BrandBlock

Simple Fabric server-side mod to block certain client brands from joining your server

Configuration

config/brandblock.json

{
  "blockedBrands": [                             // All brands that will be blocked
    "forge",
    "vanilla"
  ],
  "kickMsg": "This client brand is not allowed"  // The disconnection reason shown to the player, can be in Raw JSON text format
}

How it works?

When joining a server, during the configuration phase, both the server and client send a minecraft:brand custom payload packet (aka plugin message) containing the brand name. For unmodified clients and servers, this brand is always "vanilla." However, custom clients (such as Forge, Fabric, etc.) typically change this brand identifier.

This server-side mod listens when a brand payload is received and checks if it matches any blocked brands specified in the configuration file. If a blocked brand is detected, the player is disconnected from the server. However, this can be easily bypassed and modified clients may not to send this packet at all.