10 Essential Port Scanners Every Network Admin Should Know

Building a Custom Port Scanner with Python: Step-by-Step

This guide walks through building a simple, effective TCP port scanner in Python. It’s intended for educational use (network troubleshooting, asset discovery on networks you own or have permission to test). Do not scan networks without authorization.

What you’ll build

A command-line Python script that:

• Accepts a target IP or hostname
• Scans a range of TCP ports (configurable)
• Returns open, closed, and filtered status (basic)
• Runs scans concurrently for speed

Requirements

• Python 3.8+
• Modules: socket, argparse, concurrent.futures, datetime (All are in the standard library; no external packages required.)

Step 1 — Script outline

Create a file named scan.py and structure it with:

• argument parsing
• a worker function to test one port
• a concurrent executor to run many workers
• result aggregation and simple reporting

Step 2 — Argument parsing

Use argparse to accept:

• target (positional): IP or hostname
• –start (optional, default 1): start port
• –end (optional, default 1024): end port
• –timeout (optional, default 1.0): socket timeout in seconds
• –workers (optional, default 100): concurrency level

Example code:

python
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#!/usr/bin/env python3
import argparse 
def parse_args():
p = argparse.ArgumentParser(description=“Simple TCP port scanner”)
    p.add_argument(“target”, help=“IP address or hostname to scan”)
    p.add_argument(”–start”, type=int, default=1, help=“Start port (default 1)”)
    p.add_argument(”–end”, type=int, default=1024, help=“End port (default 1024)”)
    p.add_argument(”–timeout”, type=float, default=1.0, help=“Socket timeout seconds”)
    p.add_argument(”–workers”, type=int, default=100, help=“Concurrent workers”)
    return p.parse_args()

Step 3 — Port test worker

Use socket.createconnection to attempt a TCP connect. Return a simple status string.

python
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import socket 
def scan_port(target, port, timeout):
    try:
        with socket.createconnection((target, port), timeout=timeout):
            return port, “open”
    except socket.timeout:
        return port, “filtered”
    except ConnectionRefusedError:
        return port, “closed”
    except Exception:
        return port, “filtered”

python
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from concurrent.futures import ThreadPoolExecutor, as_completed 
def run_scan(target, start, end, timeout, workers):
    results = []
    with ThreadPoolExecutor(max_workers=workers) as exe:
        futures = {exe.submit(scan_port, target, port, timeout): port for port in range(start, end + 1)}
        for fut in ascompleted(futures):
            results.append(fut.result())
    return sorted(results, key=lambda x: x[0])

python
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import socket import sys 
def resolvetarget(target):
    try:
        return socket.gethostbyname(target)
    except socket.gaierror as e:
        print(f”DNS resolution failed: {e}“)
        sys.exit(1)

python
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from datetime import datetime 
def report(results, target, ip, start, end, elapsed):
    open_ports = [p for p, s in results if s == “open”]
    print(f”Scan of {target} ({ip}) ports {start}-{end} completed in {elapsed:.2f}s”)
    if open_ports:
        print(“Open ports:”)
        for p in openports:
            print(f”  - {p}“)
    else:
        print(“No open ports found.”)

python
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def main():
    args = parse_args()
    ip = resolve_target(args.target)
    start, end = args.start, args.end     if start < 1 or end > 65535 or start > end:
        print(“Invalid port range.”)
        return
    t0 = datetime.now()
    results = run_scan(ip, start, end, args.timeout, args.workers)
    elapsed = (datetime.now() - t0).total_seconds()
    report(results, args.target, ip, start, end, elapsed)

if name == “main”:
    main()