has an incoming_data method which is called from the dispatcher handle_read method, and handles the acknowledgements from the remote client.
Puts data read from the file into tx_data

17. ReceiveData class Inherits fromn the Connection class
Opens the requested file for writing
Has an incoming_data method which is called from the dispatcher handle_read method, and handles the file data received from the remote client.
Puts acknowledgements into tx_data

18. If remote ipaddress/port is a key in the connections dictionary
Dispatcher
handle_read()
If remote ipaddress/port is a key in the
connections dictionary
Call the connection.incoming_data()
If not
Create a new connections object
either a SendData
or a ReceiveData connection
dependent on header in received packet
handle_write()
For each Connection object in the
Dictionary
If the connection object has data in its tx_data
Attribute
Call the connection.send_data()

19. If no response received within a timeout period
Connection.poll()
Check timer
If no response received within a timeout period
re-transmit by setting tx_data
to previous contents
Or if too many timeouts, close the connection
Stopwatch class
Measures average response
time between packets, and sets a TTL
value used to measure if a timeout
has occurred.
So each connection instance uses
a Stopwatch instance.

20. GUI
Used Tkinter – note the font and general looks are much better with later versions of Python
The gui loop is started in the main thread and has access to the ServerState instance to read/write attributes.
The ServerState instance poll is called in a loop in a second thread.
Any change to attributes are then acted upon in the poll() method, to keep everything thread safe.

21. Python 2 to 3
Bytes far better, much more clarity as the received data is a bytes object
One gotcha which surprised me:
>>>x = b'hello'
>>>x[0] == b'h'
False
>>>x[0]
104

22. Tkinter structure changed
Python 2:
import Tkinter, tkFileDialog, tkMessageBox
Python 3:
import tkinter, tkinter.filedialog, tkinter.messagebox

23. Threading
Python 2:
thread.start_new_thread(tftp_engine.loop, (server,))
Python 3:
threading.Thread(target=tftp_engine.loop, args=(server,)).start()