I made plugin for secure transfers.

https://github.com/ChefMC/SecureTransfer
(Secure) Transfer packets plugin for Minecraft 1.20.5+ servers [Paper & Velocity] with total online players count support via Redis.

When anybody join Velocity proxy (and e.g. passing limbofilter plugins checks) and it trying to forward player to backend server, that's cancelling and then player moving to the backend server directly via Transfer packet - but right before this, Velocity storing in Redis an encoded transfer token.

When player after all of this joining backend server, the token is verifying (about player was succesfully passed the entry point IP) and deleting. And then player can transfer himself to any other backend server (backend-backend, many-to-main). But if he was disconnected from any backend server, he can't join them directly again, and must join server again via Velocity (entry point / main IP). And player is prohibited to join servers from the same account if he is already online.

So Velocity doesn't overloaded to keep & handle a lot of connections, and all connections to backend servers are secure.

It also sending total online players data to Redis, so you can write any wrapper that reading it on Velocity side and summarizing online from all your backend server. I will provide my implementation later. I sent to FastMOTD my code of Redis support ( the already coded implementation for FastMOTD). I was tried to add this feature directly into my plugin, but it's force overriding by another MOTD plugins (e.g. FastMOTD which I using and recommend a lot).

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The plugin is new and fresh, so it does not have a lot of functionality yet, but it already copes with secure transfers, as well as collecting and summing up online from backend servers (the display is currently implemented by me personally, because there is one problem with MOTD plugins on Velocity, so I rewrote the FastMOTD plugin itself to implement this into actual MOTD).

I using Velocity because of limbofilter plugin and some features, but decided to migrate to Transfer packets and use Velocity only as entry point. Direct join to backend servers is prevented (or if Redis is down).

I almost falling asleep, so may forget to say something.

Just inspect the code for yourself,

and let's make it better, so more people can use this kind of approach. Because I wasn't found any suitable solutions utilizing Transfer packet the way I was need it, so I made it by myself.

Feel free to send PRs!

E.g. for Bungee support or
e.g. /transfer IP:рort command
I already implemented method for this command:

but currently not added the /command itself.

And so on.

Installation

Just drop the downloaded plugin both into Velocity & Spigot plugins folder.

Then, after server started, perform changes in config.yml:

1. Configure Redis URL to match yourself
2. Configure your front/proxy IP address (to display it to player when he trying to connect directly to backend server)
3. Ensure that auto-generated secure token is equal in all configs of the plugin (both your proxy + servers behind it)