mc-multilogin-compat-mod

A Fabric server-side mod for Minecraft 1.21.1 (Mojang mappings) that integrates with MC-MultiLogin-service.

一个用于 Minecraft 1.21.1（Mojang 映射表）的 Fabric 服务端模组，用于与 MC-MultiLogin-service 集成，可以实现多源登录方式。

We suggest you to use this with Authlib-Injector. This mod only change the login method and the login-failed message.

建议配合 Authlib-Injector 使用。此模组仅修改登录方式和登录失败时的提示信息。如果您使用Authlib-Injector，请记得设定它的链接为 MC-MultiLogin-service提供的API地址。

Velocity Plugin Version

Config can be found at plugins\multilogin-auth-compat\multilogin-compat-config.json.

配置可以在plugins\multilogin-auth-compat\multilogin-compat-config.json找到。

Fabric Mod Version

Features

• On every player login the mod calls the configured MC-MultiLogin-service endpoint with detail=true, so authentication failures return a structured JSON error instead of the generic "Failed to verify username!" message.
• Detailed error reasons (DUPLICATE_NAME, BANNED, NOT_FOUND, …) are forwarded directly to the connecting client in the disconnect screen.
• Auto-rename on DUPLICATE_NAME (configurable): when the service reports that a player name is already taken by another skin-site account, the mod automatically retries the login with the service-suggested availableId (e.g. Steve_2) and records the original → new-name mapping in config/mc-multilogin-renames.json.

Setup

1. Place the mod JAR in your server's mods/ directory.
2. Start the server once – a default config file will be generated at config/mc-multilogin-compat.json and the server will stop with an error asking you to fill in the API URL.
3. Edit the config and set apiUrl to the base URL of your MC-MultiLogin-service method entry (e.g. http://127.0.0.1:25600/login/my).
4. Restart the server.

Configuration (config/mc-multilogin-compat.json)

{
  "apiUrl": "http://127.0.0.1:25600/login/my",
  "autoRename": true
}

Field	Type	Default	Description
apiUrl	string	""	Required. Base URL of the MC-MultiLogin-service login method (e.g. the path configured under method[].url). The mod appends /session/minecraft/hasJoined?… automatically.
autoRename	boolean	true	(Not working at present) When true, a DUPLICATE_NAME rejection triggers an automatic retry using the availableId returned by the service. The rename is persisted in config/mc-multilogin-renames.json. Set to false to disable and show the error instead.

Building

# Requires Java 21 and internet access to maven.fabricmc.net
./gradlew build

The output JAR will be in build/libs/.