TCP Packet Builder & Parser

by Pranav Prashant Shewale 24BCE1226 & Lavanya Vaidya 24BCE5393

This project provides a set of Python scripts to build, send, receive, and parse TCP packets using the Scapy library. It is designed for cross-platform execution between a Windows sender and a macOS receiver, demonstrating fundamental concepts of network programming and the TCP/IP protocol suite as defined in RFC 9293.

Features

• RFC 9293 Compliant: Constructs TCP packets with all required header fields.
• Full Flag Support: Crafts packets with any combination of TCP flags (SYN, ACK, FIN, RST, PSH, URG, CWR, ECE).
• Cross-Platform: Designed for sending from Windows and receiving on macOS.
• Layer 2 Sending: Uses Layer 2 (Ethernet) packet construction to bypass Windows raw socket restrictions.
• Real-Time Parsing: The receiver script captures packets live and provides a detailed breakdown of headers.
• Checksum Validation: Automatically validates the integrity of received TCP packets.

File Structure

The project is organized into a modular architecture:

.
├── sender.py           # Main script to send packets (Windows)
├── receiver.py         # Main script to receive packets (macOS)
└── tcp_builder.py      # (Optional) A modular library for building TCP packets

• sender.py: The primary script for the sending machine. It resolves the destination MAC address, constructs a complete Layer 2 (Ethernet) frame containing the IP and TCP segments, and transmits it.
• receiver.py: The primary script for the receiving machine. It sniffs the network for incoming TCP packets on a specific port, parses the headers, validates the checksum, and prints a detailed analysis.
• tcp_builder.py: A helper module that provides a function to construct TCP packets. While sender.py contains its own logic, this module can be used for other testing scenarios.

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Prerequisites

1. System Requirements

• Two machines on the same local network (e.g., connected to the same Wi-Fi router).
  • Sender: Windows 10/11
  • Receiver: macOS
• Python 3.8+ installed on both machines.

2. Dependencies

Install the necessary Python library on both systems:

pip install scapy

3. Windows-Specific Setup (Sender Machine)

Scapy requires a special driver on Windows to perform raw packet operations.

1. Download Npcap: Go to the official Npcap website and download the latest installer.
2. Install Npcap: Run the installer as Administrator and ensure the following option is checked during installation:
   • Install Npcap in WinPcap API-compatible Mode
3. Restart your computer after the installation is complete.

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Execution Guide

Follow these steps to run the project.

Step 1: Configure the Scripts

1. Find IP Addresses:

   • On the Windows machine (Sender), open Command Prompt and run ipconfig. Note the IPv4 address.
   • On the macOS machine (Receiver), open Terminal and run ifconfig | grep "inet ". Note the IP address.

2. Edit sender.py:

   • Open sender.py in a text editor.
   • Update the SRC_IP and DST_IP variables with the IP addresses you found.

   # sender.py
   SRC_IP = "192.168.1.10"  # Your Windows IP
   DST_IP = "192.168.1.15"  # Your Mac IP
   

Step 2: Run the Receiver (macOS)

On the macOS machine, open a Terminal window and run the receiver.py script with sudo to grant it the necessary permissions to sniff network packets.

# Navigate to the project directory
cd /path/to/your/project

# Run the script with root privileges
sudo python receiver.py

The terminal will display Listening on TCP port 4444... and wait for an incoming packet.

Step 3: Run the Sender (Windows)

On the Windows machine, you must run the script from a Command Prompt with elevated privileges.

1. Click the Start Menu, type cmd.

2. Right-click on Command Prompt and select "Run as administrator".

3. In the administrator Command Prompt, navigate to your project directory.

   cd C:\path\to\your\project
   

4. Execute the sender.py script.

   python sender.py
   

Step 4: Observe the Results

• On the Sender (Windows), you will see output confirming the packet was built and sent:

  --- Packet Built (Layer 2) ---
  ###[ Ethernet ]###
    dst= aa:bb:cc:dd:ee:ff
  ###[ IP ]###
    src= 192.168.1.10
    dst= 192.168.1.15
  ...
  Sent 1 packets.
  

• On the Receiver (macOS), the script will immediately print the parsed details of the received packet:

  --- TCP Packet Received ---
  Source IP: 192.168.1.10
  Destination IP: 192.168.1.15
  Source Port: 12345
  Dest. Port: 4444
  Sequence Num: 1000
  Flags: S
  Window Size: 2048
  Checksum: 0xabcd (Valid)
  Payload: Hello from Windows!
  

Troubleshooting

• Permission Denied / WinError 10013: You did not run the script as an administrator (Windows) or with sudo (macOS).
• Scapy_Exception: No libpcap provider available: Npcap is not installed correctly on Windows. Re-install it, ensuring the "WinPcap API-compatible mode" is enabled, and restart your PC.
• No Packet Received:
  • Verify that both IP addresses are correct and both devices are on the same network.
  • Temporarily disable the firewall on both the sender and receiver to ensure it is not blocking the packets.
  • Ensure the DST_IP in sender.py is reachable. Try running ping <DST_IP> from the sender machine.