Build a Simple Python Port Scanner: Step‑by‑Step Guide

Overview

Network scanning tools discover open ports and services. Python's standard library enables lightweight port scanners.

Implementation Principle

The scanner iterates over a range of TCP ports, attempts to establish a TCP connection, and treats a successful connection as an open port. The socket module provides the network I/O, while threading enables parallel scans.

Class‑Based Scanner

import socket
import threading

class PortScanner:
    def __init__(self, host):
        self.host = host
        self.open_ports = []

    def scan(self, port):
        try:
            sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            sock.settimeout(1)
            sock.connect((self.host, port))
            sock.close()
            self.open_ports.append(port)
        except:
            pass

    def scan_range(self, start, end):
        for port in range(start, end + 1):
            t = threading.Thread(target=self.scan, args=[port])
            t.start()

    def scan_all(self):
        self.scan_range(1, 65535)
        while threading.active_count() > 1:
            pass
        for port in sorted(self.open_ports):
            print(f"Port {port} is open")

if __name__ == "__main__":
    scanner = PortScanner("localhost")
    scanner.scan_all()

Usage

Replace "localhost" with the target IP or hostname.

Run the script; it scans ports 1‑65535 and prints open ports.

Argument‑Parser Version

This variant accepts host and port range via command‑line arguments.

import socket
import argparse

def scan_port(host, port):
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.settimeout(1)
        sock.connect((host, port))
        print(f"Port {port} is open")
        sock.close()
    except:
        pass

def scan_range(host, start_port, end_port):
    for port in range(start_port, end_port + 1):
        scan_port(host, port)

def main():
    parser = argparse.ArgumentParser(description="Python Port Scanner")
    parser.add_argument("-H", "--host", type=str, required=True,
                        help="Hostname or IP address to scan")
    parser.add_argument("-p", "--port", type=str, required=True,
                        help="Port range, e.g., 1-65535")
    args = parser.parse_args()
    start_port, end_port = map(int, args.port.split("-"))
    scan_range(args.host, start_port, end_port)

if __name__ == "__main__":
    main()

Run example:

python scanner.py -H localhost -p 1-65535

Installation

Python 3.7 or newer is required. Execute the script from a terminal; root/administrator privileges may be needed for privileged ports.

Extensibility

The basic scanner can be extended with:

More threads or a thread pool to improve performance.

UDP probing using socket.SOCK_DGRAM.

Asynchronous I/O (e.g., asyncio) for higher concurrency.