"Can't serialize unregistered packet" with ViaVersion and BungeeCord

[Maxi451]

What happened?

Trying to log in with a 1.20.4 premium vanilla client to a 1.12.2 PaperSpigot server causes to be disconnected with error "Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet"

What did you expect?

I expected the 1.20.2+ premium client to be sent to the "hub" server without any issue

Steps to reproduce

Server setup:

Latest version BungeeCord (git:BungeeCord-Bootstrap:1.20-R0.3-SNAPSHOT:7606d44:1801) with the following plugins:

• AuthMeBungee-2.2.0-SNAPSHOT
• FastLoginBungee.jar (1.12-SNAPSHOT-a65a5f3 (? latest anyway))

"login" server: PaperSpigot 1.12.2 with the following plugins:

• AuthMe-5.6.0-SNAPSHOT-legacy
• FastLoginBukkit (1.12-SNAPSHOT-a65a5f3)
• ViaVersion-4.9.3
• ViaBackwards-4.9.2
• ViaRewind-3.0.6-SNAPSHOT

"hub" server: PaperSpigot 1.12.2 with the following plugins:

• ViaVersion-4.9.3
• ViaBackwards-4.9.2
• ViaRewind-3.0.6-SNAPSHOT

Clients below 1.20.2 don't seem to experience any issue
Steps:

• 1.20.2+ premium vanilla client connects for the first time to the BungeeCord
• player is sent to the "login" server
• everything good, player automatically registered as premium as expected
• player sent to "hub" server
• client logs in again. Three possibilities:
  • everything goes fine
  • client is locked in the "login" server with the "Already authenticated" message, but AuthMe doesn't allow chat or commands
    • if the player logs in again this issue is gone
  • client is successfully authenticated and sent to the "hub" server, but then gets kicked with the message "Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet"
    • if the player is not sent to the "hub" server (i.e. banned) and don't get the "Already authenticated" issue he can freely join (with /server) any 1.20.4 server without issues, but should he try to join an older version (tried 1.12.2 and 1.8.8) he will get kicked for the "Internal exception" error

If I remove FastLogin these errors disappear
This error seems to appear when a 1.20.2+ player switches server to an older version, and seems to be some sort of malformed json (maybe 1.20.2+ players have issues with jsons starting with square brackets). Also, a friend of mine with the same client experiences this issue a lot less

Plugin list

No response

Configuration file

https://pastebin.com/ygXqyZDi

Server log

Bungee:

23:48:19 [INFO] [/-] <-> InitialHandler has pinged
23:48:20 [INFO] [TheSniper99,/-] <-> InitialHandler has connected
23:48:20 [INFO] [FastLogin] Incoming login request for TheSniper99 from /-
23:48:20 [INFO] [FastLogin] Handling player TheSniper99
23:48:20 [INFO] [FastLogin] Requesting premium login for registered player: TheSniper99
23:48:20 [INFO] [TheSniper99] <-> ServerConnector [login] has connected
23:48:21 [INFO] [TheSniper99] <-> ServerConnector [hub] has connected
23:48:21 [INFO] [TheSniper99] <-> DownstreamBridge <-> [login] has disconnected
23:48:22 [INFO] [TheSniper99] -> UpstreamBridge has disconnected
23:48:22 [INFO] [TheSniper99] <-> DownstreamBridge <-> [hub] has disconnected

Login:

[23:48:20 INFO]: UUID of player TheSniper99 is 7ae4946c-10a4-47ff-b173-2cb4a19c87f8
[23:48:20 INFO]: TheSniper99[/-] logged in with entity id 5 at ([world]0.5, 100.0, 0.5)
[23:48:20 INFO]: [AuthMe] TheSniper99 logged in -
[23:48:21 INFO]: [FastLogin] Player info LOGIN command for TheSniper99 from proxy
[23:48:21 INFO]: [FastLogin] Delaying force login until join event fired?: false
[23:48:21 INFO]: [FastLogin] Logging player TheSniper99 in
[23:48:21 WARN]: [FastLogin] Player CraftPlayer{name=TheSniper99} is already authenticated. Cancelling force login.
[23:48:21 INFO]: TheSniper99 lost connection: Disconnected

Hub:

[23:48:21 INFO]: UUID of player TheSniper99 is 7ae4946c-10a4-47ff-b173-2cb4a19c87f8
[23:48:21 INFO]: TheSniper99[/-] logged in with entity id 3 at ([hub]0.5, 160.0, 0.5)
[23:48:22 INFO]: [FastLogin] No on-going login session for player: CraftPlayer{name=TheSniper99} with ID -.
[23:48:22 INFO]: [FastLogin] Setups using Minecraft proxies will start delayed when the command from the proxy is received
[23:48:22 INFO]: TheSniper99 lost connection: Disconnected
[23:48:22 INFO]: TheSniper99 left the game

Plugin version

1.12-SNAPSHOT-a65a5f3

Platform

BungeeCord

Relevance

• I tried the latest build
  (build refers to development builds not necessary a release version; i.e. v1.10 is out of date)

• I checked for existing tickets -
  If there are, please vote them with a thumbs reaction and not create new ones

[TuxCoding]

That's interesting, because in proxy environments FastLogin doesn't do any packet manipulations like in Spigot. Could you try to reproduce it without Via?

[Maxi451]

How may I join a 1.12.2 server without viaversion with a 1.20.4 client?

[TuxCoding]

Sorry, my bad; too many version numbers. However, FastLogin still doesn't do many modifications with BungeeCord. Therefore it could be related solely to be the onlinemode.

Could you try it without FastLogin, but with onlinemode enabled? A local copy might be necessary to prevent UUID changes conversations depending on your plugins. Then also the full exception for Can't serialize unregistered packet from the client logs could be useful, so we could check which packet ID is the issue here. Nevertheless, your descriptions seems to indicate a missing translation by Via* for newer clients.

[Maxi451]

I tried with onlinemode and logged in successfully, so somehow something's actually interfering with FastLogin. Sadly there isn't much in the client log, just this line:
[18:52:23] [Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet
it's almost like the server disconnected the client with a Disconnect packet instead of simply the client crashing. Nothing new in the server's logs

[TuxCoding]

[18:52:23] [Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet

The EncoderException indicates that the packet sender terminated the connection, because it couldn't package the data. In this case it could be both BungeeCord (re-packing) or Spigot if ViaVersion didn't modified the sending process correctly.

Is the log about the second case from the first post or the third with exception message? We could also try to deactivate AuthMe's session logins to prevent conflicts with the Already authenticated error.

Furthermore, AuthMe also sends packets, but this feature is only active if ProtocolLib is enabled. Is ProtocolLib installed and do you know if there any other plugins that perform network packet manipulations?

Regarding local testing: Correct me if I'm wrong, but to reproduce this only ViaVersion from Via-* set is necessary. So connecting to an older server with a newer client.

Sorry for so many questions.

[Maxi451]

So, after many tries I found what is wrong: there's an option in the authme's config.yml file, settings.sessions.enabled, which if set to true will cause 1.20.2+ clients to disconnect in the environment I specified above. Setting it to false will resolve this issue. If I remove fastlogin this issue disappears, so I guess there's something going on between authme and fastlogin on this. It solved the "already authenticated" error too. Anyway, thank you for your time, I will just set it to false

[TuxCoding]

That sounds great, thanks for letting me know. That seems to indicate a race condition in AuthMe code base, like multiple authentication requests at once.