WIP: Forge 1.20.1 (FML3) login plugin message forwarding

[robinbraemer]

Problem

Forge 1.20.1 modded clients cannot connect to modded backend servers through Gate proxy. The backend server rejects connections with:

Channels [mantle:network,tconstruct:network] rejected vanilla connections

This happens because:

1. Modern Forge (1.13+) uses login plugin messages (fml:loginwrapper) during LOGIN phase for mod negotiation
2. Gate doesn't forward these messages - it completes client login before connecting to backend
3. By the time client connects to backend, it's already in PLAY state and can't receive LOGIN phase messages

User's Setup

• Gate proxy with BungeeCord legacy forwarding
• Forge 1.20.1 backend server with BungeeForge mod
• Forge 1.20.1 client with mods (Tinkers Construct, Mantle, etc.)

What Was Tried

1. Fixed FML marker detection

• Gate now detects FML2 and FML3 markers (not just FORGE)
• Connection type correctly identified as ModernForge

2. Added ForgeData to ping response

• Modern Forge clients expect forgeData in server ping (not legacy modinfo)
• Added fmlNetworkVersion: 3 for FML3 compatibility
• Server list now shows "Incompatible FML modded server" instead of "vanilla"

3. Added BungeeForge extraData property

• BungeeForge expects extraData property with \x01FML3 prefix
• Property now included in legacy forwarding

4. Attempted login plugin message forwarding

• Deferred ServerLoginSuccess for Modern Forge clients
• Connect to backend BEFORE completing client login
• Forward fml:loginwrapper messages from backend to client
• Fixed LoginPluginMessage packet to use raw bytes (not length-prefixed)

Current State

Stuck at "encrypting..." stage

Flow:

1. ✅ Client connects to Gate with FML3 marker
2. ✅ Gate authenticates client with Mojang
3. ✅ Gate connects to backend server
4. ✅ Backend sends LoginPluginMessage (fml:loginwrapper)
5. ✅ Gate forwards message to client (same ID, same data)
6. ❌ Client does NOT respond - no LoginPluginResponse
7. ⏳ Backend times out waiting

Logs

forwarding Forge login plugin message to client channel=fml:loginwrapper id=0 dataLen=161
encoded packet PacketType=*packet.LoginPluginMessage bytes=180
sent LoginPluginMessage to client, waiting for response id=0
... (no response from client)
timeout waiting for Forge login plugin response

Possible Issues to Investigate

1. Client state machine - Maybe client doesn't expect LoginPluginMessage after SetCompression?
2. Packet format - The forwarded packet might be malformed in some way
3. Context/timing - Client might need something else before it will respond
4. Different approach needed - Maybe proxy needs to act as a Forge server itself (like Velocity does)

Files Changed

• pkg/edition/java/ping/pong.go - Added ForgeData struct for modern Forge ping
• pkg/edition/java/proto/packet/login.go - Fixed LoginPluginMessage to use raw bytes
• pkg/edition/java/proxy/login_inbound.go - Added Forge response forwarding
• pkg/edition/java/proxy/session_backend_login.go - Forward login plugin messages
• pkg/edition/java/proxy/session_client_auth.go - Defer login for Modern Forge
• pkg/edition/java/proxy/session_status.go - Use ForgeData in ping response

References

• Minecraft Forge Handshake - wiki.vg
• BungeeForge mod for 1.20.1

Test Environment

Config used for testing:

config:
  bind: 0.0.0.0:25565
  announceForge: true
  compression:
    threshold: -1  # Disabled for testing
  servers:
    forge-backend: localhost:25566
  try:
    - forge-backend
  forwarding:
    mode: legacy

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