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Tutorial: Writing a Python Port Scanner (Step by Step for Beginners)

Introduction
A port scanner is one of the most important tools in a hacker’s or penetration tester’s toolbox. Before you exploit a system, you first need to know what services it’s running. Services = ports, and ports = attack surface.

While tools like Nmap are industry standards, writing your own port scanner in Python is one of the best beginner projects in cybersecurity. It teaches you:

• How networking works under the hood.
  


• How sockets and TCP connections are established.
  


• How to use Python for offensive security scripting.
  

In this tutorial, we’ll write a working port scanner in Python step by step, test it, and then discuss improvements you can add to make it more powerful.

What is a Port Scanner?
Every service on a computer (web, FTP, SSH, DNS, etc.) communicates through a port. Some common ports include:

• Port 21 → FTP
  


• Port 22 → SSH
  


• Port 25 → SMTP
  


• Port 80 → HTTP
  


• Port 443 → HTTPS
  


• Port 3306 → MySQL
  

A port scanner attempts connections to a range of ports and reports which ones are open.

Why this matters:

• Open ports = potential entry points.
  


• It reveals what technologies the target is running.
  


• It’s the foundation of every penetration test.
  

Step 1: Import the Required Library
Python includes the

Code

socket

library for handling network connections. We’ll use it to connect to different ports.

Code

import socket

This is all we need for a basic TCP scanner.

Step 2: Define the Target
We need to specify the IP address or hostname of the machine we want to scan. For safety, always test on your own machine or lab.

Code

target = "127.0.0.1"   # localhost for testing

Tip: Replace this with the IP of a VM or test environment you control. Scanning unauthorized systems is illegal.

Step 3: Create a Function to Scan a Single Port
We’ll write a function that tries to connect to one port and reports if it’s open.

Code

def scan_port(port):  
    try:  
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)  
        s.settimeout(1)  # Prevents hanging  
        s.connect((target, port))  
        print(f"[+] Port {port} is OPEN")  
        s.close()  
    except (socket.timeout, ConnectionRefusedError):  
        pass  
    except Exception as e:  
        print(f"Error on port {port}: {e}")

How it works:

• Code

  socket.AF_INET
  → IPv4.
  


• Code

  socket.SOCK_STREAM
  → TCP connection.
  


• Code

  settimeout(1)
  → limits wait time to 1 second.
  


• If it connects successfully → port is open.
  


• If it fails → port is closed (we ignore it).
  

Step 4: Loop Through Multiple Ports
Instead of checking just one port, let’s scan a whole range.

Code

for port in range(1, 1025):   # Scan ports 1–1024  
    scan_port(port)

Ports 1–1024 are the “well-known ports” used by common services.

Step 5: Full Script (dummy/testing)
Here’s a complete, working Python port scanner.

Code

import socket  

target = "127.0.0.1"   # Replace with a legal target you control  

def scan_port(port):  
    try:  
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)  
        s.settimeout(1)  
        s.connect((target, port))  
        print(f"[+] Port {port} is OPEN")  
        s.close()  
    except (socket.timeout, ConnectionRefusedError):  
        pass  
    except Exception as e:  
        print(f"Error on port {port}: {e}")  

print(f"[*] Scanning target: {target}")  

for port in range(1, 1025):  
    scan_port(port)

Run it with:

Code

python3 scanner.py

You’ll see open ports printed in the terminal.

Step 6: Improvements
Our scanner works, but we can make it better:

• Threading: Use
  Code

  threading
  or
  Code

  concurrent.futures
  to scan ports faster.
  


• Banner Grabbing: Try reading service banners to identify software versions.
  


• Custom Input: Ask the user for a target and port range instead of hardcoding.
  


• Logging: Save results to a file.
  


• UDP Scanning: Extend functionality to scan UDP ports.
  

Example: Banner Grabbing

Code

def scan_port(port):  
    try:  
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)  
        s.settimeout(1)  
        s.connect((target, port))  
        try:  
            banner = s.recv(1024).decode().strip()  
            print(f"[+] Port {port} OPEN | Banner: {banner}")  
        except:  
            print(f"[+] Port {port} OPEN (no banner)")  
        s.close()  
    except:  
        pass

This can reveal information like “Apache/2.4.49” or “OpenSSH 8.2” which can be checked for vulnerabilities.

Real-World Uses

• Pentesters use port scans during reconnaissance.
  


• Admins use them to find misconfigured or unnecessary services.
  


• Learners use them to understand network behavior.
  

Conclusion
By building your own Python port scanner, you gain more than just a tool, you gain an understanding of how network services respond, how connections are made, and why scanning is so fundamental to hacking.

Tools like Nmap will always be superior in features, but writing your own scanner builds the foundation for every other security project you’ll do in the future.

[ttbr0]

i truly believe that to be successful as a hacker you should first be learning to build something. when you have built something, you gain understanding of how something works. when you gain understanding of a system, you might realize vulnerabilities in it. when you realize the vulnerabilities, you know how to attack it. when you know how to attack it, you know can figure out how to defend it. this is the crucial first step that people always skip and it is detrimental to their progress. its why i always recommend learning programming before hacking. if you cant even program, i doubt you have any meaningful knowledge of how a computer works at the fundamental level. if you dont even have this simple knowledge, i wouldnt trust you could exploit a system even if a blatant vulnerability was staring at you right in your face.

[▄︻デ══━一一一一━]

i truly believe that to be successful as a hacker you should first be learning to build something. when you have built something, you gain understanding of how something works. when you gain understanding of a system, you might realize vulnerabilities in it. when you realize the vulnerabilities, you know how to attack it. when you know how to attack it, you know can figure out how to defend it. this is the crucial first step that people always skip and it is detrimental to their progress. its why i always recommend learning programming before hacking. if you cant even program, i doubt you have any meaningful knowledge of how a computer works at the fundamental level. if you dont even have this simple knowledge, i wouldnt trust you could exploit a system even if a blatant vulnerability was staring at you right in your face.

Gem of an advice.

[Stanley]

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