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Grokking Computer Networking for Software Engineers

Improvements to UDP Programs: Avoiding Arbitrary Servers

Explore methods to improve UDP client programs by preventing replies from unknown servers. Understand how to use the connect() method to restrict communication to one server and implement address matching to handle multiple servers. This lesson helps you write more reliable and secure UDP applications in Python.

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Problem: Replies From Arbitrary Servers

Note that at the moment, our UDP client accepts replies from any machine and assumes that it’s the one that it sent the initial message to, evident in the following line,

data, address = s.recvfrom(MAX_SIZE_BYTES)

Note how the client does not check who it is receiving the message from. It just receives a message.

Fix with connect()

There are two quick ways to go about fixing this. The first of which is to use the connect() method to forbid other addresses from sending packets to the client.

Python 3.5
import socket
MAX_SIZE_BYTES = 65535 # Mazimum size of a UDP datagram
def client(port):
  host = '127.0.0.1'
  s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
  s.connect((host, port))
  message = input('Input lowercase sentence:' )
  data = message.encode('ascii')
  s.send(data)
  print('The OS assigned the address {} to me'.format(s.getsockname()))
  data = s.recv(MAX_SIZE_BYTES) 
  text = data.decode('ascii')
  print('The server replied with {!r}'.format(text))

With the ...