Trampoline
IP-Forwarding support for Fabric servers connected via Bungeecord.
Why would you use this? It's because of how Bungeecord is setup where it requires the servers to be in offline mode, but this doesn't give the player's real UUID or skin. IP-Forwarding fixes this but requires some server modifications to work.
How do I use this. First make sure people can't join your server and bypass the Bungee proxy by following the tutorial here, otherwise others could log in as others: https://www.spigotmc.org/wiki/firewall-guide/
Then just drop it into the mods folder and restart your server. The mod will automatically configure the server into offline mode to work with Bungeecord.