Simple TCP Port Scanner

A minimal, single-threaded TCP port scanner written in C. Designed for educational purposes to demonstrate the fundamentals of TCP connect-based scanning.

Overview

The scanner tests a range of TCP ports on a target IP by attempting a full TCP connection (connect()).

Key characteristics:

• Sequential: Scans ports one by one (simple, but slow for large ranges).
• Non-blocking I/O: Uses non-blocking sockets with select() to avoid hanging.
• Timeout-based: Waits up to 1 second per port before reporting a timeout.

Build & Run

Compile:

gcc -o scanner scanner.c

Run:

./scanner <IP> <Start Port> <End Port>

Example:

./scanner 127.0.0.1 80 100

Output Meaning

• [OPEN] — TCP connection succeeded (service listening)
• [TIMEOUT] — No response within timeout (likely filtered)
• [ERROR] — System or socket error
• (No output) — Port closed (connection refused immediately)

Example Use Cases

• Verify local services

  ./scanner 127.0.0.1 8080 8080

• Check common service ports

  ./scanner 127.0.0.1 22 3306

• Detect filtered or non-responsive hosts

  ./scanner 192.0.2.1 80 85

Notes & Limitations

• Scanning large ranges (e.g. 1–65535) is slow due to sequential execution.
• Uses a full TCP handshake (not stealthy).
• No service, version, or OS detection.
• Intended for learning, not production use.

---

Disclaimer: Scan only systems you own or have explicit authorization to test. Unauthorized port scanning may be illegal.