Offline Login Mod

Provides authentication for NeoForge offline-mode servers.

This mod has two main goals:

• Premium players can complete Mojang online validation on an offline-mode server.
• Offline players must register or login before they can move, interact, chat, or view their real inventory.

Features

1. Offline Player Registration and Password Login

• Unregistered offline players can use register <password> <confirmPassword> on first join.
• Registered offline players can use login <password>.
• Players who are already in-game can use auth changepassword <password> <confirmPassword> to change their offline password.

2. Premium Account Validation

• When a player connects, the mod actively starts premium handshake and Mojang session validation during the login phase.
• If validation succeeds, the player continues login as a premium account, with the experience consistent with a premium server.
• After a successful premium login, the player uses the premium UUID instead of the offline-mode server-generated UUID, so that both mods depending on premium UUIDs (e.g., Figura and FTB Chunks) and premium skins work correctly.

3. Known Player List Management

• Every successful login (premium or offline) is recorded in the known player list, including UUID, username, and login mode.
• On the next login, the player is routed directly according to their known mode, skipping mode checks.
• If a player is in the list and marked as ONLINE, but premium validation fails, they are rejected immediately.
• Administrators can use auth setmode <UUID|username> <online|offline> to manually specify a player's login mode.
• Administrators can use auth remove <UUID|username> to completely remove all stored data for a player (known player list, offline password, login blocks, and passwordless login records). The player will return to a first-login state on next join.

4. Passwordless Login Window

• After a registered offline player logs in successfully, the mod records a trusted login entry for that account UUID and IP.
• Within the configured time window, the same UUID logging in again from the same IP can skip password entry.
• If the same IP is associated with multiple UUIDs during that window, those UUIDs lose passwordless eligibility and fall back to normal password login.

5. Isolation Before Authentication

Before an offline player completes registration or login, the mod places the player into a pending-auth state. In that state, the mod:

• Switches the player to spectator mode.
• Locks the player's position and continuously applies blindness.
• Sends an empty inventory view to hide the real inventory.
• Blocks chat, attacks, block interaction, container access, item dropping, and other gameplay actions.
• Allows only the register and login authentication commands.

6. Configurable Security Policies

• Maximum password retry count.
• Temporary block duration.
• Login timeout.
• Repeated prompt interval.
• Minimum password length.
• Password blacklist (auto-created on first startup, directly editable).
• Mojang network request timeouts.
• Default language and automatic player-language detection.

Configuration

Config File Location

• The server config file is named mixauth-server.toml.
• NeoForge loads this file as a SERVER config.
• After changing the config, restarting the server is recommended so that the new authentication parameters are fully applied on the next startup.

Default Configuration

[database]
path = "mixauth/mixauth"

[offline_login]
max_login_attempts = 3
temporary_block_minutes = 5
trusted_login_window_hours = 24
login_timeout_minutes = 5
prompt_interval_seconds = 5
bcrypt_cost = 12
min_password_length = 1
max_password_length = 72
password_blacklist_path = "mixauth/password_blacklist.txt"

[online_validation]
connect_timeout_seconds = 10
request_timeout_seconds = 10
pending_handshake_ttl_seconds = 120

[localization]
default_language = "en_us"
auto_detect_player_language = true

Key Configuration Options

Option	Description
database.path	Base path of the H2 database. Relative paths are resolved from the server root, and the default produces mixauth/mixauth.mv.db.
offline_login.max_login_attempts	Maximum number of wrong password entries allowed during a pending-login phase.
offline_login.temporary_block_minutes	Temporary block duration after the failed-attempt limit is reached.
offline_login.trusted_login_window_hours	Passwordless login window for the same UUID and IP.
offline_login.login_timeout_minutes	Timeout for a registered offline player while waiting in pending-login state.
offline_login.prompt_interval_seconds	Interval for repeating register or login prompts during pending authentication.
offline_login.bcrypt_cost	BCrypt cost factor used for offline password hashes.
offline_login.min_password_length	Minimum password length (default 1, range 1–72). BCrypt input is limited to 72 bytes.
offline_login.max_password_length	Maximum password length (default 72, range 1–72). BCrypt input is limited to 72 bytes.
offline_login.password_blacklist_path	Path to the external password blacklist file. File format is one password per line; lines starting with # are comments. When the file does not exist, it is automatically created from built-in resources on first startup. Relative paths are resolved from the server root. Default produces mixauth/password_blacklist.txt.
online_validation.connect_timeout_seconds	Timeout for connecting to Mojang services.
online_validation.request_timeout_seconds	Timeout for Mojang service requests.
online_validation.pending_handshake_ttl_seconds	Retention time for a pending premium handshake during the login phase.
localization.default_language	Default prompt language. Supported values: zh_cn, en_us, es_es, pt_br, ru_ru.
localization.auto_detect_player_language	Whether to use the player's reported client language after login when it matches a supported locale.

Additional notes:

• The player's language cannot be determined reliably before login, so that phase always uses localization.default_language.
• Offline player UUIDs are forced to be generated by the server based on the username (hash of OfflinePlayer:<username>), ensuring the same UUID for the same user on every login.
• Changing an offline password, or resetting one through an administrator action, clears previous trusted-login records.

Commands

Regular Player Commands

Command	Description
register <password> <confirmPassword>	Register an offline password for the first time. If the player is already online in-game but has not set an offline password yet, this command can also create one.
login <password>	Complete login with the offline password.
auth changepassword <password> <confirmPassword>	Change the player's own offline password.

Administrator Commands

Command	Description
auth setpassword <UUID|username> <password> <confirmPassword>	Set or reset the offline password for the specified player.
auth setmode <UUID|username> <online|offline>	Set the login mode for the specified player, forcing premium or offline login for future joins.
auth remove <UUID|username>	Completely remove all stored data for the specified player (known list, offline password, login blocks, passwordless records). They will return to a first-login state on next join.