Nytheria Shield

Check which mods a player has before they join your server.

Nytheria Shield is a mod list check for Fabric 1.21.1 servers. When a player connects, the server asks their client for its mod list during login, before they spawn. It then checks the list against your whitelist, blacklist and required mods, and either lets them in or disconnects them with a clear message saying what to change.

All rules live on the server. The client part only answers the question.

✨ Features

• Checked during login. Players who fail are disconnected before they enter the world.
• Whitelist, blacklist and required mods. Allow only your modpack, ban specific mods, or require mods every player must have. Optional version ranges per mod.
• Build your whitelist in game. Join with your reference modpack and run /nytheriashield whitelist capture. The server writes the list for you.
• Dry run mode. Test your rules on real players first. Everything is logged and nobody is kicked.
• Clear disconnect messages. Players see exactly which mods to remove, install or update. All message texts can be edited.
• Discord notifications (optional). Webhook embeds for rejected (or all) logins, sent in the background so they never slow down logins.
• Reload without restart. /nytheriashield reload applies config edits right away.
• Status command. /nytheriashield status shows the active rules and warns about risky settings.

📦 Two files: which one do I need?

The client file does nothing anywhere else. In singleplayer and on servers that don't run Nytheria Shield, it sends nothing and checks nothing. Add it to your modpack and players never notice it.

🔧 Requirements

• Minecraft 1.21.1
• Fabric Loader 0.16.10 or newer (built and tested with 0.19.5)
• Fabric API
• Java 21

🚀 Quick setup

1. Put the server jar in your server's mods/ folder and start the server once. It creates config/nytheria_shield.json5 with comments on every setting.
2. Set dryRun: true and run /nytheriashield reload.
3. Join with your complete modpack (including the client jar) as an operator and run: /nytheriashield whitelist capture
4. Watch the log while players join. Anyone who would have been rejected shows up as [DRY RUN] would have rejected .... Add any missing mods to the config by hand.
5. When everything looks right, set dryRun: false and reload. The rules are now enforced.

Example config

{
  dryRun: false,
  requireWhitelistOnly: true,
  allowedMods: [
    { id: "sodium" },
    { id: "lithium", minVersion: "0.12.0" }
  ],
  forbiddenMods: [
    { id: "xray", reason: "X-ray is not allowed" }
  ],
  requiredMods: [
    { id: "voicechat" }
  ]
}

💬 Commands

All commands need permission level 4.

The config file is backed up (.bak) before a capture writes to it, and your comments are kept.

⚠️ Please read: what this mod can and cannot do

Nytheria Shield checks the mod list a client reports. A player who modifies their client can report anything they like, and no mod can fully prevent that.

Good for:

• Making sure everyone is on your modpack
• Blocking casual misuse: forbidden mods, missing pack mods, wrong versions
• Keeping honest players honest without a third-party anti-cheat service

Not good for:

• Stopping a determined cheater with a modified client
• Replacing a real anti-cheat that checks what players do in game (movement, reach, packets)

Tip: a blacklist (forbiddenMods) matches exact mod ids, so a renamed cheat is not caught by it. For real enforcement use the whitelist (requireWhitelistOnly: true).

❓ FAQ

Does the client jar work on other servers? Yes. It only answers servers that run Nytheria Shield and does nothing on any other server.

Can I let admins bypass the check? Yes, with exemptPlayerUuids. Only rely on this with online-mode=true (or a proxy that verifies players). On offline-mode servers, anyone can claim a UUID.

What happens if my config has a typo? The server refuses all logins and logs an error until the file is fixed and reloaded. It never silently lets everyone in.

Do I need Discord? No. Discord notifications are optional and off by default.