Custom auth server

The authliberty plugin, from @opys/minecraft.

Points the game at your own account server.

authliberty({
  version: 'latest',
  hosts: {
    auth: 'https://auth.example.com/authserver',
    session: 'https://auth.example.com/sessionserver',
  },
});

The game normally asks Mojang about accounts. AuthLiberty is a small Java agent that redirects those calls. Your server must speak Yggdrasil, Mojang's account protocol.

Options

Option	What it does
version	An exact version ('0.3'), or 'latest'.
hosts	Which servers to redirect, and where.
project, gitlab, token	Where the agent jar is published, if not the default.

hosts keys: auth, account, session, services. One you leave out stays on Mojang's. It may also be a function of the key.

What it adds

Kind	What
Files	The agent jar.
Launch	jvmArgs.
Variables	None.
Environment	None.

Each one below: what it is, and how it ends up in the manifest.

Files · the agent jar

One jar, in the libraries folder.

// opys.config.mjs
export default defineConfig({
  plugins: [
    minecraft({ version: '1.21.1' }),
    java({ version: '21' }),
    authliberty({
      version: 'latest',
      hosts: {
        auth: 'https://auth.example.com/authserver',
        session: 'https://auth.example.com/sessionserver',
      },
    }),
  ],
  manifest: {
    command: '@minecraft.command',
    args: [
      '@authliberty.jvmArgs',
      '@minecraft.jvmArgs',
      '@minecraft.mainClass',
      '@minecraft.gameArgs',
    ],
    workdir: '${game_directory}',
  },
});

// in the manifest
{
  "path": "${library_directory}/net/harmoniya/authliberty/latest/authliberty-latest.jar",
  "source": { "url": "https://gitlab.com/api/v4/projects/harmoniya%2Fauthliberty/packages/generic/authliberty/latest/authliberty-latest.jar" },
  "size": 601888,
  "integrity": { "sha256": "aca98855bf83000fb48e3854929a83a7d4785785e4580dfa72c5727b0d3bdaaa" }
}

Launch · @authliberty.jvmArgs

Loads the agent, then one line per host you set.

// opys.config.mjs
export default defineConfig({
  plugins: [
    minecraft({ version: '1.21.1' }),
    java({ version: '21' }),
    authliberty({
      version: 'latest',
      hosts: {
        auth: 'https://auth.example.com/authserver',
        session: 'https://auth.example.com/sessionserver',
      },
    }),
  ],
  manifest: {
    command: '@minecraft.command',
    args: [
      '@authliberty.jvmArgs',
      '@minecraft.jvmArgs',
      '@minecraft.mainClass',
      '@minecraft.gameArgs',
    ],
    workdir: '${game_directory}',
  },
});

// in the manifest
"args": [
  "-javaagent:${library_directory}/net/harmoniya/authliberty/latest/authliberty-latest.jar",
  "-Dminecraft.api.auth.host=https://auth.example.com/authserver",
  "-Dminecraft.api.session.host=https://auth.example.com/sessionserver"
  // … then the game's own JVM arguments
]

Why you place it yourself, and first: the agent must load before any account code runs. Only the launch line decides order, and the launch line is yours.

No variables.

Good to know

• It decides where the game asks. It does not decide who the player is. That is the token: see bifrost.
• 'latest' is pinned to the jar that is latest when you build.