AuthBack

Allows you to play as normal even when Mojang's authentication servers are down. Additionally also improves the networking behavior.

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 authentication servers/APIs are down.

You can find more information - like usage guides, FAQ and a list of all options - in the corresponding repository.