## Understanding and Building a Network Port Scanner with Python

This article explores the development of a port scanner using Python 3. Before diving into the code, it's essential to understand what a port scan is and its underlying mechanics.
## What is a Port Scan?
Essentially, a port scan involves sending data packets to specific ports on a target system (like a server or computer) to determine which ports are open, closed, or filtered. This process primarily operates at the transport layer of the network stack, most commonly using the TCP protocol.
In a typical TCP SYN scan, the scanner sends a packet with the SYN (synchronize) flag set to a specific port on the target server. The server's response indicates the port's status:
1.  **SYN-ACK (Synchronize-Acknowledge):** If the server responds with a SYN-ACK packet, it signifies that the port is open and listening for connections.

2.  **RST (Reset):** If the server replies with an RST packet, the port is considered closed.

3.  **ICMP Destination Unreachable:** Receiving this type of ICMP message often means the port is filtered, possibly by a firewall. This indicates the port might be accessible only internally or from specific authorized networks.
While TCP scans rely on establishing a connection (the "three-way handshake"), UDP scans are also possible but are connectionless, meaning they don't guarantee packet delivery or reception confirmation in the same way.
Here's a simplified view of the TCP SYN scan interaction for an open port:
```plaintext

[Scanner] --------- SYN ----------> [Server Port X]
[Scanner] <------ SYN-ACK ---------- [Server Port X]   (Port is open!)

[Scanner] -------- RST ------------> [Server Port X]   (Scanner closes the potential connection)

</code></pre>
With this understanding, let's proceed to build a basic port scanner using Python.
<h2>Crafting the Python Port Scanner</h2>
The Python script utilizes standard libraries to perform network socket operations. The code will be presented and explained in logical sections.
<strong>1. Importing Necessary Libraries</strong>
<pre><code class="language-python">import socket

import sys

from datetime import datetime

from colorama import init, Fore
# Initialize Colorama for colored terminal output (resets automatically)

init(autoreset=True)

</code></pre>
This initial block imports:

*   <code>socket</code>: For low-level network communication.

*   <code>sys</code>: To handle system-specific parameters and functions, like exiting the script.

*   <code>datetime</code>: To measure the duration of the scan.

*   <code>colorama</code>: A library to add colored text output to the terminal, making results easier to read. <code>init(autoreset=True)</code> ensures colors reset after each print statement.
<strong>2. The Port Scanning Function</strong>
<pre><code class="language-python">def scan_port(ip, port):

    try:

        # Create a new socket using IPv4 (AF_INET) and TCP (SOCK_STREAM)

        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

        # Set a timeout for the connection attempt (0.5 seconds)

        sock.settimeout(0.5)
        # Attempt to connect to the target IP and port

        # connect_ex returns 0 if the connection is successful (port open)

        result = sock.connect_ex((ip, port))

        # Close the socket connection

        sock.close()
        if result == 0:

            print(Fore.GREEN + f"[+] Port {port} is open")

        else:

            # Optional: You could refine this to distinguish closed vs filtered later

            # For simplicity, we'll mark non-open as closed/filtered here.

            # Using Fore.RED provides visual feedback for closed ports.

            # If you want silent output for closed ports, comment the next line.

            print(Fore.RED + f"[-] Port {port} is closed or filtered")

    except socket.gaierror:

        print(Fore.RED + f"[!] Hostname could not be resolved: {ip}")

        # Optionally exit if the hostname is invalid for the first port

        # sys.exit(1)

    except socket.error:

        print(Fore.RED + f"[!] Could not connect to server: {ip}")

        # Optionally exit if the server is unreachable

        # sys.exit(1)

    except Exception as e:

        # Catch any other unexpected errors during the scan of a specific port

        print(Fore.RED + f"[!] Error scanning port {port}: {e}")
</code></pre>
The <code>scan_port</code> function is the core of the scanner.

*   It takes the target IP address (<code>ip</code>) and the <code>port</code> number as arguments.

*   It creates a TCP socket (<code>socket.SOCK_STREAM</code>) for IPv4 (<code>socket.AF_INET</code>).

*   <code>sock.settimeout(0.5)</code> prevents the script from hanging indefinitely on unresponsive ports.

*   <code>sock.connect_ex((ip, port))</code> attempts the connection. Unlike <code>connect()</code>, <code>connect_ex()</code> returns an error code instead of raising an exception if the connection fails. A return value of <code>0</code> indicates success (port open).

*   The socket is always closed after the attempt using <code>sock.close()</code>.

*   Based on the <code>result</code>, it prints whether the port is open (green text) or closed/filtered (red text).

*   Basic error handling is included for issues like hostname resolution (<code>socket.gaierror</code>) or general connection problems (<code>socket.error</code>).
<strong>3. Main Execution Logic</strong>
<pre><code class="language-python">def main():

    print("-" * 50)

    print(" Simple Python Port Scanner")

    print("-" * 50)
    # Get target IP address or domain name from user

    target = input("Enter the target IP address or domain: ")
    # Get ports to scan from user

    ports_input = input("Enter ports separated by comma (e.g., 80,443), or leave blank to scan ports 1-1024: ")
    ports_to_scan = []
    if ports_input.strip() == "":

        # Default to scanning common ports 1-1024 if no input is given

        ports_to_scan = range(1, 1025)

        print("\nScanning default ports 1-1024...")

    else:

        try:

            # Parse comma-separated input into a list of integers

            ports_to_scan = [int(port.strip()) for port in ports_input.split(",")]
        except ValueError:

            print(Fore.RED + "[!] Invalid port format. Please use numbers separated by commas.")

            sys.exit(1) # Exit if port input is invalid
    print(f"\nInitiating scan on {target}...\n")

    start_time = datetime.now() # Record start time
    # Resolve hostname to IP address once before the loop

    try:

        target_ip = socket.gethostbyname(target)

        print(f"Resolved {target} to {target_ip}\n")

    except socket.gaierror:

        print(Fore.RED + f"[!] Hostname could not be resolved: {target}")

        sys.exit(1) # Exit if the target hostname can't be resolved
    # --- Scanning Loop ---

    open_ports = []
    for port in ports_to_scan:

        # The scan_port function now handles printing open/closed status directly

        # We modify scan_port slightly to return True if open, False otherwise

        # to potentially collect open ports if needed later.

        # For this version, scan_port prints directly.

        scan_port(target_ip, port)
    # --- Scan Completion ---

    end_time = datetime.now() # Record end time

    total_time = end_time - start_time # Calculate duration
    print("\nScan finished!")

    # Consider listing open ports found here if collected

    print(f"Total scan duration: {total_time}")
# Ensure main() runs only when the script is executed directly

if __name__ == "__main__":

    main()

</code></pre>
The <code>main</code> function orchestrates the scan:

*   It displays a simple banner.

*   It prompts the user for the target (IP or domain) and the ports to scan.

*   It parses the port input. If the input is empty, it defaults to scanning ports 1 through 1024 (a common range for well-known services). Otherwise, it converts the comma-separated string into a list of integers. Input validation ensures only numbers are provided.

*   It records the start time using <code>datetime.now()</code>.

*   Crucially, it attempts to resolve the target domain name to an IP address <em>before</em> starting the loop using <code>socket.gethostbyname(target)</code>. This prevents repeated DNS lookups inside the loop and handles resolution errors early.

*   It iterates through the <code>ports_to_scan</code> list.

*   Inside the loop, it calls the <code>scan_port</code> function for each <code>port</code> on the resolved <code>target_ip</code>.

*   After the loop finishes, it records the end time, calculates the total scan duration, and prints a completion message along with the time taken.

*   The <code>if __name__ == "__main__":</code> block ensures that the <code>main()</code> function is called only when the script is run directly (not when imported as a module).
This script provides a foundational understanding of how port scanners work and how they can be implemented using Python's built-in socket library. Remember that scanning networks without explicit permission is unethical and potentially illegal. Always obtain proper authorization before scanning any system you do not own.
<hr />
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