Network programming Using Python

Network Programming
Using Python

Contents
● Sockets
● Socket Types
● Python socket module
● Socket Object Methods
● Applications
○ Echo Server
○ Network Sniffer
○ File Transfer

Sockets
● The endpoints of a network connection
● Each host has a unique IP address
● Each service runs on a specific port
● Each connection is maintained on both
ends by a socket
● Sockets API allows us to send and receive
data
● Programming Languages provide modules
and classes to use this API

Socket Types
● Stream Sockets (SOCK_STREAM):
○ Connection-oriented
○ Use TCP
● Datagram Sockets (SOCK_DGRAM):
○ Connectionless
○ Use UDP
● Other types:
○ E.g. Raw Sockets
● We will use stream sockets

Python socket module
● socket.socket(family, type, proto)
○ Create new socket object
● socket.SOCK_STREAM (default)
● socket.SOCK_DGRAM
● socket.gethostname()
○ returns a string containing host name of the machine
● socket.gethostbyname(hostname)
○ Translates hostname to ip address
● socket.gethostbyaddr(ip_address)
○ Translates ip address to host name

Socket Object Methods (Server)
● socket.bind(address)
○ e.g. socket.bind((host, port))
● socket.listen(backlog)
○ backlog specifies wait queue size
○ e.g. socket.listen(5)
● socket.accept()
○ Blocks until a client makes a connection
○ Returns (conn, address) where conn is a new socket object usable to send and receive data
○ address is the address bound to the socket on the other end of the connection

Socket Object Methods
● socket.connect(address) - used by client
○ e.g. socket.connect((host, port))
● socket.send(bytes, flags)
○ e.g. socket.send(b‘Hello, World!’)
● socket.recv(bufsize, flags)
○ e.g. socket.recv(1024)
○ bufsize specify maximum amount of data in bytes to be received at once
● socket.close()
○ close connection

Example 1: Echo Server
# Echo server program
import socket
HOST = socket.gethostname()
PORT = 50007 # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print('Connected by', addr)
while True:
data = conn.recv(1024)
if not data: break
conn.send(data)
conn.close()

Example 1: Echo Server
# Echo client program
import socket
HOST = 'localhost'
PORT = 50007 # The same port as used by the server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.send(b'Hello, world')
data = s.recv(1024)
s.close()
print('Received', repr(data))

Example 2: Basic Network Sniffer
#Packet sniffer in python
#For Linux
import socket
#create an INET, raw socket
s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_TCP)
# receive a packet
while True:
print s.recvfrom(65565)
Full Example with Packet Parsing

Example 3: File Transfer
# file_transfer_server.py
import socket
host = socket.gethostname()
port = 6000
s = socket.socket()
s.bind((host, port))
s.listen(5)
print('Server listening..')
...

# file_transfer_server.py
...
while True:
conn, addr = s.accept()
print('New Connection from {}'.format(addr))
with open('test.txt', 'r') as f:
while True:
l = f.read(1024)
if not l: break
conn.send(l.encode())
print('Sent {}'.format(l))
print('Finished Sending.')
conn.close()
print('Connection closed')

# file_transfer_client.py
import socket # Import socket module
s = socket.socket() # Create a socket object
host = ‘localhost’ # Get local machine name
port = 6000 # Reserve a port for your service.
s.connect((host, port))
with open('received.txt', 'w') as f:
print('Downloading file..')
while True:
data = s.recv(1024)
if not data: break
f.write(data.decode())
print('Received: {}n'.format(data.decode()))
print('File downloaded successfully.')
s.close() # Close the socket when done
print('Connection closed')

Code
The previous examples and more can be found at:
https://github.com/ksonbol/socket_examples

Useful Resources
● Python socket module documentation
● Python Network Programming Cookbook
● Simple Client-Server Example
● Packet Sniffer Example
● File Transfer Example
● Unix Sockets Tutorial

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