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.

Comparison with Nmap

Feature	This Scanner	Nmap
Scan Speed	Sequential	Parallel
Scan Technique	TCP connect	Multiple
Service Detection	No	Yes
OS Detection	No	Yes
Extensibility	No	NSE scripts

Troubleshooting

If you see [ERROR]:

Verify the IP address
Check network connectivity
Ensure outbound connections are allowed
Confirm you have permission to scan the target

Future Improvements

Possible extensions to improve functionality and performance:

Parallel Scanning: to scan multiple ports concurrently.
Configurable Timeout: Allow users to set custom timeout values.
Scan Modes: Add support for half-open (SYN) scans instead of full TCP connects.
Service Identification: Perform basic banner grabbing to identify common services.
IPv6 Support: Extend the scanner to handle IPv6 addresses.
Output Options: Support machine-readable formats (e.g., JSON) for automation and scripting.

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