Sonar

Sonar is a lightweight and easy-to-use anti-bot plugin for your Minecraft server. Sonar provides a multi-layered defense against bots, keeping your server safe from common and even sophisticated bot attacks without hindering the experience for real players.

Design and Goal

Effective, lightweight, and easy-to-use
No unnecessary features and clean code
Protection against all kinds of bot attacks
No player should be annoyed by any sort of CAPTCHA
No sort of checking for VPNs or proxies
Multi-platform support (See supported versions)

Checks

Sonar analyzes a player's behavior before joining the actual server, therefore stopping malicious traffic from ever reaching the backend. It is supposed to be an instant, powerful, and simple method of verification that should prevent all typical and advanced types of bots.

Sonar sends the player to a lightweight fake server when they connect for the first time.
Sonar verifies that players obey the laws of Minecraft's physics, including gravity and proper block collision.
Sonar verifies that players send legitimate packets when interacting with vehicles (e.g. boats).
Sonar makes sure that players send legitimate packets according to the vanilla Minecraft protocol.

Sonar also protects against spambot attacks since it queues the incoming connections, therefore making it technically impossible to have a ton of bots join the server at the same time.

False positives

Sonar is unlikely to ever falsely prevent a player from joining the server since Minecraft uses the TCP protocol, which means that packets are always sent in the correct order. Therefore, lag or ping should not affect the bot check.