AuthBack

Allows you to play as normal even when Mojang's authentication servers are down.

How does it work?

1. The responses from Mojang's webservices/API are cached locally for some time.
   If an outage occurs the previously cached responses are used.
   For example this allows for your skin to be displayed properly.
2. Every client locally stores (or generates if not present) a keypair, which will be used as a fallback to authenticate with servers when Mojang's authentication servers are down (see Public-key cryptography).
   Please note:
   • The server also needs to have the mod installed
   • You must have already joined the server once (during the past 36 days)

The mod is optional for the client and server.
However if one side lacks the mod joining a server will be impossible when Mojang's centralized authentication servers/APIs are down.

Usage

Command

You can use the /authback command to e.g. manage your public keys or those of other players if you are an admin.

Configuration

Client

You can edit the most important configuration in the game options UI via

• Options > Online... > AuthBack...
• If ModMenu is installed: Mods > AuthBack > Click Icon/Settings

General

The configuration is dynamically loaded from (sorted by highest priority)

• Environment variables
  • prefixed with AUTHBACK_<variant>
    • where variant is either SERVER OR CLIENT
  • prefixed with AUTHBACK_
  • all properties are in UPPERCASE and use _ (instead of .) as delimiter
• System properties
  • prefixed with authback.<variant>
    • where variant is either server OR client
  • prefixed with authback.
• A configuration file located in .config/authback-<variant>.json
  • where variant is either server OR client

Full list of configuration options

Please note that the preconfigured values usually work out of the box.
You should know exactly what you're doing when doing modifications.

Common (Client and Server)

Property	Type	Default	Notes
global-public-keys-cache.default-reuse-minutes	int	120 (2h)	The response for the global public keys of the central Mojang API barely ever changes. As of writing this documentation it has stayed the same the past 2+ years. This option instructs the mod to re-use the last saved response for the specified amount of minutes instead of contacting the API again. Using the cached response is a lot faster and saves network traffic during frequent game restarts.
game-profiles.delete-after-days	int	36	36 days was choosen as the default because when a player changes their username the name will be unavailable for 37 days
game-profiles.max-cache-size	int	250	Maximum amount of game profiles to keep in the cache. If the size exceeds the maximum the oldest entries will be removed until the list is at 90% of the configured maximum.
skip-extract-profile-action-types	bool	false	Debug-Option

Server

Property	Type	Default	Notes
keys.max-keys-per-player	int	3	Maximum amount of public keys to store per player
keys.delete-after-unused-days	int	36	If a user does not login with a public key for this amount of time the key will be deleted. 36 days was choosen as the default because when a player changes their username the name will be unavailable for 37 days
fallback-auth.allow-always	bool	true	Always allows the use of fallback authentication. Set to false if you only want to allow fallback authentication when the server can't communicate with the central Mojang API during a player login
fallback-auth.rate-limit.requests-per-ip-per-minute	int	20	The default allows for requests every 3s. If the value is set to 0 or less the rate limiter will be disabled
fallback-auth.rate-limit.bucket-size	int	1000	Amount of IP addresses to store (in memory)
fallback-auth.rate-limit.ignore-private-addresses	bool	true	Should private IP addresses be rate limited?
fallback-auth.rate-limit.ipv6-network-prefix-bytes	int	8	Network prefix bytes (not bits!) for IPv6. The default 8 resolves to /64.
skip-old-user-conversion	bool	true	Skips the migration of user files used by servers before 1.7.6 (released 2014-04). It's extremely unlikely that this is required by a server and requires contacting the central Mojang API therefore the migration is skipped by default

Client

All client specific options can be changed / are described in the game options UI (see above).

FAQ

Where can I find my public key?

The public key can be found/copied under

• Options > Online... > AuthBack... > Key management
• or if ModMenu is installed: Mods > AuthBack > Click Icon/Settings > Key management

The authentification servers are down, I installed the mod on my server but I can't join

This is because the server doesn't know your public key.

You can fix this in the following way:

1. Make sure that you logged onto the server less than a month ago
   • that is because the entries in usercache.json - which will be used in such a case - will expire after a month
2. Make sure that you can run commands via the server console/RCON
3. Add the mod to your local client/game and launch it
4. and copy the public key from there (see above for details)
5. Then associate the public key on the server with your account:
   • /authback public_key add name <yourName> <yourPublicKey>
   • or /authback public_key add id <yourUUID> <yourPublicKey>

You should now be able to log in as usual.
Note however that profile related content like skins will not not be displayed.

It's also recommended to restart the server once the outage is over to correctly load existing profiles.

The authentification servers are down, my friend got the mod but they can't join my server

This is because they never joined with the mod before.

You can fix this in the following way if you are an Admin:

1. Tell your friend to launch the game with the mod
2. and to send their public key to you
3. Associate the public key on the server with them by running /authback public_key add name <yourFriendsPlayerName> <yourFriendsPublicKey>

Where does the mod store it's data?

In the game directory (e.g. %APPDATA%\.minecraft) inside the .authback directory.