HailWall — Mod Whitelist

HailWall checks the mods installed on a player's client when they join, and kicks them if they have disallowed mods (or are missing a required one). It's a modern, lightweight take on the classic Mod Whitelist idea.

⚠️ Install it on BOTH the server and every client. A purely server-side mod cannot read a client's full mod list — vanilla simply doesn't send it. So, like every mod of this kind, HailWall needs a tiny companion running on the client.

✨ Features

• Whitelist mode (default) — block everything except the mods you allow. The essentials (minecraft, the loader, java, hailwall) are always allowed.
• Blacklist mode — allow everything except specific mods (cheat clients, x-ray, …).
• Required mods — kick players who are missing a mandatory mod.
• Operator bypass — server operators are never kicked, even with extra mods (like a usePermission toggle).
• Transition / monitor mode (enforce: false) — kick nobody, but still record everyone's mods. Perfect for rolling HailWall out without disrupting players while you build your list.
• Access log — one JSON-Lines file per day (kept under config/, so it's reachable even on hosts like Aternos) listing who joined and with which mods. Old files are pruned automatically.
• Signed handshake — HMAC + per-connection challenge to stop trivial packet spoofing.
• Zero gameplay overhead — everything happens once, during login; the log is written off-thread. No per-tick cost.

⚙️ How it works

During the login handshake the server asks the client for its mod list. The client replies with its installed top-level mods (id + version), signed with HMAC. The server verifies the signature, compares the list against your config, and either lets the player in or disconnects them — all before they enter the world.

Situation Result Installed on client and server ✅ Full verification Installed only on the client You can still play on other servers; here it protects nothing Installed only on the server A client without HailWall can't join (when requireCompanionMod is on)

📦 Setup

1. Install the correct version of your chosen mod loader (e.g., Fabric, Forge, or NeoForge) and any required APIs on the server and on every client.
2. Drop the hailwall-x.y.z.jar into the mods/ folder of the server and of each client.
3. Start the server once to generate config/hailwall.json, edit it, and restart.

🔧 Configuration — config/hailwall.json

{ "mode": "whitelist", // "whitelist" or "blacklist" "enforce": true, // false = monitor mode (log only, kick nobody) "operatorsBypass": true, // operators are never kicked "requireCompanionMod": true, // kick clients that don't have HailWall "verifySignature": true, "logModLists": true, "enableAccessLog": true, "accessLogRetentionDays": 5, "whitelist": ["sodium", "iris", "modmenu"], // YOUR allowed mods "blacklist": ["meteor-client", "wurst", "baritone"], // used only in blacklist mode "requiredMods": [] // + fully customizable / translatable kick messages }

Find any mod's id in its mod.json (the id field). Easier: read it straight from the access log, or from the client's logs/latest.log, where HailWall prints the reported mod list.

🔒 Privacy & data

• The client sends only mod ids + versions of top-level mods — no file paths, no system info, no personal data.
• That data is used to allow/deny the join and is recorded only in the server's own access log (default 5-day retention). It's never sent anywhere else, and other players can't see it.
• Open source — you can verify exactly what it does.

🛡️ Honest security note

Like every client-side mod check, HailWall is not bulletproof: a determined user could still bypass it. The HMAC + challenge stops casual spoofing, not an expert. It works great against normal players and is best paired with online-mode=true. Treat it as one solid layer, not a silver bullet.