Server PreCheck

A Fabric mod for Minecraft that allows server administrators to control which mods clients can use when connecting.

Overview

Required on both client and server. The client sends its mod list to the server, which validates it against configured rules before allowing connection.

Installation

1. Download the mod JAR file
2. Place it in your mods folder (both client and server)
3. Start Minecraft/Server
4. Configure the server settings (see below)

Quick Start

1. Generate your whitelist: Have a client with acceptable mods run /serverprecheck build whitelist to copy the mod list to clipboard
2. Configure: Edit config/server_precheck-config.json on your server
3. Choose mode: Set USE_WHITELIST_ONLY to true (whitelist) or false (blacklist)
4. Optional: Add player UUIDs to EXEMPT_PLAYERS to bypass checks

Configuration

Validation Modes

Blacklist Mode (default, USE_WHITELIST_ONLY = false)

• Allows all mods except those in CLIENT_MOD_BLACKLIST
• Default blacklist: aristois, bleachhack, meteor-client, wurst

Whitelist Mode (USE_WHITELIST_ONLY = true)

• Only allows mods in CLIENT_MOD_WHITELIST
• Stricter control for curated modpacks

Key Configuration Options

| Option | Type | Description |
|——–|——|————-|
| USE_WHITELIST_ONLY | Boolean | false = blacklist mode, true = whitelist mode |
| CLIENT_MOD_NECESSARY | Array | Mods required to join (default: ["server_precheck"]) |
| CLIENT_MOD_WHITELIST | Array | Allowed mods in whitelist mode |
| CLIENT_MOD_BLACKLIST | Array | Banned mods in blacklist mode |
| EXEMPT_PLAYERS | Array | Player UUIDs that bypass all checks |
| MODPACK_NAME | String | Shown in disconnect messages |
| CUSTOM_DISCONNECT_MESSAGE | String | Overrides default disconnect message |

Message customization options (MSG_HEADER, MSG_UNAUTHORIZED_HEADER, MSG_MISSING_HEADER, etc.) support i18n translation keys.

Client Commands

| Command | Description |
|———|————-|
| /serverprecheck list | Lists all installed mods |
| /serverprecheck count | Shows the number of installed mods |
| /serverprecheck build whitelist | Copies mod list to clipboard for config |

Utilities

Finding a Mod ID: Open the mod’s JAR file → fabric.mod.json → look for the "id" field

Finding a Player UUID: Use NameMC or check server logs

Getting Mod List from Logs: Check .minecraft/logs/latest.log for:

Server Pre-Check vx.x.x from the client! Modlist: ["fabric-api", ...]

Example Configuration

{
    "MODPACK_NAME": "My Server Modpack v1.0",
    "USE_WHITELIST_ONLY": false,
    "CLIENT_MOD_NECESSARY": ["server_precheck"],
    "CLIENT_MOD_BLACKLIST": ["aristois", "bleachhack", "meteor-client", "wurst"],
    "EXEMPT_PLAYERS": ["069a79f4-44e9-4726-a5be-fca90e38cmf5"]
}

Issues

Please report all issues here