NeoVelocity

Without a compatibility layer, a NeoForge server behind a Velocity proxy often sees every player connect from 127.0.0.1 and fails the login. NeoVelocity implements Velocity's native forwarding protocol, so the proxy passes the real IP address, UUID and game profile to the server, and the server validates UUIDs and skins correctly. The mod is server-side only, so players join without installing anything, and it works with NeoForge builds for 1.21 – 1.21.11.

Setup is handled in the server config. The forwarding secret can be pasted directly into the config, or the config can point to a file that holds it. Changes to the secret apply immediately, so the server does not need a restart when the secret is updated. The mod also documents the Velocity startup flag that must be set to a number higher than 64 for the forwarding to work.

Mods built on the Fabric Networking API send their own custom login packets. For those to work, the server's login-custom-packet-catchall setting must be false. With that setting false, NeoVelocity checks the signature of each packet first, claims the packets that belong to Velocity, and passes the rest to the other mods. If the secret does not match, NeoVelocity ignores the Velocity packet. If the server receives unknown data it cannot handle, it disconnects the player with an Incompatible mod detected error, which is written to the logs.